BC Measurement Overview, and Considerations for a

Community Approach to Measurement

2nd Workshop on Marine Black Carbon Emissions

Utrecht, Netherlands, September 16-17

Dr Daniel Lack

danielalack75@gmail.com +61 (0) 428 452 511

Outline

Commercial measurement options for BC light absorption

Potential biases for all measurement options

Removal of semi-volatile material

Why the community needs BC light absorption (mass)

Commercial measurement options for BC light absorption

�  Photoacoustic Absorption Spectrometer �  Independently validated à used to validate lower

accuracy measurements. �  Accuracy: 5% �  Cost: $$$ �  Biases: NO2, Coatings

�  Filter-Based Absorption – MAAP �  Accuracy: ~10% �  Cost: $$ �  Biases: Scattering, coatings

�  Filter-Based Absorption – Aethelomter, PSAP, COSMOS. �  Accuracy: 30-40%

�  Cost: $

�  Biases: Scattering, coatings, particle size, filter type, particle loading.

�  Filter-Based Extinction – FSN, bosch number etc. �  Accuracy: Not applicable

�  Cost: $

�  Biases: Correction to absorption, scattering, coatings, particle size, filter type, particle loading.

Light Absorption Biases �  Scattering Material + Filter Fibers

�  SO4, Organics

�  Particles and filter material scatters light à measured as excess absorption

�  Can bias filter-based by >100% �  Particle loading progressively increases bias.

�  Liquid Material �  Organics

�  Liquid coats filter fibers �  Changes optics of fibers.

1038 D. A. LACK ET AL.

FIG. 4. (a) Ratio of PSAP to PAS measured aerosol absorption coefficient asa function of Q-AMS OA mass. OA mass concentrations measured for differentregions of the globe are included from data from Zhang et al. (2007) and thisstudy. (b) Ratio of PSAP to PAS absorption versus ratio of OA to estimatedLAC mass. The laboratory data of Cappa et al. (2008a) are included as redcircles (LAC) and squares (Nigrosin). For both Figure 4a and Figure 4b graphsthe solid black line is a least-square fit to the data, the dashed line is the expected1:1 agreement between PSAP and PAS, the grey points are individual data pointsfrom which bin averages are obtained. Size of the binned data points relates toabundance of data in each OA mass bin and uncertainty bars are 1 σ . (c) Areproduction of Figure 4b showing the individual data points color coded byfraction of OOA to total OA. Black solid line is the linear fit to all data, dashedblack lines are linear fits to the 25% of data having the lowest and highestfractions of OOA. Expected 1:1 line is also included.

bias (due to evaporation or otherwise) was found to be smallcompared to the total absorption (Cappa et al. 2008a).

The Optical Nature of Organic AerosolWhen considering the nature of OA, it is important to under-

stand the extent to which the OA is light absorbing in nature,which is currently not well established at the mid-visible wave-lengths considered here. The PSAP data were used to determinethe Angstrom exponent of absorption (Aabs), defined as:

Aabs = In (bx/by)In (x/y)

[1]

where bX and bY are the measured absorption at wavelengths xand y (467 nm and 660 nm). Kirchstetter et al. (2004) showedthat some compounds in OA emitted by biomass combustion(termed CBROWN due to their brown or yellow appearance) havea spectral dependence of light absorption that is stronger thanthat due to soot (i.e., Aabs for soot = 1; Aabs for CBROWN > 1).Subramanian et al. (2007) showed that OA containing particlesfrom biomass combustion (including CBROWN) are often liquid-like and can form beads and coatings around the filter fibers afterdeposition. They suggested that this could alter absorption andscattering by the deposited OA compared to its in-situ state andaffect filter-based measurement of absorption. If the depositionand spreading of CBROWN is the source of the bias we mightexpect to see Aabs > 1 for much of the data. For OA > 2 µg m−3

(majority of data) Aabs was near 1 (Figure 5), which suggests thatthe influence of OA on the PSAP absorption is not from CBROWN.At lower OA concentrations, where HOA was a larger fraction ofthe OA, the Aabs is noisy due to lower absorption levels but doesdisplay a small upward trend to a Aabs of ∼1.5, suggesting thatthe HOA measured may be mildly absorbing. However, the OAin this study appears to be dominated by non-absorbing OOA.

3.0

2.5

2.0

1.5

1.0

0.5

0.0

Åab

s,P

SA

P (4

67n

m -

660

nm

)

20151050AMS OA ( g m-3)

1.000.900.800.70OOA / (OOA + HOA)

FIG. 5. Angstrom exponent (Aabs) measured by the PSAP (476 nm–660 nm)as a function of OA mass concentration. Data classified by color using the massfraction of OOA to OOA + HOA.

�  Other Biases �  Dilution: Affects gas-particle partitioning of semi-

volatile scattering particles �  RH/Pressure: affects properties of filters

�  Flow Rate: �  Filter Size: �  Particle Size

Brown Carbon (BrC)

�  Brown carbon is likely to contribute <10% of light absorption.

�  Brown carbon contributes to biases in measurement.

�  For the purposes of accurate measurements, BrC should be removed through sample heating.

Filter-Based Absorption Correction Methods

�  PSAP, MAAP, Aethelometer, COMOS have undergone multiple laboratory and field inter-comparison and validation trials. � Particle type. � Particle size. � Scattering contributions à Single Scatter

Albedo �  Filter loading

1.Collaud Coen M, et al. (2010) Minimizing light absorption measurement artifacts of the Aethalometer: evaluation of five correction algorithms. Atmos. Meas. Tech. 3(2):457-474. 2.Virkkula A (2010) Correction of the Calibration of the 3-wavelength Particle Soot Absorption Photometer (3λ PSAP). Aerosol Science and Technology 44(8):706 - 712. 3.Petzold A, et al. (2005) Evaluation of multiangle absorption photometry for measuring aerosol light absorption. Aer. Sci. Tech. 39(1):40-51. 4.Irwin M, Kondo Y, & Moteki N (2015) An empirical correction factor for filter-based photo-absorption black carbon measurements. Journal of Aerosol Science 80(0):86-97. 5.Arnott WP, Hamasha K, Moosmuller H, Sheridan PJ, & Ogren JA (2005) Towards Aerosol Light-Absorption Measurements with a 7-Wavelength Aethelometer: Evaluation with a Photoacoustic Instrument and 3-Wavelength Nephelometer. Aerosol Science and Technology 39(1):17-29. 6.Hansen ADA, Rosen H, & Novakov T (1984) The aethelometer - An instrument for the real-time measurement of optical absorption by aerosol particles. Science of the Total Environment 36:191–196. 7.Weingartner E, et al. (2003) Absorption of Light by Soot Particles: Determination of the Absoprtion Coefficient by means of Aethