Final Report

Construction of a DOAS Instrument for Installation at CARB for Low Level Measurement of SO2 to Investigate the

Relation between SO2 and Sulfate

ARB Contract No. 10-312

Prepared for:

Mr. Winston Potts Dr. John Collins

Research Division California Air Resources Board

1900 14th St. Sacramento, CA 95811

Email: wpotts@arb.ca.gov

May 2014

Submitted by:

Dr. Thomas D. Durbin Mr. John T. Pisano Mr. Kurt Bumiller

University of California CE-CERT

Riverside, CA 92521 951-781-5791

951-781-5790 (fax)

Dr. Gervase Mackay Dr. Alak Chanda

Mr. Keith Mackay

Unisearch Associates Inc. 96 Bradwick Dr.

Concord, Ontario Canada L4K 1K8

905-669-3547

Disclaimer

The statements and conclusions in this report are those of the contractor and not necessarily those of the California Air Resources Board. The mention of commercial products, their source, or their use in connection with material reported herein is not to be construed as actual or implied endorsement of such products.

Acknowledgments

This report was prepared at the University of California, Riverside, Bourns College of Engineering-Center for Environmental Research and Technology (CE-CERT). The authors thank the California Air Resources Board for funding for this study under contract No. 10-312. The authors also thank Mr. Winston Potts and Dr. John Collins for their technical and programmatic input into this study.

ii Development and Installation of an SO2 DOAS

 

 

 

 

 

 

 

 

 

   

   

   

   

   

   

   

 

   

   

   

   

   

   

   

 

Table of Contents Disclaimer ............................................................................................................................................ ii Acknowledgments ............................................................................................................................... ii Table of Contents ............................................................................................................................... iii List of Figures ..................................................................................................................................... iv List of Tables ........................................................................................................................................v Abstract............................................................................................................................................... vi Acronyms and Abbreviations .......................................................................................................... vii Executive Summary ......................................................................................................................... viii

Recommendations for Future Work ................................................................................................ ix

1 Introduction ..................................................................................................................................1 2 Theory of DOAS Operation.........................................................................................................3

2.1 Spectroscopy .......................................................................................................................... 3

2.2 DOAS (Differential Optical Absorption Spectroscopy) ........................................................ 3

2.3 List of Pertinent Species Capable of Being Measured By DOAS ......................................... 4

3 Description of DOAS Instrument ................................................................................................7 4 Test Laboratory Configuration.................................................................................................11

Calibration and Gas Delivery System......................................................................................... 11 5 Performance Tests at UC Riverside ..........................................................................................14

5.1 Minimum Detection Limits .................................................................................................. 14

5.2 Linearity Tests...................................................................................................................... 15

5.3 Interference Tests ................................................................................................................. 17

5.4 Vehicle Emission Tests ........................................................................................................ 26

6 Installation/Verification at CARB Facility ...............................................................................32 7 Summary and Conclusions ........................................................................................................33

Recommendations for Future Work ............................................................................................... 34

iii Development and Installation of an SO2 DOAS

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

List of Figures

Figure 2-1 DOAS technique, in which I0 is replaced by I0' , the light intensity in the absence of a

structured absorption band............................................................................................................ 4 Figure 2-2 Absorption cross-sections of typical species measured by the DOAS technique; note SO2

Figure 2-3. Absorption cross-sections overlapped of typical species measured by the DOAS technique; note SO2 and HCHO responses which are highly important for the designed DOAS

Figure 3-1. Components of the DOAS system developed for the measurement of low level (~20

Figure 5-4. Shows initial DOAS configuration file and reference spectra for NO, notice strong NO

Figure 5-5. Shows DOAS configuration file and reference spectra for NH3 where the strongest

Figure 5-6. Shows DOAS SO2, NH3 and NO 5-second background response with only zero air

Figure 5-7. Shows enlarged DOAS SO2 5-second background response with only zero air flowing. Notice measured SO2 background again is slightly negative (-3.4ppbv) with standard deviations

Figure 5-8. Shows DOAS SO2 5-second response at 80 ppbV, notice that NH3 and NO spectra and

Figure 5-9. Shows DOAS SO2 5-second response at 680 ppbV. Notice that NH3 and NO spectra, as

Figure 5-10. Shows new DOAS configuration file and reference spectra for SO2 with a change in spectral range of analysis to minimize NO and NH3 interferences. The new range selected is

Figure 5-11. Shows DOAS SO2 5-second background response with new fit limits to minimize NO interference NO is injected at 300 ppmV. Notice strong measured spectral lines for NO where the blue lines (measured) lay on top of the red lines (reference). The interference is below the

Figure 5-12. Shows DOAS SO2 5-second background response with new fit limits to minimize NH3

as well as NO interferences. In this example 16.5 ppmV of NH3 is injected. Notice strong measured spectral lines for NH3 where the blue line (measured) lays on top of the red line

and HCHO responses which are highly important for the proposed DOAS system .................... 5

system ........................................................................................................................................... 6

ppbV) SO2 from vehicle exhaust .................................................................................................. 7 Figure 3-2. UV lamp and transmitter .................................................................................................... 8 Figure 3-3. DOAS receiver, with grating.............................................................................................. 9 Figure 4-1. DOAS in the Laboratory at UC Riverside ....................................................................... 11 Figure 4-2. Overview picture of DOAS in the Laboratory at UC Riverside ...................................... 11 Figure 5-1. DOAS SO2 low dilution linearity plot ............................................................................. 16 Figure 5-2. DOAS SO2 high dilution linearity plot ............................................................................ 17 Figure 5-3. Shows initial DOAS configuration file and reference spectra for SO2. ........................... 18

absorption from 214 nm to 216 nm............................................................................................. 18

absorption is. ............................................................................................................................... 19

flowing. Notice measured SO2 background is slightly negative (-5 ppbv) with standard deviations between 4-5 ppbV...................................................................................................... 20

again between 4-5 ppbV. ............................................................................................................ 20

response are unaffected by this level of SO2. ............................................................................. 21

in the case of the 80 ppbV of SO2, have responses that are unaffected by this level of SO2...... 22

206-214 nm, which leaves interferences to NO of below a single standard deviation (less than 5 ppbV) of SO2 at 300 ppmV (NO) and less than 10 ppbV of SO2 at 16.6 ppmV (NH3). ............ 23

detection limit with new fit limits. .............................................................................................. 23

(reference). The inteference to SO2 at this NH3 level is between 5-10 ppbV............................. 24

iv Development and Installation of an SO2 DOAS

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Figure 5-13. Shows DOAS SO2 5-second response at 680 ppbV using the original configuration file and spectral range of analysis between 203 and 230 nm. All linearity tests were done with this spectral range of analysis. ........................................................................................................... 25

Figure 5-14. Shows DOAS SO2 5-second response at 680 ppbV, using the new configuration file and spectral range of analysis between 206 and 214 nm. The response is similar at this narrower

Figure 5-18. DOAS spectral response under high NH3 conditions showing a negative bias to SO2

Figure 5-19. DOAS measured values for the 2012 Nissan Versa emissions test. Note the drop off of the signal once NH3 levels reached 90 ppmV, and the significant negative bias to SO2 under

Figure 5-20. NH3 emissions measurements for the DOAS along with concurrent measurements with a tunable diode laser system configured for NH3 for the 2012 Nissan Versa. Both are raw exhaust measurements. Note the high similarity in response between the two separate and

Figure 5-21. Regression plot NH3 emissions for the 2012 Nissan Versa for the DOAS compared with

Figure 5-22. DOAS Spectral response under low NH3 conditions, typical to diesel engines, the

spectral range, which is where the primary SO2 absorptions take place. ................................... 25 Figure 5-15: Diesel test at engine turn on, note there is no detectable SO2 (-1.105 ppbV)................ 26 Figure 5-16: progression of diesel test still SO2 measured below detection limit .............................. 27 Figure 5-17: Progression of diesel test with SO2 now measured above the detection limit. .............. 27

measured values. ......................................................................................................................... 28

these high NH3 conditions. ......................................................................................................... 29

distinct spectral techniques. ........................................................................................................ 30

an NH3 tunable diode laser system. Both are raw exhaust measurements. Note the high correlation between the two techniques for values up to 90 ppmV with an R2 of 0.951............ 31

spectral response and the calculated numbers for SO2 show no negative bias. .......................... 31

List of Tables

Table 4-1. SO2 test concentrations used for low dilution ................................................................... 12 Table 4-2. SO2 test concentrations used for high dilution .................................................................. 13 Table 5-1. SO2 low dilution concentration measurements .................................................................. 14 Table 5-2. SO2 high dilution concentration measurements ................................................................ 15 Table 5-3. SO2 low dilution concentration linearity measurements ................................................... 15 Table 5-4. SO2 high dilution concentration linearity measurements .................................................. 16

v Development and Installation of an SO2 DOAS

Abstract Understanding the nature of the sulfur and sulfate in combustion and lubricant-derived particles is important to characterize to better understand the relative contribution of fuel vs. oil sulfur, to understand storage vs release of sulfur from catalyzed surfaces, and measure the sulfur fraction partitioning into the PM nucleation particles. The objective of this research is to construct, test and provide to ARB laboratories a differential optical absorption spectrometer (DOAS) that can measure down to 20 ppbV in real time and determine a mass balance between SO2 and sulfate. The DOAS developed under this program was evaluated using a series of laboratory performance tests at the CE-CERT laboratory. The DOAS will be installed and operated in the ARB Heavy Duty Emissions Test Laboratory.

The differential optical absorption spectrometer (DOAS) developed for this project was specially designed and built for rapid, extractive detection of sulfur dioxide at low ppb levels for the ARB. The integrated system consists of the extractive sampling train, the sampling cell, and the spectrometer. The sample cell uses a single-path design with beam expanding and focusing optics to minimize cell volume. The sample cell for the UC Riverside system is cylindrical with a length of 1.0 m and a total volume of approximately 0.51 liters. To avoid possible interferences from other exhaust constituents, the spectral range was restricted to the UVB region (200–230 nanometer (nm)). The DOAS software also allows for additional subsections of the spectral range to be selected to minimize interferences from species such as NH3. A deuterium lamp was used as the light source. The cell is designed to operate over a full 8-10 hour testing day at a temperature of 110°C, with a temperature range from 25°C up to 120°C (250ºF).

Laboratory tests were conducted to evaluate the response of the SO2 monitor to a range of operating conditions, from ambient conditions to those most representative of exhaust gas emanating from vehicles. These laboratory tests showed detection limits were approximately in the range of 12 to 15 ppbV. The instrument showed good linearity over SO2 concentration ranges of 20 to 953 ppbV. A number of interference tests were conducted, focusing on potential interferences between SO2, NO, and NH3. Initial tests showed some interference with either NO or NH3, at levels of 80 ppbV and 680 ppbV. Initial tests at 300 ppmV of NO and 16.6 ppmV of NH3 confirmed that at these high levels they would interfere with the SO2 response over the spectral range specified. With a modification of the spectral range to 206 to 214 nm, the DOAS interference SO2 response was less than 1.5 ppbV with 300 ppmV of NO injected with, and was slight less than 8 ppbV with 16.6 ppmV of NH3

injected.

The DOAS was evaluated over vehicle emissions tests in CE-CERT’s light-duty chassis dynamometer laboratory. The results showed no appreciable SO2 interferences for the tests on the diesel truck, which had very low NH3 emissions. Clearly, in a low NH3 environment, as tested here the SO2 detection is unbiased and meets the laboratory tested detection limits of around 20 ppbV. If the DOAS is used in a low NH3 environment, as in this test, it should achieve the expected detection limits. The gasoline vehicles, on the other, had higher levels of NH3, and showed a significant interference, especially when the NH3 values exceeded 11 ppmV. If it is to be used for vehicles with higher levels of NH3, an NH3 scrubber would likely be needed to scrub the level of NH3 in the exhaust to no greater than 10 ppmV, as well as perhaps some additional updates to the software that are currently being developed. If an ammonia scrubber is used, it is recommended that SO2

measurements be made both with and without the scrubber in order to determine whether there is any appreciable effect on the measurements.

vi Development and Installation of an SO2 DOAS

Acronyms and Abbreviations

ARB ...................................................Air Resources Board CARB .................................................California Air Resources Board CE-CERT ...........................................College of Engineering-Center for Environmental Research

and Technology (University of California, Riverside) CFO ....................................................critical flow orifice CCD ...................................................charge-coupled device CPU ....................................................central processor unit DECSE ...............................................Diesel Emission Control – Sulfur Effects project DOAS .................................................differential optical absorption spectroscopy DPF ....................................................diesel particulate filter mm .....................................................millimeter NIST ...................................................National Institute of Standards and Technology nm ......................................................nanometer NOx ....................................................nitrogen oxides PM ......................................................Particulate matter SCR ....................................................selective catalytic reduction SO2 .....................................................sulfur dioxide SSR ....................................................solid-state relays UCR ...................................................University of California at Riverside UV ......................................................ultraviolet

vii Development and Installation of an SO2 DOAS

Executive Summary Since sulfur and sulfate are some of the main components of combustion and lubricant-derived particles, it is important to characterize sulfur species and understand the composition and potential health impacts of particles. Sulfate levels in vehicle exhaust particulates can be readily measured, but it is important to understand the relative contribution between combustion and oil-derived particles and the conversion rates found for SO2 to sulfate. With the low level of sulfur present in the fuel and oil products used in modern vehicles, detection limits in the ppb range are needed to measure the SO2 contribution.

One technique that has been shown to provide real-time, low-level measurement of engine-out SO2, is differential optical absorption spectroscopy (DOAS). DOAS technology provides the potential for very low detection limits, along with the capability of measuring SO2 in real-time. Over the past decade, there have been numerous advances in DOAS technology that have allowed for the development of instruments with lower detection limits in conjunction with smaller sample cells to provide improved residence times. The objective of this study was to construct, test, and provide the ARB with a state-of-the-art DOAS that can measure SO2 down to 20 ppbV in real-time. The DOAS developed under this program was evaluated using a series of laboratory performance tests at the CE-CERT laboratory. The DOAS will be installed and operated in the ARB Heavy Duty Emissions Test Laboratory. The major conclusions from the findings are summarized in the following:

The differential optical absorption spectrometer (DOAS) developed for this project was specially designed and built for rapid, extractive detection of sulfur dioxide at low ppb levels for the ARB. The integrated system consists of the extractive sampling train, the sampling cell, and the spectrometer.

The sample cell uses a single-path design with beam expanding and focusing optics to minimize cell volume. This allows the highest amount of throughput for a given amount of initial light energy and is essential with a small diameter sample cell. The small diameter was a design parameter to minimize the cell volume which yields reduced residence time of the exhaust gas in the cell. The sample cell was designed as an open cell with the sample cell residing between the windows, rather than having the transmitting and collecting optics inside the sample cell.

The sample cell for this system is cylindrical with a length of 1.0 m. The total volume of the DOAS sample cell is approximately 0.51 liters.

The cell is designed to operate over a full 8-10 hour testing day at a temperature of 110°C, with a temperature range from 25°C up to 120°C (250ºF).

To avoid possible interferences from other exhaust constituents, the spectral range was restricted to the UVB region (200–230 nanometer (nm)). For the 200–230 nanometer (nm) region, a deuterium lamp was used as the light source. Additional adjustments were also made to allow for subsection of the spectral range to be selected for analysis, to minimize interferences from species such as NH3, such as narrowing the spectral range to 206 nm to 214 nm.

The spectrometer/receiver consists of a single working element, a concave 1200 groove/millimeter (mm) holographic diffraction grating optimized for 200–230 nm.

The DOAS software, LasIRView 2009, is a user-friendly Windows-based based package with outputs for data logging and a software manual. The DOAS software was upgraded to provide an adjustable window over the spectral range, such that interferences from species such as NH3 can be minimized.

viii Development and Installation of an SO2 DOAS

Laboratory tests were conducted to evaluate the response of the SO2 monitor to a range of operating conditions, from ambient conditions to those most representative of exhaust gas emanating from vehicles.

Detection limits were approximately in the range of 12 to 15 ppbV at three times the standard deviation of zero gas response.

The linearity of the instrument was evaluated over SO2 concentration ranges of 20 to 192 ppbV and 51 to 953 ppbV. The regression analysis showed a slope of 1.011 and 0.995, respectively, and R2 values of 1.0 and 0.999, respectively, for these ranges.

A number of interference tests were conducted, focusing on the potential NO/NH3 DOAS interference with SO2, and vice versa. SO2 was not found to interfere with either NO or NH3, at levels of 80 ppbV and 680 ppbV. Initial tests at 300 ppmV of NO and 16.6 ppmV of NH3

confirmed that at these high levels they would interfere with the SO2 response over the spectral range specified. To mitigate the impact of these interferences, the spectral range was modified to focus on a narrower range of 206 to 214 nm. With the new spectral range of analysis, the DOAS interference SO2 response was less than 1.5 ppbV with 300 ppmV of NO injected with, and was slight less than 8 ppbV with 16.6 ppmV of NH3 injected.

The DOAS was evaluated over vehicle emissions tests in CE-CERT’s light-duty chassis dynamometer laboratory. A 2012 Nissan Versa, a 2013 VW Jetta GLI, and a 2013 Dodge Ram 2500 diesel pickup truck were tested. The results showed no appreciable SO2

interferences for the tests on the diesel truck, which had very low NH3 emissions. Clearly, in a low NH3 environment, as tested here the SO2 detection is unbiased and meets the laboratory tested detection limits of around 20 ppbV. If the DOAS is used in a low NH3

environment, as in this test, it should achieve the expected detection limits. The gasoline vehicles, on the other, had higher levels of NH3, and showed a significant

interference. Essentially, when the NH3 values exceeded 11 ppmV, the SO2 measured values go negative. This interference is also non-linear as once the NH3 values approach 80 ppmV, the SO2 measured values are greater than -900 ppbV. If it is to be used for vehicles with higher levels of NH3, an NH3 scrubber would likely be needed to scrub the level of NH3 in the exhaust to no greater than 10 ppmV, as well as perhaps some additional updates to the software that are currently being developed. If an ammonia scrubber is used, it is recommended that SO2 measurements be made both with and without the scrubber in order to determine whether there is any appreciable effect on the measurements.

This DOAS also has the capability of measuring NH3 in real-time. Comparisons were made between the DOAS and an NH3 tunable diode laser. The results showed a good correlation between the TDL and DOAS results for NH3, at NH3 levels below 90 ppmV, with a regression coefficient of R2 =.951, but once the NH3 levels exceed 90 ppmV the DOAS UV light source is too attenuated to make a measurement.

Recommendations for Future Work

Based on the results of this study, several recommendations for future work can be provided. This includes recommendations for further development of the instrument, as well as recommendations based on the laboratory testing that was conducted. For the DOAS itself, the initial work report here suggests that this instrument could be further developed/optimized to provide concurrent measurements of NO and NH3. To expand the measurement capability for the NH3, some additional development work would be needed to address the fact that the DOAS UV light source is too attenuated to make a measurement once the NH3 levels exceed 90 ppm. The possibility of expanding the DOAS for use in other applications or for other species is also potentially of value. Another interesting finding from the laboratory tests was the higher than expected NH3 emissions for some of

ix Development and Installation of an SO2 DOAS

the test vehicles. In particular, both of the gasoline vehicles tested for this study produced higher than expected NH3 emissions, even though both vehicles represented late model vehicle technologies. While these vehicles were tested under a very narrow range of conditions, these initial results suggest that further investigation over a wider range of vehicles and conditions to see how prevalent the higher NH3 emissions levels might be and if this could be an important issue with respect to tailpipe emissions.

x Development and Installation of an SO2 DOAS

1 Introduction Improving air quality and meeting increasingly more stringent ambient air standards is an important factor in improving human health and overall quality of life. Emissions from vehicles and internal combustion engines have long been one of the most important contributors to the emissions inventory. Governmental regulations have targeted the reduction of emissions from such sources for a number of decades now, with advances in aftertreatment systems being a critical factor in the continuing reduction of the emission level standards. For gasoline vehicles, catalysts have been an integral part of their emissions control systems since the mid-1970s. More recently, aftertreatment systems have also been implemented for diesel engines, with the most recent regulations for diesel engines in 2007 and 2010 requiring significant reductions in particulate matter (PM) and oxides of nitrogen (NOx), respectively, to levels that have essentially required the implementation of aftertreatment systems. This includes diesel particle filters (DPFs) for PM and selective catalytic reduction (SCR) for NOx.

One of the most important considerations with the implementation of aftertreatment systems is the potential for sulfur to poison the catalyst surface and reduce its efficiency. It is well known, from studies dating back to the 1970s, that the adsorption of sulfur onto a catalyst surface inhibits the performance of the catalyst, leading to increases in overall emissions [1–4]. From a fuel perspective, fuel sulfur is one of the most important properties to control in order to facility low emissions standards. For gasoline vehicles, the reduction of fuel sulfur has been the subject of a number of studies over several decades [5–12]. Further studies have indicated that the sulfur inhibition and subsequent loss of catalyst activity have an effect on all the regulated as well as other emissions under a range of operating conditions, although the sensitivity of various catalyst formulations to sulfur inhibition can vary [13]. More recent studies of gasoline vehicles have focused on other aspects, such as impact of sulfur in terms of immediate impact, the irreversible impact of sulfur due to permanent catalyst damage, the potential for additional emission reductions in lowering sulfur levels below 30 ppm, and the impact of sulfur for Tier 4 vehicles.

The impact of sulfur on diesel aftertreatment systems is also important. A series of studies were conducted to evaluate the potential impact of fuel sulfur on diesel aftertreatment control systems prior to the implementation of the 2007 emissions standards, as part of the Diesel Emission Control – Sulfur Effects (DECSE) project. These studies showed significant sulfur impacts of DPFs, even at sulfur levels as low as 30 ppm, and that the DPF PM conversion efficiency was nearly zero for 150 ppm sulfur fuel. Additional efforts were also undertaken to evaluate the impacts of lubricant sulfur levels on aftertreatment systems as part of the Advanced Petroleum-Based Fuels – Diesel Emission Control project. Fuel sulfur levels were reduced nationwide to 15 ppm.[14–17]

Since sulfur and sulfate are some of the main components of combustion and lubricant-derived particles, it is important to characterize sulfur species to understand the composition and potential health impacts of particles. Sulfate levels in vehicle exhaust particulates can be readily measured, but it is important to understand the relative contribution between combustion and oil-derived particles and the conversion rates found for SO2 to sulfate. With the low level of sulfur present in the fuel and oil products used in modern vehicles, detection limits in the ppb range are needed to measure the SO2 contribution. Wet chemical methods are commonly used for the measurement of SO2 in stationary source applications, and have also been used in applications with vehicle exhaust. This includes the collection of SO2 using hydrogen peroxide impinger solutions or potassium carbonate coated filters with subsequent analysis by either titration methods or ion chromatography [18–21]. These techniques have limitations for vehicle exhaust

1

as vehicle decline to ultralow emission levels. In particular, such techniques may not provide sufficient detection limits when measured through a dilution tunnel. For measurements made in the raw exhaust, on the other hand, there are other issues since these techniques provide integrated totals over fixed periods of time that cannot be correlated with exhaust flow rates to determine mass emission rates.

An alternative technique that has been shown to provide real-time, low-level measurement of engine-out SO2, is differential optical absorption spectroscopy (DOAS). DOAS technology provides the potential for very low detection limits, along with the capability of measuring SO2

in real-time. DOAS technology has been applied in a number of different applications, including ambient and source measurements and ships. Pisano et al. previously developed and demonstrated a DOAS as part of an earlier study to evaluate the impact of lubricants oil on exhaust emissions. The DOAS developed by Pisano et al. had a time resolution of 3 seconds and an optical path length of 19.6 m, providing a detection limit of 75 ppbv SO2 at three times the standard deviation of the noise level. These levels were below those of any existing methods at the time for measuring SO2 in exhaust in real-time. With this detection limit, the contribution of SO2 from lubricant oil sulfur was measurable over the range of oil sulfur levels utilized in the study [22].

Over the past decade, there have been numerous advances in DOAS technology that have allowed for the development of instruments with lower detection limits in conjunction with smaller sample cells to provide improved residence times. The objective of this study was to construct, test, and provide the ARB with a state-of-the-art DOAS that can measure SO2 down to 20 ppbV in real-time. The instrument can be used to evaluate the mass balance between SO2 and sulfate, which provides information about the relative contribution between combustion and lubricant-derived particles. This instrument can provide interesting testing possibilities as ARB seeks to understand the workings of advanced catalyzed aftertreatment systems and their interactions with the fuel/lube systems and the potential to attenuate or produce sulfate particulate matter. This could be of benefit to fuel vs lube oil sulfur contribution assessments, understanding storage vs release of sulfur from catalyzed surfaces, and measuring sulfur fraction partitioning into the PM nucleation particles. The instrument is designed to be run independently in the ARB’s laboratories by ARB staff. The system was initially be utilized in the ARB’s Heavy Duty Emissions Test Laboratory, but could also be used at ARB’s Haagen Smit Laboratory for vehicle test programs involving light duty diesel or gasoline direct injection vehicles. This report describes the DOAS instrument, the results of performance tests, and the installation of the DOAS at the ARB Heavy Duty Emissions Test Laboratory.

2

2 Theory of DOAS Operation

2.1 Spectroscopy

Spectroscopy can be defined as the investigation of the interaction that occurs between a defined source of electromagnetic radiation and a target sample. The way emissive properties of the radiative source, usually some sort of light source, are perturbed depends upon the sample being investigated. For example, molecules of gas can absorb electromagnetic radiation leading to molecular vibrations or rotations, depending on the absorption frequencies unique to each different type of molecule in the target sample. Therefore, each molecule has its own characteristic absorption pattern over the electromagnetic spectrum. For the investigation of low-level SO2, the best available spectroscopic technique is Differential Absorption Spectroscopy (DOAS).

Spectroscopic methods evaluate the concentration of the molecule investigated through Beer-Lambert’s Law, which states that the fraction of light intensity transmitted through a gas is given by:

I/Io = exp (-()NL) Equation 2-1

where I and Io are the transmitted and incident powers respectively, L is the absorption path length (cms), and N would then be the concentration of the absorbing molecules in number of molecules per cubic centimeter. In this example, () is the wave-number dependent absorption cross section in square centimeters per molecule.

2.2 DOAS (Differential Optical Absorption Spectroscopy)

Differential Optical Absorption Spectrometry (DOAS) is a UV/VIS optical measurement technique based on Beer-Lambert’s Law. Since absorption coefficients in the UV are generally orders of magnitude higher than in the IR region, sensitivity is usually enhanced, which makes DOAS a method for measuring relatively low concentrations. On the other hand, UV absorption features are much broader than those typically associated with the infrared spectra, which leads to two potential problems:

1) Only species with absorption features that are considerably narrower than the typical measuring window of 15–40 nm can be detected and quantified; this constraint significantly reduces the number of compounds that can be measured.

2) The light intensity without absorbing species in the optical path, I0 , has to be

known to apply Beer-Lambert’s Law. Broad absorption features are affected by numerous species in the light path, and with light scattering are often indistinguishable from the background, making it difficult to measure I0 .

One approach to address potential problems incorporates differential optical absorption, in which I0 is replaced by I 0

' , where I 0' is the light intensity in the absence of a structured absorption

band (refer to Figure 2-1). The DOAS technique solves for the concentration using a modified form of the Beer-Lambert’s Law, in which a differential absorption coefficient [ ' ] replaces the absolute absorption coefficient [()]. C is the species concentration, L is the optical path length, and I is the intensity at an absorption peak.

3

-- · -· - · - · -·--·-·-·-·-·-·-· .­

. _,,.,. . --- . ---

1 I 0'

C ln Equation 2-2 'L I

Inte

nsity

"true" I0

broad absorption band

I'0

I

"differential"

Wavelength

Figure 2-1 DOAS technique, in which I0 is replaced by I 0' , the light intensity in the absence

of a structured absorption band

2.3 List of Pertinent Species Capable of Being Measured By DOAS

There are many gases, with UV-vis signatures, to which the DOAS method can be applied, including HCHO, NH3, NO and NO2 for automotive applications. The absorption cross sections for a range of species are provided in Figure 2-2. Developing the DOAS method for different species is primarily a question of software development; however, an additional consideration is tailoring the light source to the desired species for better sensitivity and also examining potential interferences. For example, there is some overlap of the spectral absorption cross-section between SO2 and HCHO, as is evident from Figure 2-2.

4

HO o

j .!R HOHO

'

I CHOCHO

1.1 ll

l, I~ ... BrO C CJ

OD

OBIO IJ

"' w Cl

c; CIO C [l = e- 100 ~ ~ 0 '

OCIO

~1 ll,

,LO

I® j

J '----LO ':: l _J\ °" J.-".i.

' A /\_

2P) WO 400 500 ~ 00 Wa l,mi;itn n m

.

Plot shows spectral response of SO2 and HCHO, which are cross-over species and need to be both accounted for in the method development for this proposed SO2

DOAS.

Figure 2-2 Absorption cross-sections of typical species measured by the DOAS technique; note SO2 and HCHO responses which are highly important for the proposed DOAS system

The spectral range selected for the designed DOAS was 200-230 nm, to avoid possible interferences from other exhaust constituents. A closer view of the absorption cross sections in this region is provided in Figure 2-3. It can be seen that in this region that while the absorption cross section for SO2 is still fairly strong, this is a weak absorption region for HCHO. Thus, HCHO concentration would likely have to be 100 times higher in concentration than SO2 to interfere. In this region, it is also known that both NO and NH3 absorb and they will be measured concurrently with SO2, however, their cross-sections are also typically much weaker (100 times or more). It should be noted that an absorption cross-section is also found for ClO in the same region, but the levels of this gas would be expected to be negligible in vehicle exhaust, and hence it would not interfere with the SO2 readings.

5

Region for enhanced grating response 210 to 230 nm. Far more sensitive to SO2

then HCHO

 

ptical depth

,- 1 ~ c.. ~

- --==- =- :- - - -- - ------- - -'° - -- - --I.I ".C: "' 0..

10·1

300 400 500

"" I ngth / nm Figure 2-3. Absorption cross-sections overlapped of typical species measured by the DOAS

technique; note SO2 and HCHO responses which are highly important for the designed DOAS system1

1 http://www.meted.ucar.edu/EUMETSAT/atmos_comp/navmenu.php?tab=1&page=3.0.0&type=text

6

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S02measurement r.mgl!: Low: 0-2000 ppm; Hii!,h: 0·15% (No sofl:Wilre swl:chlni or settl111 c hil111e reqJlred for 502. HIGH or LOW) ML".is url!ffll!nt rntt-: 2 !ll"C tor t, ie;her).

Manual Rotameter {to adjust ga5 flow rate; 0-6SPLM)

3 Description of DOAS Instrument

The differential optical absorption spectrometer (DOAS) developed for this project was specially designed and built for rapid, extractive detection of sulfur dioxide at low ppb levels for the ARB. The integrated system consists of the extractive sampling train, the sampling cell, and the spectrometer. An overall schematic layout of the system is shown in Figure 3-1, and a full explanation of the system follows.

Figure 3-1. Components of the DOAS system developed for the measurement of low level (~20 ppbV) SO2 from vehicle exhaust

To measure reactive molecules, such as SO2, as well as to allow fast response times, the residence time in the sample cell, and thus the volume of the cell, was made as small as possible. The sample cell uses a single-path design with beam expanding and focusing optics to minimize cell volume. This allows the highest amount of throughput for a given amount of initial light energy and is essential with a small diameter sample cell. The small diameter was a design parameter to minimize the cell volume which yields reduced residence time of the exhaust gas in the cell. Since exhaust gases are very corrosive, hot, and humid, and potentially contain higher levels of particles, direct exposure of the mirrors to the sample gas was avoided. Therefore, the cell was designed as an open cell with the sample cell residing between the windows, rather than having the transmitting and collecting optics inside the sample cell. The sample cell is constructed of electro-polished 316 SS for inertness, with an inlet and outlet provided for the

7

sample gas. The sample cell is equipped with coated anti-reflective quartz windows at each end to allow the UV light to enter and exit. Quartz windows were used with the sample cell, since they are easy and fast to remove and clean.

The sample cell for the UC Riverside system is cylindrical with a length of 1.0 m. The total volume of the DOAS sample cell is approximately 0.51 liters. The cell is designed to operate over a full 8-10 hour testing day at a temperature of 110°C, with a temperature range from 25°C up to 120°C (250ºF). The cell is heated radiatively with heating tapes and the temperature is maintained with heating tapes. A tunable PID controller maintains the cell temperature at a user-defined level. During operation, gas pressure inside the sample cell is slightly below atmospheric due to the extractive sampling technique.

A specially ordered deuterium lamp was used as the light source for the 200–230 nanometer (nm) region. The spectral output of this light source was tested and shown to drop off significantly below 230 nm. The light from the UV lamp source, with peak emission in the 200- 230 nm range is collimated to a beam size just under the interior diameter of the sample cell. The UV lamp transmitter is shown in Figure 3-2.

Figure 3-2. UV lamp and transmitter

The spectrometer/receiver consists of a single working element, a concave 1200 groove/millimeter (mm) holographic diffraction grating optimized for 200–230 nm. The spectrometer is shown in Figure 3-3. The concave diffraction grating both wavelength-disperses the light and focuses it onto the single-stage, thermoelectric-cooled, charge-coupled device (CCD) detector array. The effective resolution of the spectrometer is ~0.1 nm. Spectra are measured every 0.25 milliseconds (ms), and are integrated over 5 seconds for each sample point. The optical elements windows, transfer optics, collector optics and retroreflector are integrated with the spectrometer to provide yield capture efficiencies of greater than 80%.

8

i F

igu

re 3

-3. D

OA

S r

ecei

ver,

wit

h g

rati

ng

The

DO

AS

is

desi

gned

to

be a

ttac

hed

via

a sa

mpl

ing

trai

n to

a s

ampl

e po

rt f

rom

the

veh

icle

’s

exha

ust

syst

em.

A p

ump

can

used

to

extr

act

the

vehi

cle/

engi

ne e

xhau

st t

hrou

gh t

he s

ampl

ing

trai

n an

d th

e D

OA

S. T

ypic

al e

lem

ents

of

a sa

mpl

ing

trai

n in

clud

e a

heat

ed p

arti

cle

filt

er (

300º

F;

150º

C)

and

a he

ated

PFA

-lin

e (3

00ºF

; 15

0ºC

). C

are

shou

ld b

e ta

ken

to a

void

any

col

d sp

ots

in

the

sam

plin

g pa

th. A

ll in

teri

or s

urfa

ces

of th

e sa

mpl

e tr

ain

shou

ld b

e ei

ther

sta

inle

ss s

teel

or

PFA

te

flon

, wit

h th

e ex

cept

ion

of t

he p

arti

cle

filt

er, w

hich

cou

ld c

onsi

st o

f a

mat

eria

l su

ch a

s bo

nded

qu

artz

fib

ers.

The

DO

AS

sof

twar

e, L

asIR

Vie

w 2

009,

is

a us

er-f

rien

dly

Win

dow

s-ba

sed

base

d pa

ckag

e w

ith

outp

uts

for

data

log

ging

and

a s

oftw

are

man

ual.

The

sof

twar

e pr

ovid

es d

ata

outp

uts

via

ethe

rnet

or

RS

-232

for

eas

y in

corp

orat

ion

into

CA

RB

dat

a lo

ggin

g re

quir

emen

ts,

as w

ell

as 4

-20

mA

ou

tput

for

up

to 4

par

amet

ers.

The

pro

gram

com

mun

icat

es d

irec

tly

wit

h an

alyz

er s

o us

ers

can

mon

itor

the

ana

lyze

r in

rea

l-ti

me.

The

pro

gram

has

var

ious

fun

ctio

ns i

nclu

ding

las

er s

etup

, lo

cati

on s

etup

, se

nsor

inp

ut s

etup

, ba

ckgr

ound

sub

trac

tion

, va

riou

s co

rrec

tion

set

ting

and

dat

a pr

oces

sing

, etc

. The

bas

ic p

acka

ge d

oes

not h

ave

addi

tion

al a

nalo

g in

puts

to r

ecor

d su

ch it

ems

as

spee

d, e

xhau

st f

low

, or

pres

sure

s fr

om o

ther

sou

rces

, whi

ch c

an b

e ob

tain

ed s

epar

atel

y by

AR

B.

The

sof

twar

e in

clud

es a

reg

ress

ion

prog

ram

for

the

spe

ctra

l an

alys

is.

A s

ever

al s

tep

proc

ess

is

used

for

the

spec

tral

ana

lysi

s fo

r th

e D

OA

S. T

he s

oftw

are

firs

t no

rmal

izes

the

inp

ut s

igna

l to

the

la

mp

back

grou

nd. T

he r

egre

ssio

n so

ftw

are

then

use

s a

line

ar l

east

squ

ared

app

roac

h to

sol

ve f

or

the

coef

fici

ents

rel

ated

to

the

refe

renc

e sp

ectr

a fo

r th

e di

ffer

ent

spec

ies

of i

nter

est.

The

se

refe

renc

es

are

obta

ined

fr

om

cyli

nder

ga

ses

at

spec

ific

co

ncen

trat

ions

. T

he

mea

sure

d co

ncen

trat

ions

can

the

n be

det

erm

ined

fro

m t

he r

egre

ssio

n co

effi

cien

ts. T

he f

inal

ized

ver

sion

of

the

soft

war

e is

als

o de

sign

ed s

uch

that

the

reg

ress

ion

can

be p

erfo

rmed

ove

r su

bran

ges

of t

he

spec

tral

reg

ion.

Thi

s al

low

s po

rtio

ns o

f th

e sp

ectr

al r

ange

tha

t ar

e su

bjec

t to

int

erfe

renc

es t

o be

re

mov

ed f

rom

the

reg

ress

ion

anal

ysis

. T

he d

etai

ls o

f th

e al

gori

thm

s an

d th

e st

eps

take

n to

opti

miz

e th

e al

gori

thm

s ar

e pr

opri

etar

y.

The

fol

low

ing

has

been

pro

vide

d an

d te

sted

:

1)

Fib

er o

ptic

link

to e

xtra

ctiv

e ce

ll

2)

AC

-DC

pow

er s

uppl

y

3)

A h

eati

ng c

ontr

ol f

or t

he s

ampl

e ce

ll,

incl

udin

g th

erm

al s

enso

rs a

nd t

herm

al c

ontr

olle

rs,

as t

he s

ampl

e ce

ll w

ill

be h

eate

d ab

ove

the

dew

poi

nt o

f ty

pica

l ve

hicl

e ex

haus

t. T

he

DO

AS

met

hod

is w

ritt

en f

or th

e sa

mpl

ing

tem

pera

ture

and

has

a th

erm

al c

orre

ctio

n cu

rve

for

the

vari

ous

tem

pera

ture

s se

lect

ed.

Tem

pera

ture

dir

ectl

y af

fect

s bo

th t

he l

ine

stre

ngth

9

of the molecule investigated as well as the number of molecules being counted via the ideal gas law correction.

4) A pressure transducer for pressure input into the system analyzer, which in turn provides input for the software to correct for pressure variations of the spectral response. The system is designed for the signal specific to the pressure transducer for the sample cell.

5) Launch and receive optics to couple the UV spectrometer to the cell

6) System analyzer, including a laptop computer for display and control functions

7) DOAS vehicle exhaust-specific software with method written for the analysis of low level SO2 in the presence of typical diesel exhaust streams. Windows-based software will be installed on a laptop that will be provided by the ARB.

8) The DOAS will be transportable, so that it can measure either engine out or dilution tunnel values, as well as be moved to other labs within the ARB.

10

4 Test Laboratory Configuration

Prior to installation at the ARB facility, the proposed DOAS will be evaluated at UC Riverside. Testing will be done at a UC Riverside laboratory specifically designed for evaluating spectroscopic instruments – a laboratory initially funded by the Electric Power Research Institute (EPRI) for the evaluation of commercially available spectroscopic instruments that measure flue gases typical of combustion boilers (gas turbine, coal, and wood). The following section provides a description of the test facility. Pictures of the DOAS in the test laboratory are provided in Figure 4-1 and Figure 4-2.

Figure 4-1. DOAS in the Laboratory at UC Riverside

Figure 4-2. Overview picture of DOAS in the Laboratory at UC Riverside

Sample Cell Measurements

Since the DOAS is an extractive system the laboratory was equipped with a calibrated source that provided a constant flow through the extractive cell of 7.5 standard liters per minute (slpm). The source of the gas was from the TECO 42 dilution calibrator.

Calibration and Gas Delivery System

The Laboratory employs a TECO 146C Dynamic Gas Calibrator with central processor unit (CPU) and controlled precision mass flow controllers. The calibration system provides precise concentrations of target gas by utilizing gas dilution, through a precision, low flow mass flow controller. For the proposed project, a supply of “zero” air is required to dilute the higher concentration calibration gases in order to attain the target gas concentration to be delivered to the sample cell. Zero air refers to the purity of the diluent gas, indicating a non-detectable concentration of the species under investigation.

11

   

 

The system flow rates are determined by the CPU from the electrical signals from each mass flow controller. The CPU calculates the necessary flows to attain the desired pollutant concentration. Equation 3-1 is used to calculate the required zero air and pollutant gas flows.

Cf= Ci *{Qgas /(Qgas+Qair)} Equation 3-1

Where: Cf = final concentration of the diluted gas

Ci = source gas concentration

Qg a s = volumetric flow rate of the source gas

Qa i r = volumetric flow rate of the zero air source

The mass flow controller used to control the diluent air has a higher volume capability (approximately 1.8 scfm (50 slpm)) than the mass flow controller used to control the source gas (approximately .0036 scfm (0.1 slpm)). This approach allows the control system to develop a broad range of gas concentrations, from trace amounts to high concentrations, limited only by the initial NIST traceable gas source concentration. The measured gas species and the high flow diluent are mixed in an inert (Teflon®) chamber prior to entering the exhaust manifold. The uncertainty of the calibration source is dependent primarily on the accuracy of the TECO 146C calibrator. The calibrator has a purported accuracy of better than 2% of the full scale reading.

The gas supply consisted of a single certified SO2 cylinder with concentration of 96.3 ppmV, which was mixed at different ratios with zero air, at two predetermined dilution flow rates, to achieve the desired concentrations. The nominal concentrations for the SO2 measurements are listed in Table 4-1 for the low dilution (9500 cc/min) and Table 4-2 for the high dilution (47,500 cc/min) to encompass a range from 20 ppbV (low detection) to 953 ppbV (high detection). Two concentrations were tested for both dilution flows (i.e., 51 and 151 ppbV) to provide data for overlapping concentrations under each of the dilution conditions. These are highlighted by being in italics in the two tables. Other calibration gases were National Institute of Standards and Technology (NIST) certified gas cylinders, each containing concentrations of the target species in an inert (nitrogen) carrier. The gas standards were certified by Scott Marrin Gases, and a supplier’s certification sheet was obtained for each of the gases purchased.

Dilution SO2 Cylinder Total Dilution SO2 SO2

Flow Flow Air Flow Ratio Concentration Concentration (cc/min) (cc/min) (cc/min) (ppmV) (ppbV)

9500 96.0 9596.0 9.90E‐03 0.953 953 9500 75.7 9575.7 7.83E‐03 0.754 754 9500 55.6 9555.6 5.76E‐03 0.554 554 9500 35.4 9535.4 3.67E‐03 0.353 353 9500 25.4 9525.4 2.62E‐03 0.253 253 9500 20.4 9520.4 2.10E‐03 0.202 202 9500 15.3 9515.3 1.58E‐03 0.152 152 9500 10.4 9510.4 1.05E‐03 0.101 101 9500 5.4 9505.4 5.26E‐04 0.051 51

Table 4-1. SO2 test concentrations used for low dilution

12

   

 

Dilution SO2 Cylinder Total Dilution SO2 SO2

Flow Flow Air Flow Ratio Concentration Concentration (cc/min) (cc/min) (cc/min) (ppmV) (ppbV)

47500 96.0 47596.0 2.00E‐03 0.192 192 47500 75.7 47575.7 1.58E‐03 0.152 152 47500 55.6 47555.6 1.16E‐03 0.111 111 47500 35.4 47535.4 7.36E‐04 0.071 71 47500 24.4 47524.4 5.26E‐04 0.051 51 47500 15.3 47515.3 3.16E‐04 0.030 30 47500 10.4 47510.4 2.10E‐04 0.020 20

Table 4-2. SO2 test concentrations used for high dilution

13

   

         

5 Performance Tests at UC Riverside

The following section provides an overview of the results of the laboratory testing that was conducted at UC Riverside. This included tests to determine the minimum detection limits of the system, tests to evaluate the linearity of the system, and test to evaluate the potential impact of interferents on the SO2 measurements.

5.1 Minimum Detection Limits

According to the accepted convention the limit of detection (LOD) is the lowest analyte concentration that is significantly different from the blank which is a known sample that does not have any of the analyte (basically zero).

The signal intensity (yD) corresponding to the detection limit represents the size of the signal that is equivalent to the blank value.

The signal is determined by the mean value B and the standard deviation b of a blank signal described in equation (1), (Grant et al, 1992)

yD = B + KDb (1)

where KD = 3.

Measurements at a full range of SO2 concentrations are shown in Table 5-1 and Table 5-2. These measurements include the target concentration, the measured concentration, the measured concentration corrected for background, and the standard deviation of the measurements over 10 minutes. The results show a standard deviation at the blank level of 4 ppbV and 5 ppbV, respectively, for the low and high dilution measurements. Based on these measurements, the minimum detection limits are approximately in the range of 12 to 15 ppbV. It should be noted that similar are lower standard deviations are found for the higher concentrations, indicating a roughly similar level of detection throughout the measurement range evaluated.

Dilution SO2 SO2 DOAS SO2 DOAS Standard Correlation

Flow Concentration Concentration Concentration Deviation Coefficient

(cc/min) Calibrator raw 5‐ second 5‐sec with bkg sub 10 min R2

(ppbV) (ppbV) (ppbV) (ppbV)

9500 953 940 945 1.6 0.99 9500 754 745 750 2.2 0.99 9500 554 545 550 1.9 0.98 9500 353 342 347 2.2 0.97 9500 253 244 249 1.7 0.96 9500 202 192 197 2 0.95 9500 152 142 147 1.9 0.94 9500 101 93 98 2 0.85 9500 51 42 47 3 0.64 9500 0 ‐5 0 4 0.14

Table 5-1. SO2 low dilution concentration measurements

14

   

         

   

 

Dilution SO2 SO2 DOAS SO2 DOAS Standard Correlation

Flow Concentration Concentration Concentration Deviation Coefficient

(cc/min) Calibrator raw 5‐ second 5‐sec with bkg sub 10 min R2

(ppbV) (ppbV) (ppbV) (ppbV)

47500 192 185 191 5 0.96 47500 152 148 154 5 0.93 47500 111 102 108 2.1 0.87 47500 71 64 70 4 0.71 47500 51 43 49 3 0.57 47500 30 20 26 5 0.36 47500 20 11 17 2 0.23 47500 0 ‐6 0 5 0.11

Table 5-2. SO2 high dilution concentration measurements

5.2 Linearity Tests

Additional tests were conducted to evaluate the linearity of the systems over a range of concentrations for both low and high dilution conditions. The data for these tests are tabulated in Table 5-3 and Table 5-4 and shown graphically in Figure 5-1 and Figure 5-2. The tests showed good linearity over the concentration ranges for both the low and high dilution measurements, respectively. The slopes for the low and high dilution measurements were 0.995 and 1.011, respectively, and R2 values of 0.999 and 1.0, respectively.

Dilution SO2 Cylinder Total Dilution SO2 SO2

Flow Flow Air Flow Ratio Concentration Concentration (cc/min) (cc/min) (cc/min) (ppmV) (ppbV)

9500 96.0 9596.0 9.90E‐03 0.953 953 9500 75.7 9575.7 7.83E‐03 0.754 754 9500 55.6 9555.6 5.76E‐03 0.554 554 9500 35.4 9535.4 3.67E‐03 0.353 353 9500 25.4 9525.4 2.62E‐03 0.253 253 9500 20.4 9520.4 2.10E‐03 0.202 202 9500 15.3 9515.3 1.58E‐03 0.152 152 9500 10.4 9510.4 1.05E‐03 0.101 101 9500 5.4 9505.4 5.26E‐04 0.051 51

Table 5-3. SO2 low dilution concentration linearity measurements

15

   

 

      

 

  

  

     

 

■ ■ ■

■

■

■

♦

■

Dilution SO2 Cylinder Total Dilution SO2 SO2

Flow Flow Air Flow Ratio Concentration Concentration (cc/min) (cc/min) (cc/min) (ppmV) (ppbV)

47500 96.0 47596.0 2.00E‐03 0.192 192 47500 75.7 47575.7 1.58E‐03 0.152 152 47500 55.6 47555.6 1.16E‐03 0.111 111 47500 35.4 47535.4 7.36E‐04 0.071 71 47500 24.4 47524.4 5.26E‐04 0.051 51 47500 15.3 47515.3 3.16E‐04 0.030 30 47500 10.4 47510.4 2.10E‐04 0.020 20

Table 5-4. SO2 high dilution concentration linearity measurements

1000 1

900 0.8

800 0.6 y = 0.995x ‐ 2.854

R² = 1

SO2 (ppbV) R2

700

600 0.2

500 0

400 ‐0.2

300 ‐0.4

200 ‐0.6

100 ‐0.8

0 ‐1 0 200 400 600 800 1000

SO2 Calibrator Source Value (ppbV)

Figure 5-1. DOAS SO2 low dilution linearity plot

DOAS Response

to SO

2 (ppbV)

Correlation

Coefficient

16

0.4

 

      

 

  

  

     

 

■

■

■

■

■

♦

■

■ ■

y = 1.011x ‐ 2.474 R² = 0.999

‐1

‐0.8

‐0.6

‐0.4

‐0.2

0

0.2

0.4

0.6

0.8

1

0

50

100

150

200

250

0 50 100 150 200 250

Correlation

Coefficient

DOAS Response

to SO

2 (ppbV)

SO2 Calibrator Source Value (ppbV)

SO2 (ppbV) R2

Figure 5-2. DOAS SO2 high dilution linearity plot

5.3 Interference Tests

A series of tests were conducted to evaluate potential interferences with the SO2 absorption. This included tests with different concentrations of SO2 with NO and NH3. The results of these tests are provided in the following subsection.

The initial configuration files of the three species, SO2 (Figure 5-3), NO (Figure 5-4) and NH3

(Figure 5-5), show that the spectral range of measurement was from 203 nm to 230 nm. This range was chosen as it encompasses the best available spectral range for SO2 measurements, with regard to sensitivity. Note on the figures, the reference spectra are in red and the measured spectra are in blue, with all tests were done with the instrument configured for 5 second response. Also note on Figure 5-4, that there is a strong NO absorption starting at 214 nm and ending at 216 nm, which will likely interfere with the SO2 response. For NH3 (Figure 5-5), the strongest absorption occurs from 203 nm to 204 nm. Although this absorption is significantly weaker than that of SO2, it is still a likely interferent.

17

 

 

L.aslRView 2013 File • I Setup DOAS

Analyzer: 120801SU1 · [), .. Species: 502 .. Telescope: 1

ProcesqSettings

Wa- U>- ~ '- 2'°j M

Telescope Settrio, 0esa¥)1:ion

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Caibration Fi!cior _36 __

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100

~ Ii Path~ 1.000

Dwel Tme 00:05~

,.,,.,,.., , f:

Lhb ppb

Low Power~~

~Conea10r1D

203 0 208 4 213 8 219.2 224 6 230.0

H.egatm 250 Samples 10 [ Save P.ef -~ @ Altomatic Error Mode Hold~)

PortNo 1 t: Oip..t Concertraoon

Species 502 Telescope 1

lookup Tables Relays

~Enabled ~ --­Raw Spectrum

Como,.

Bac:ic!JCU'l(l s...tllrac:tionSetti'lgs -~ "'"""""' r,,.,n..,. ltJ [m-OJ K

Rx.eel Pres. @ 1.013.0 mbar

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Fwning A-ve,age

~ ~ Oteria~ ~ r~ [ooooosooo '

Reset Tme 00:00:00.000 Clear

100

75

,f\l\(\f\/\ ,o li e:, ~ -= ftV\jlTVl/ v 77 v~ v

IJ.13 % I I I ,,J: 203.0 208 4 213 8 2192 2246 2300

RawWave!e,¢ UlWef" ~ Lippef 230

Setup DOAS

Analyzer. 1208015U1-01 .. Species: llmJ H Telmope: 1 ~ eg ~ ~ Proceu/ng Settngs TelMccpe Settng,

W11velen¢1 Low« 203

Calmition Fi!cior 28S

Reference

~ tJJ:iple RegeSSIC(l Plllh Len¢, 1.000

100 Dwel Tme 00·05 000 J

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.~ .Ii 2030 2084 2138 2192 2246 2300

wegen 250 -~ li1l ,..__, &ro, - """'i::«,c,ri-)

&,­

~ Temp. ~ 298.0

F«ed Pm ~ l.013.0 IT"Car

IO Settnga ..... 1)

l.hl:1 ppm

Low Pow« ~ ~

-eoo.-.lt]

Port No. 1 : OJ;,u Concenraoon 4-20mA lower 0.0 ~ 100.0 ppm

Species 502

~ Table1

~Enabled Coriigu"e --­Raw Spectrum

,9.5 % 2030 ,.., ' 2138

RawWavelength Low« 203 Lwer 230

....,.,,,_ □ Enabled OteriaCouit Off

Trne 0000-05000

Re,ei Trne 00:00-00 000

2192 ' 12'6

100

-,

150

l.l

86.19t. ! "J

2300

Figure 5-3. Shows initial DOAS configuration file and reference spectra for SO2.

Figure 5-4. Shows initial DOAS configuration file and reference spectra for NO, notice strong NO absorption from 214 nm to 216 nm.

18

 

OOAS

~n.1lyi.cr: 1Z08015U1 ~0, • Sp~~1- H Telescope 1 ITdcscopc Su n l ! , ~ ~ Setmgs T~ ~

Wi!1Velen¢, low""" 203 Upper ~ Dcsc:tp,on

CalbrabonFactor ~

Reference 100 Dwd Tme 00:05.000 low"- loo''t

~ : . , , , , 35.0J: 2030 208 4 213;; 219:t 2246 Z](ll,

Hegation 250 ~ 1()' SaveRcf

"""°""""" ~ At~ Em;,r Mode Hold f,:oncenr?llorl)

Port No. 2.._J: <>..qiu: Conarintion

Species $02 T..._,,, 1

'°"""' T'""' Relay,

~ Enabled ~

Raw Spectrum

213 S

Rw1 W11vdef"9h lower ~ ~ ~

Sq,a!Ga.'1 I CanelationCorrewon ~

&dtgou,c!Si.birad:JonSettngs

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4·20mA LDwec- 0.0 1.-wer 100.0 ppm --□ Enabled Oteoa Coi.,t OIi

T.-ne 00.00:05000

Reset Tme 00:0000CXIJ

2192 2246

Stam llcg

100

"5

I"' "5

35.0 % 1

D 23:10

Figure 5-5. Shows DOAS configuration file and reference spectra for NH3 where the strongest absorption is.

Initial tests were done to measure the backgrounds of SO2, NO, and NH3 by running zero air through the sample cell. Note that all measurements are with a 250 ms response time integrated for a period of 5 seconds. Figure 5-6 shows the background spectra and values for the three species, with SO2 being around -5 ppbV, and NO being less than 0.000 ppmV and NH3 being -0.0055 ppmV. These backgrounds were measured during the linearity tests and were reproducible over both dilution ranges.

In Figure 5-7, the SO2 spectra is enlarged to show that the background is flat over the entire range of measurement (203 nm to 230 nm). Also note that the background in this case was -3.4 ppbV, standard deviations over this test period (over an hour) were 4.3 ppbV, yielding a minimum detection limit (MDL) of 12.9 ppbV at 3 times the standard deviation of zero air response.

Ozone is also measured since it is in the spectral region and is presented here on the plots. Ozone is included since the instrument can currently measure up to four species. We expect that the ozone will be replaced with HCHO once we have conducted the work for the HCHO response and interference testing. Since the HCHO response drops off significantly over this range, we do not expect a significant interference contribution from HCHO.

19

 

 

File • Plugins •

o, NH3

NO

-18.9057 ppm r1 = -0.437

OJXXX> ppm

-0.0055 ppm

70

• 1 1

~ 0 C.

35 - ~ C

L <l' - ~

0

r1 :c 0.046

r1 :c -0.130

NectV.,tue 03

, 0.5 % 245 0

50 Wednesday, iJanuacy 8, 2014

laslRView 2013

25 .J

-+-­I !

' 2570

Jj

L -JO A 100

38 ·············+---4----+----+----+--····· so

2' >---+---+---+----+----+----<60

26H

€ C

~ ~ 0 u

' 269.0

S02

0.3 %

- □ --• Species; NO • Telescope: 11 ·~

1.7% , J 2750

I

00

"

25

35.0%1 0

1l3.8 219.2 224.6 230.0

Cocnb.don: -0.1302 Concentr~tion: -0.006 ppm

lww Cocnb.tion: --0. 1302 fQw COtKentr~tion; -0.oo6 ppm

100

75

n tr so

r !5

, 0.3% 2030 ' 208.4 .213.8

I

I 66.1 % 1 0 219.2 224.6 230.0

:2 0.

~

,,..., __ ! I

5. g_ 203 0 208J f---+----t---+----+----t----;40 CL a.

213.8 I

2192 2246

rS 1/l

-• ·8

;; -;: 03

>---··; .. I : '1 !r-_~1· ;ir-20 2 0 lf\ (\ (\ ('\ (\ ~A~ A ~+fl : I ~1,L(1.,M.. V • \fJl}V v v rvv v v \/'V7 l ,2

soj: -0.2786i ± 5.86 PP.b (-18.8~ ppb, 22!~3).pli) : , : : I ll : I

······-··~ 1 : -5.6994 ~ 4.60 pp~ 1-18.25 .~.P..i:!!, 2.~ ppml ... _ ,_ 1 0.5 %

1 1 1 1 1.7 % ,

ti : 0.0000!± 0.00 PP,m (0.00 p'pm, 0,0ffl"N• 41 Z45.0 251.0 mo m.o 269.0 m.o NO: 0.0010 ± 0.00 pp~ (-0.01 p1pm, 0.03 ppm~!

-20 ~ __ 1 -~I __ I __ I __ 1 _~ 20 os.Jo oscSo 09 no 09 o ONO 09 10 19 40

Telescope S02 ·l.4379 ppb r' = -0.033 2014·01·080'J:36:21.885

Figure 5-6. Shows DOAS SO2, NH3 and NO 5-second background response with only zero air flowing. Notice measured SO2 background is slightly negative (-5 ppbv) with standard

deviations between 4-5 ppbV.

Figure 5-7. Shows enlarged DOAS SO2 5-second background response with only zero air flowing. Notice measured SO2 background again is slightly negative (-3.4ppbv) with

standard deviations again between 4-5 ppbV.

The next two figures, Figure 5-8 and Figure 5-9, show the DOAS response to two discreet levels of SO2 calibration gas. One is at 80 ppbV (Figure 5-8) and the other is at 680 ppbV (Figure 5-9).

Note that the response to the SO2 gas does not affect the background levels of either NH3 or NO, so basically at this level of SO2 there is no interference to either NO or NH3.

20

 

 

 

 

aslRView 2013

File ~ Plugi~o,=•=====:;======::;::============:::;=~;;~ ~~~~~~~~~ [~! species: NO ~ Telescope: L_

!( o::===~ ====:=======:=:=====:=======:=======:=:======11,:,•_0

0

5 I~ 1~! l _ ,Lt~ ~ yV -, • , , 27% , o 120 I : A ..,.. 100 203.0 208.4 213.8 219,2 224.6 230.0

, ednesday, Ji nuary 8, 2014' ,-03

100

! , /\/\f\/'\/1/V\ArJ\.Ai\" f\75

v'lfi IJVV\)VVV° Vv'v'V , 1:: I Q.4 % I I I I i ,]%1 0 245.0 251.0 257.0 263.0 269.0 275.0 -7' f-------jl ·t------'j------1----V---f-------t----'·

NH3 100

Ee \ ,o ,-------+------+------+--------,f------+------i,O I }-,\-u.,n- .\-v-l~- ,,_v..,.~=~.,,~-------1::

I Q.Y% I I I JS.Q% 1 0

75

2' t------+-------+--------+-------+-------+-------1,20 2030 208.4 213.8 219 2 2246 230.0

NO

- 'V - 0.4 %

20 2030

78.Sn3 ppb t' = 0.S62 NH3 0.0005 ppm r' = 0.109 2014-01-08 11:19:57.785

o, -12.7458ppm r' = -0.378 NO -0.0064ppm r':: -0.133 Ntxt Valut 02

V ' 208.4 2H8 219.2

100

75

v=1so 25

, 86.1 % , 0 ;!24.6 2300

For Figure 5-8, looking at the measured (blue) response, it can be seen that other than for SO2 it is a flat line for NH3 and NO. Although the measured line for SO2 is fairly weak (just 5 times over the detection) line, the correlations were typically between 0.55 and 0.75 at this level. Again it is important to point out that this is with a 5 second integration and with longer integration times the sensitivity would improve.

Looking at Figure 5-9, where 680 ppbV of SO2 is being injected, the response of the measured signal (blue) more closely matches the reference (red), and correlations are much higher, typically greater than 0.85 and approaching 1.0 on occasions. Also, note that the response to the SO2 gas does not affect the background levels of either NH3 or NO at this higher level of SO2, so there is not expected to be any interference to either NO or NH3 over the expected range of SO2

measurements.

Figure 5-8. Shows DOAS SO2 5-second response at 80 ppbV, notice that NH3 and NO spectra and response are unaffected by this level of SO2.

`

21

 

 

slRView 2013 - " ~ Species: NO ~ Telescope:: L ~~

100

:

:o•.oos - so, . } 1AA~1:: ,tt ~ ~ V V ·, • , , '"' , 0

"0

Wednesday, J' nuary 8, 2014i 203-0 208.4 213.8 219.2 224.6 230.0 A 100

o,

, /IA(\ /\A(\;\ AAA, A/\~ fl 75

100

52'>------>----+---+------+------+-------+-------160

V'lN V'1l\}\l VlJV v ljv'V7

1::

I Q.-4% I l ! I , .]% 1 Q

245,0 251.0 257.0 263.0 269.0 275.0

NH3

! 390 f-------+---+---t------+------t------+-------j40 J! 1fv=v= <>

lo.Y% i

253 f-------t--c,"r''l---+------+------+-------+-------j20

--·20

10:55 tl 00 Telescope, 1 S02 682.45-41 ppb r' = 0.985 NHl 0.0001 ppm 2014-01-08 10:55:42.678

a, -13.76TT ppm r' = -0393 NO 0.0357 ppm r' = 0.233 Next Value 02

203.0 208.J

NO

11 0.4%

' 203.0 208-4

' 213.8

V 213.8

' 219.2

219.2

100

75

I:: I 35.0% 1 Q

224 6 230.0

100

75

v=1•0 25

I 86J% 1 Q

;!24.6 2300

Figure 5-9. Shows DOAS SO2 5-second response at 680 ppbV. Notice that NH3 and NO spectra, as in the case of the 80 ppbV of SO2, have responses that are unaffected by this

level of SO2.

Initial tests at 300 ppmV of NO and 16.6 ppmV of NH3 confirmed that at these high levels they would interfere with the SO2 response over the spectral range specified. At 300 ppmV of NO, the SO2 interference response was at levels higher than 100 ppbV. At 16.6 ppmV of NH3, the SO2

interference response was between 20-25 ppbV.

Given these results, the spectral range of measurement was changed from (1) to (2)

1) 203 nm to 230 nm 2) 206 nm to 214 nm

Refer to Figure 5-10 which shows the revised configuration file for SO2, as well as the spectra (reference in red) for the narrower region.

Figure 5-11 shows DOAS SO2 response being less than 1.5 ppbV with 300 ppmV of NO injected with the new spectral range of analysis.

Figure 5-12 shows DOAS SO2 response being less 8 ppbV with 16.6 ppmV of NH3 injected with the new spectral range of analysis.

22

 

 

 

Setup DOAS ltill An.ilynr. 120801SU1 - 0, • Spr ci= 502 • Tr l5cop~ 1

l'rocessi,g Settilgs Telescope Sfflhgs

Wavelength Lower ~ ~ _214 __ nm Desaition

Calibnition Facic;,r 36 ~~le~ Pathlen!th 1.000 "'' e______:] Reference

tOO IM~ Tme 0005.000 LcwPDVl'er~4

.~ Ii SignalG.sn !,______l: CotrelationUlffedion D

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Mode()ff

2060 207.6 20'J2 2108 2124 2140

""''""""' H~ 250 """"" fixedl~. ~ 1298.0

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~ h.tomatic Eiror Mode !Hold p,noentration) _i] IO~(tris)

Pot No r,-,:a 0upu °""""- 4-20mA lower 0:0-~ 100.0 ppb

Species S02 Telesoope 1 ~ Av,inge

~&.!lbled Oteri!IC<x.rt OIi y

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I,']'"""' --­Raw Spectrum

Tme 00:00:05.000

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lrVlfV7JI/ v 7777 v 0

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RawWaveler-¢ lower 203 ~ ~

224.6

100

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150

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laslRView 2013 - cl -File • Plugirn • • Sprcte5: S02 • T e li!'SCope: 1

so, g ~!

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,20 Wednesday, Ja/iuary 8 ,12014 A

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J I\ l} (\ 11 . fflfr" ... . ,,.I· 60 206.0 207.6 209.2 210,8 212.4 214 0

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v' vrv\llj V V \fV V V l/""71:: , 0 .3 % 1.7% 1 ~ .:'4S.O ,251 0 2S7.0 263.0 269.0 27S.O

;...,...,.,...,..,....--l_ so;:9:55~.38.p. _b·[,..;:;: pp~4~ pb

...s --- -'~- ,o ~--=0:02~8--Y-0:1·1-pJ,-m-r=o:21-p j,m-:-o:2slif/~ ----1ai­

i NI : 0.0402 ! 0 .00 ~pm (0.00 p~m. 0 .00 ppm]

-20_ j NO: 302} 278 ! 0 .03!p pm [302~ 0 ppm. 302~59 ppml

20 1 S600 56:10 11:56:20 "56,30 56·.tO P S6:SO 1'57:00

Tel5CO(H' 1 SQ2 1A360 ppb .-Z = 0.017 2014-0l-081M6:56.085

0 3 -0.0149 ppm f = -0.046 Next Value 01

NH3 0.OC02 ppm .-l = 0.032 DOAS13001 - USB - 1208015U1

NO 302.5090 ppm .-Z = 0.999

208.4

NO

100.1 %

203.0 208A

' 213.8

213.8

219.2

' 219.2

• I Cycle Displ.ly j

00

75

I:: I 35.Q %1

.)

224.6 .:'30.0

100

I 86.1 % 1 J 224.6 230.0

Figure 5-10. Shows new DOAS configuration file and reference spectra for SO2 with a change in spectral range of analysis to minimize NO and NH3 interferences. The new range

selected is 206-214 nm, which leaves interferences to NO of below a single standard deviation (less than 5 ppbV) of SO2 at 300 ppmV (NO) and less than 10 ppbV of SO2 at 16.6

ppmV (NH3).

Figure 5-11. Shows DOAS SO2 5-second background response with new fit limits to minimize NO interference NO is injected at 300 ppmV. Notice strong measured spectral lines for NO where the blue lines (measured) lay on top of the red lines (reference). The

interference is below the detection limit with new fit limits.

23

 

aslRView 2013

_,. _ Fil~ • Plugin~ • Anatyz~r. if:M llii!IMllehih 1H ~

so, 100 NH, 100

[ g

~ -3 "~ t:

120 Wedne~day, Jailuary 8, 2014 A

100 so 203.0 208.4 213.8 219.2 224.6 230.0

97 f------+----+---f----+----+----<60

73 f------+----+---f----+----+------<'60 11A%

206.0 ' 207,6 209 .2 ' 210.8 2.7 % I Q

214,0

E E 03 50 l-----+----+---f----+----+----140 li: g: f\ f\ 75

100

gs,/\A f\r,Afv\1\(111 , ArL {\ v'liTVvVV V ITVVV\)li'V41::

-NH : 1:6.36112 ± 0 .~ ppm (16]31 ppm, 16:.43 ppm)

NO: 0.~ 158 ± 0.0~ ppm [0 .01 '.ppm, 0.02 ~pm)

-~~lLl.~00~-1~2~:11~,1~0- ~1~2,~11~20~ ~ 12~,1~1:3~0- ~172,~11~470 - ~12~ 1~17:50~ ~ 12~;1/.~

T~lescopt' 1 S02 7.6.Bl ppb r' = 0.Q35 2014-01-0812:11:51.488

0 3 -0.1461 ppm r' = -0.100 NruValu" 02

NH3 16.4279 ppm r' = 1.00'.)

NO 0.02SO ppm r' : 0.182

10.2% I 1.7% 1 Q

245.0 251 0 257.0 263 .0 269.0 275.0

NO

1.3% 203.0 208.4 213.8 219.2

100

15

t 86.1 % 1 D 224.6 230.0

Figure 5-12. Shows DOAS SO2 5-second background response with new fit limits to minimize NH3 as well as NO interferences. In this example 16.5 ppmV of NH3 is injected.

Notice strong measured spectral lines for NH3 where the blue line (measured) lays on top of the red line (reference). The inteference to SO2 at this NH3 level is between 5-10 ppbV.

To confirm that the new spectral range of analysis yielded a similar overall response to the target gases, measurements were again conducted at 80 ppbV and 680 ppbV of SO2. Figure 5-13 shows the response with the original spectral fit, and Figure 5-14 shows the response with the new spectral fit which minimizes interferences for the 680 ppbV test. Note that the measured values for both analysis yielded similar results.

Additional work was done to revise the software to permit masking of spectral regions influence by interferences, as is done in an FTIR. This allows additional regions to be selected or excluded from the spectral analysis, in order to avoid regions of interference. This may yield an even lower interference response, especially for NH3.

24

 

 

laslRView 2013 Fil, .. Plugins .. .. Species; S02 .. Telescope: I 1

€ so, 100 NH,

t: j " l7V~ = ,o.Y%

"0 Wedne~day, Jaituary 8,i2014 A

100 so 203.0 208.4

i " §

-663

m

390

m

,,,

lie

f-----+-------1--,,--------,---~ ----+------;80

l---+------l+ ---+-----t---+----l60 12.0 % 203.0 ' 208.4 ' 213.8 ' 219,2 ' 224,6

25

2.7 % 1 0 230.0

100 E E 03

40 lt ,t fl (\ 75 t----+----+---+---------1 ° 6 , 11 o n I\ A fv\ u A u , A fl ~ A

z v , VJ vvv v vv VVlTIJ'H ·1:: 20 10.2% 1.7% 1 0

S02 : 43511010 ± 300[41 ppb (60117 ppb, 71~.46 ppb) 1---+-,03:· o:00~2:1:o:101>P,m-1:o:s9·pp'm:-o:s9p'p1'1J·············· a.a ,.....,,..i,,,..._.,N.~ it70":-ob'16 ± 0.00 Jpm [0.00 pfJm. 0.01 pp'.mJ

NO: -0.0~65 ± 0.03 Jpm [-0.10 ppm. 0.03 p~m) ~--~ --~i --~ i _ __ ~i ___ •~---~ 20

245.0 251 0 257.0 263.0 269.0 275.0

13:40 13:50 14:00 14:10 14,20 14:30 14:40 T, l~ copt 1 so, 687.4072 ppb r' = 0.990 2014-01-0814;30;33.263

03

0.02S4ppm r':O.OS4 Ntxt Valut 03

NH3 0.0016 ppm ,; = 0.1n

NO 0.0093 ppm ,J = 0.171

laslRView 2013

NO

, 0.3% 203.0 208.4

' 213.8

' 2B.8

~ Plugins • Analyztr itM ifill@IJlf,Hl{i 1H • Sptcie~ S02 • T tie-scope: [ 1

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'00 Wedne~day, Jaituary 8, 2014

. o.o A 100 so 203.0 208.4

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100

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l---+-- --+---H---+----+---120

S02

: 36718431 ± 3041 4 ppb (60:17 ppb, 71~.46 ppb) >-- - -+,-0 3:·o:002;1-t~o:10·pp1.,Jq-:o:s9~piim;-o:96·ppr!r,-1-~- -

l'W',__,.i•·~Ni'fi';:10':bo23"-ro'.oO~ m (0.00 p~m. 0.01 pp'.mJ "

10.2% I I 1.7% 1 Q

245.0 251 0 257.0 263.0 269.0 275.0

· NO: -0.0?41 :t 0.03 Jpm (-0.10 ~pm, 0.03 p~ml ~ ~--~--~! ___ i~--~i ___ ~t __ ~ ,o

13:30 13:40 13:50 14:00 14:10 14:20 14-:30 Ttl~copt 1 S02 688.9847ppb rt=0.992 2014-01-08 14:28:57.041

o3 0.11 06ppm r': 0.090 NtxtValut 02

NH3 0.0020ppm r'=0.184

NO 0.0146ppm r' =0.198

NO

,0 .3% 203.0 ' 208.4

' 213.8

' 213.S

' 219.2

219.2

219.2

' 219.2

-0-~~ 100

75

I:: I 35.0% 1 Q

224.6 230.0

100

"

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• J Cyclt Displ;,y l

100

75

I:: I 35.Q % I 0 224.6 230.0

100

75

I 86.1 % I 0 224.6 230.0

Figure 5-13. Shows DOAS SO2 5-second response at 680 ppbV using the original configuration file and spectral range of analysis between 203 and 230 nm. All linearity tests

were done with this spectral range of analysis.

Figure 5-14. Shows DOAS SO2 5-second response at 680 ppbV, using the new configuration file and spectral range of analysis between 206 and 214 nm. The response is similar at this

narrower spectral range, which is where the primary SO2 absorptions take place.

To test for potential interferences with formaldehyde, a permeation tube was used. The permeation tube provides high concentrations of formaldehyde, which are well above those that would be found in typical vehicle exhaust. Thus, this represents an extreme upper limit of interference that might be seen in an actual application. For the present tests, sampling from the permeation tube did show any impacts on the spectral lineshape or SO2 concentration. Thus, it is

25

           

LaslRView 2013 rile • Plu9ffl$ • Analy-..er: mmm!ammml - Species: SOZ · leles(ope: r

1.lrui.:11\,ll!11t/m,u,.,d- )tl.,,,f - - ·- -L,t::iJ_,_ lie Wednesday, F-;bruary iz. 2014 I

i A I' l ! I

I

I "' ~

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I ~ l

I I I ! ' i I

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I I I i I I i i

' l ! i

I_, sol : _g_onz t 13.76 ppb l-51. JZ ppb, 39.78 ppb/ NH-, : o.00;1s t 0.00 ppm l-0.07jppm, 0.0 1 ppmf

j~Jj jl jt !I. ~'(~" llJ 02' ~pm l->3/ ppm.oo, 'ppml NO: 1.21~3 t g_73 ppm (0.00 Pltm·C"-26 ppml . ,I Ji ~I,. '-A•- LIA , I JJ I , 11 .1. 1 • 1 i1 J

'-

' . t4.;0 ,~ )0 4 ~I) H;.50 " .. 0 ,, . ·1.105£ppb r' ..0.01~ NH, -0.0,mppm t" E -0.1&1 20l4-02-ll1S:0):),U82

.().>670ppm --0.07Z NO

-~

concluded that formaldehyde would not be an interferent for the SO2 measurement for this DOAS.

5.4 Vehicle Emission Tests

Vehicle emissions measurements were conducted in CE-CERT’s Vehicle Emissions Research Laboratory (VERL). The centerpiece of the VERL is a 48-inch Burke E. Porter single-roll electric chassis dynamometer, capable of testing vehicles weighing up to 12,000 lbs. A Pierburg Positive Displacement Pump-Constant Volume Sampling (PDP-CVS) system is in conjunction with a Pierburg/AVL AMA-4000 emissions bench for emissions measurements.

Several vehicles were tested in this portion of the program, including a 2012 Nissan Versa, a 2013 VW Jetta GLI, and a 2013 Dodge Ram 2500 diesel pickup truck. The primary purpose of the vehicle exhaust measurements was to evaluate if any additional interfering species needed to be accounted for, outside of those used in the calibration gas verification tests. Overall, the results were consistent with the results from the calibration gas tests. In general, for the vehicles without NH3, there did not appear to be any significant interferences to the SO2 measurements. On the other hand, vehicles for which NH3 was found in the exhaust at levels above 11 ppm, did show a significant interference.

The best performance for the DOAS was on 2012 Dodge Ram diesel pickup truck. The DOAS results for the diesel Dodge Ram are provided below. Figure 5-15 shows the DOAS output after the diesel vehicle is turned on. The results show a significant amount of NO, greater than 50 ppmV, but very little SO2 (-1.105 ppbV) and NH3 (.025 ppmV). These results show that NO does not bias the SO2 at these levels over the fit limits chosen (209 to 213 nm), so NO is not an interferrent. The SO2 levels are very low, and the SO2 measurement is at the expected detection limits. Figure 5-16 shows that as the test progresses, the NO level decreases, while the SO2

initially remains at/below the detection limit. Finally, near the latter stages of the test as shown in Figure 5-17, SO2 is measured above the detection limit, at a level of approximately 26 ppbV. Clearly, in a low NH3 environment, as tested here the SO2 detection is unbiased and meets the laboratory tested detection limits of around 20 ppbV. If the DOAS is used in a low NH3

environment, as in this test, it should achieve the expected detection limits.

Figure 5-15: Diesel test at engine turn on, note there is no detectable SO2 (-1.105 ppbV).

26

         

 

PMlt

.,

uor=============== -! 1 I ; ~I---+---+---+---+-,---+---,,,

"

!

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1---f---+, ---+--•r- I

S01: .9.1701 f B.52 ppb (-51.32.pP.b. J9.~S ppb J

O1.,d~~~~-~y pm (-1.45 , ,Pm]o.oz ppm!_ ,: Nl'°'J~ .0.0012 t 0.0 1 pp_m 1-0.10 ppm. O.Ol~ppm ]

~o :j7.9013 ± i23.92 PPT [O.00 ~•~m. 186.l ;f ppml ~0

.-.._11 -. ..U -1 D 11,U !~,:JO ,.,.,1,1 1) 4 1J Ttl,mo~ I ~ 1 2s...9mppti I 0JSJ. ZOl4-02-12 IS:11);2l691

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~ J Cye:le 0.Sf)l,iy I ,.,

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Figure 5-16: progression of diesel test still SO2 measured below detection limit

Figure 5-17: Progression of diesel test with SO2 now measured above the detection limit.

The gasoline vehicles, on the other hand, showed higher levels of NH3, and as such showed periods where a significant negative bias was found. The 2012 Nissan Versa emitted up to 100 ppm or more of NH3 during various parts of the cycle. The laboratory tests showed that once the levels exceeded 11.0 ppmV of NH3, there became a statistically significant negative bias to the SO2 response.

Figure 5-18 shows the DOAS spectral response for the three species measured, NO, NH3 and SO2. As can be seen, that the raw NH3 spectrum over the range measured (203-215 nm) from the vehicle (blue) lies virtually on top of the reference NH3 spectrum (red). At this point in the test,

27

.. . ,

"' Thursday, Fe in , 1 2014 ® ' "' + ,__ 130

-- --

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spectrum over the same range measured (209-213 nm), an overall negative bias caused by the NH3 interference, which results in measured SO2 values of – 38.898 ppbV.

This trend is shown in the overall test run, as shown in Figure 5-19, where once the values exceed 11 ppmV of NH3, the SO2 measured values go negative. This interference is also non-linear as once the NH3 values approach 80 ppmV, the SO2 measured values are greater than -900 ppbV.

The DOAS is designed to measure SO2 out of diesel engines where NH3 levels are typically less than 10 ppmV. If it is to be used for vehicles with higher levels of NH3, an NH3 scrubber would likely be needed to scrub the level of NH3 in the exhaust to no greater than 10 ppmV, as well as perhaps some additional updates to the software that are currently being developed. If an ammonia scrubber is used, it is recommended that SO2 measurements be made both with and without the scrubber in order to determine whether there is any appreciable effect on the measurements.

It should be noted that in addition to the capability of measuring SO2 in the exhaust, as per the original program scope, this DOAS also has the capability of measuring NH3 in real-time. For the Nissan Versa, a tunable diode laser system (TDLAS) that was designed to measure NH3 in raw vehicle exhaust was run concurrently with the DOAS. Figure 5-20 shows the measured results from the DOAS and the TDLAS. The results show a good correlation between the TDLAS results and the DOAS results for NH3, at NH3 levels below 90 ppm. A regression plot (Figure 5-21) shows a high correlation between the two measurements, with an R2 =.951, but once the NH3 levels exceed 90 ppm the DOAS UV light source is too attenuated to make a measurement. Also, looking at the regression plots, it can be seen that the DOAS is reading slightly lower than the TDLAS at higher levels, likely from saturation of the UV spectra.

Figure 5-22 shows the DOAS response under conditions where NH3 levels were much lower, in this case 1.486 ppmV. Here, the SO2 is measured at 8.997 ppbV, and does not show any negative bias in the measured spectra. At these lower NH3 levels the DOAS appears to meet the objectives of better than 20 ppbV detection limits.

Figure 5-18. DOAS spectral response under high NH3 conditions showing a negative bias to SO2 measured values.

28

 

  

 

 

           

 

  

    

             

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200 4000

150 3000

100 2000

NO

and

NH3

(ppmV)

SO2

(ppbV)

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0 0 14:00

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Figure 5-19. DOAS measured values for the 2012 Nissan Versa emissions test. Note the drop off of the signal once NH3 levels reached 90 ppmV, and the significant negative bias to SO2 under these high NH3 conditions.

29

 

 

 

 

    

    

                

                

 

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DOAS Calibration Gas Response

TDL Calibration Gas Response

13:45 13:50 13:55 14:00 14:05 14:10 14:15 14:20

Time (PDT)

Figure 5-20. NH3 emissions measurements for the DOAS along with concurrent measurements with a tunable diode laser system configured for NH3 for the 2012 Nissan Versa. Both are raw exhaust measurements. Note the high similarity in response between the two separate and distinct spectral techniques.

30

          

  

   

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Figure 5-21. Regression plot NH3 emissions for the 2012 Nissan Versa for the DOAS compared with an NH3 tunable diode laser system. Both are raw exhaust measurements. Note the high correlation between the two techniques for values up to 90 ppmV with an R2

of 0.951.

Figure 5-22. DOAS Spectral response under low NH3 conditions, typical to diesel engines, the spectral response and the calculated numbers for SO2 show no negative bias.

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100

6 Installation/Verification at CARB Facility Once the instrument is completed and tested at UC Riverside, it will be taken to, and installed at, one of CARB’s laboratories. Installation will include verification and on-site calibration of the instrument, training, and an evaluation of the system on one test vehicle, which will be provided by CARB. The on-site installation, verification, and training for the DOAS is expected to take one week or less.

The installation of the DOAS will be done in coordination with CARB staff from the laboratory where it will be installed. UC Riverside will provide a description and size measurements for all components of the DOAS system to CARB laboratory staff prior to the installation period, and will work with CARB staff to identify a suitable location for the instrument and all associated equipment. UC Riverside will also participate in meetings, as needed, to ensure the laboratory is ready for installation, both phone call/conference call meetings and site visits.

CARB will be responsible for providing a sample delivery system (from the location where sampling is done to the instrument itself) and any fabrication required for installation of the instrument in the laboratory. The sample delivery system should include a sample pump, heated lines, and a heated filter to reduce PM contamination. CE-CERT will discuss sampling requirements/parameters with CARB staff prior to the purchase of these items to ensure they meet the requirements for the instrument. Once the instrument is ready for installation, and the laboratory has completed preparations for installation, UC Riverside will bring the instrument to the CARB laboratory and set it up. Initial set-up of the instrument in the laboratory is expected to take 1 to 2 days.

Once the instrument set-up is complete, instrument operation and accuracy will be verified with calibration gas. The calibration/verification of the DOAS will be done using calibration gases in a range similar to concentration levels expected in raw exhaust; this range will be the same as that used for calibration of the instrument in the bench laboratory tests. The instrument calibration curve will consist of several points spanning the desired concentration range. The detection limits of the instruments will be established based on one standard deviation of the noise in the time resolved data under conditions where the instrument is zeroed and at several calibration points.

Following calibration/verification of the DOAS, UC Riverside will train CARB laboratory staff on the operation of the instruments. Training will occur on both the software and hardware components of the system, the general principles of instrument operation, and a discussion of any elements of the instrument that may require routine maintenance/repair. Training is expected to take approximately one day. A manual will also be provided to CARB staff, either at the time of installation or at some subsequent point, and will provide a description of the unit, a description of the scientific principles of the instrument’s operation, instructions on how the instrument is operated, and a description of any parts needed for routine maintenance. A software manual will be provided by Unisearch and will be included in the system manual.

Overall, instrument installation, set-up, verification, calibration, and training can be completed in the period of approximately one week or less. This would include testing on actual vehicle exhaust from vehicles provided by CARB (and with fuels provided by CARB). Initial testing will focus on measurements in raw exhaust. Sulfur in the fuel and oil will provide baseline SO2

measurements for vehicle testing. SO2 levels in the exhaust can also be varied to look at a wider range of SO2 concentrations, if needed. This could include direct injection of SO2 calibration gas into the exhaust stream.

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7 Summary and Conclusions

Understanding the nature of the sulfur and sulfate in combustion and lubricant-derived particles is important to characterize and better understand the relative contribution of fuel vs. oil sulfur, to understand storage vs release of sulfur from catalyzed surfaces, and measure the sulfur fraction partitioning into the PM nucleation particles. The objective of this research is to construct, test and provide to ARB laboratories a differential optical absorption spectrometer (DOAS) that can measure down to 20 ppbV in real time and determine a mass balance between SO2 and sulfate. The DOAS developed under this program was evaluated using a series of laboratory performance tests at the CE-CERT laboratory. The DOAS will be installed and operated in the ARB Heavy Duty Emissions Test Laboratory. The major conclusions from the findings are summarized in the following:

The differential optical absorption spectrometer (DOAS) developed for this project was specially designed and built for rapid, extractive detection of sulfur dioxide at low ppb levels for the ARB. The integrated system consists of the extractive sampling train, the sampling cell, and the spectrometer.

The sample cell uses a single-path design with beam expanding and focusing optics to minimize cell volume. This allows the highest amount of throughput for a given amount of initial light energy and is essential with a small diameter sample cell. The small diameter was a design parameter to minimize the cell volume which yields reduced residence time of the exhaust gas in the cell. The sample cell was designed as an open cell with the sample cell residing between the windows, rather than having the transmitting and collecting optics inside the sample cell.

The sample cell for this system is cylindrical with a length of 1.0 m. The total volume of the DOAS sample cell is approximately 0.51 liters.

The cell is designed to operate over a full 8-10 hour testing day at a temperature of 110°C, with a temperature range from 25°C up to 120°C (250ºF).

To avoid possible interferences from other exhaust constituents, the spectral range was restricted to the UVB region (200–230 nanometer (nm)). For the 200–230 nanometer (nm) region, a deuterium lamp was used as the light source. Additional adjustments were also made to allow for subsection of the spectral range to be selected for analysis, to minimize interferences from species such as NH3, such as narrowing the spectral range to 206 nm to 214 nm.

The spectrometer/receiver consists of a single working element, a concave 1200 groove/millimeter (mm) holographic diffraction grating optimized for 200–230 nm.

The DOAS software, LasIRView 2009, is a user-friendly Windows-based based package with outputs for data logging and a software manual. The DOAS software was upgraded to provide an adjustable window over the spectral range, such that interferences from species such as NH3 can be minimized.

Laboratory tests were conducted to evaluate the response of the SO2 monitor to a range of operating conditions, from ambient conditions to those most representative of exhaust gas emanating from vehicles.

33

Detection limits were approximately in the range of 12 to 15 ppbV at three times the standard deviation of zero gas response.

The linearity of the instrument was evaluated over SO2 concentration ranges of 20 to 192 ppbV and 51 to 953 ppbV. The regression analysis showed a slope of 1.011 and 0.995, respectively, and R2 values of 1.0 and 0.999, respectively, for these ranges.

A number of interference tests were conducted, focusing on the potential NO/NH3 DOAS interference with SO2, and vice versa. SO2 was not found to interfere with either NO or NH3, at levels of 80 ppbV and 680 ppbV. Initial tests at 300 ppmV of NO and 16.6 ppmV of NH3 confirmed that at these high levels they would interfere with the SO2 response over the spectral range specified. To mitigate the impact of these interferences, the spectral range was modified to focus on a narrower range of 206 to 214 nm. With the new spectral range of analysis, the DOAS interference SO2 response was less than 1.5 ppbV with 300 ppmV of NO injected with, and was slight less than 8 ppbV with 16.6 ppmV of NH3 injected.

The DOAS was evaluated over vehicle emissions tests in CE-CERT’s light-duty chassis dynamometer laboratory. A 2012 Nissan Versa, a 2013 VW Jetta GLI, and a 2013 Dodge Ram 2500 diesel pickup truck were tested. The results showed no appreciable SO2

interferences for the tests on the diesel truck, which had very low NH3 emissions. Clearly, in a low NH3 environment, as tested here the SO2 detection is unbiased and meets the laboratory tested detection limits of around 20 ppbV. If the DOAS is used in a low NH3 environment, as in this test, it should achieve the expected detection limits.

The gasoline vehicles, on the other, had higher levels of NH3, and showed a significant interference. Essentially, when the NH3 values exceeded 11 ppmV, the SO2 measured values go negative. This interference is also non-linear as once the NH3 values approach 80 ppmV, the SO2 measured values are greater than -900 ppbV. If it is to be used for vehicles with higher levels of NH3, an NH3 scrubber would likely be needed to scrub the level of NH3 in the exhaust to no greater than 10 ppmV, as well as perhaps some additional updates to the software that are currently being developed. If an ammonia scrubber is used, it is recommended that SO2 measurements be made both with and without the scrubber in order to determine whether there is any appreciable effect on the measurements.

This DOAS also has the capability of measuring NH3 in real-time. Comparisons were made between the DOAS and an NH3 tunable diode laser. The results showed a good correlation between the TDL and DOAS results for NH3, at NH3 levels below 90 ppmV, with a regression coefficient of R2 =.951, but once the NH3 levels exceed 90 ppmV the DOAS UV light source is too attenuated to make a measurement.

Recommendations for Future Work

Based on the results of this study, several recommendations for future work can be provided. This includes recommendations for further development of the instrument, as well as recommendations based on the laboratory testing that was conducted. For the DOAS itself, the initial work report here suggests that this instrument could be further developed/optimized to provide concurrent measurements of NO and NH3. To expand the measurement capability for the NH3, some additional development work would be needed to address the fact that the DOAS UV light source is too attenuated to make a measurement once the NH3 levels exceed 90 ppm. The

34

possibility of expanding the DOAS for use in other applications or for other species is also potentially of value. Another interesting finding from the laboratory tests was the higher than expected NH3 emissions for some of the test vehicles. In particular, both of the gasoline vehicles tested for this study produced higher than expected NH3 emissions, even though both vehicles represented late model vehicle technologies. While these vehicles were tested under a very narrow range of conditions, these initial results suggest that further investigation over a wider range of vehicles and conditions to see how prevalent the higher NH3 emissions levels might be and if this could be an important issue with respect to tailpipe emissions.

35

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9. Leppard WR, Koehl WJ, Benson JD, Burns VR, Hochhauser AM, Knepper JC, et al. Effects of Gasoline Properties (T50, T90, and Sulfur) on Exhaust Hydrocarbon Emissions of Current and Future Vehicles: Speciation Analysis - The Auto/Oil Air Quality Improvement. SAE Technical Paper. 1995;1995–10–01.

10. Leppard WR, Koehl WJ, Benson JD, Burns VR, Hochhauser AM, Knepper JC, et al. Effects of Gasoline Properties (T50, T90, and Sulfur) on Exhaust Hydrocarbon Emissions of Current and Future Vehicles: Modal Analysis - The Auto/Oil Air Quality. SAE Technical Paper. 1995;1995–10–01.

11. Rutherford, J.A., Burns, V.R., Leppard, W.R., Rippon, B.H., Koehl, W.J., Hochhauser, A.M., Painter, L.J., Reuter, R.M., Benson, J.D., Knepper, J.C., and Rapp LA. Effects of Gasoline Properties on Emissions of Current and Future Vehicles –T50, T90, and Sulfur– The Auto/Oil Air Quality Improvement Research Program. SAE Technical Paper. 1995;(952510).

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12. Benson JD, Burns V, Gorse RA, Hochhauser AM, Koehl WJ, Painter LJ, et al. Effects of Gasoline Sulfur Level on Mass Exhaust Emissions – Auto/Oil Air Quality Improvement Research Program. SAE Technical Paper. 1991;(912323).

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14. Diesel Emission Control – Sulfur Effects (DECSE) Program. Phase I Interim Data Report No. 1; 1999.

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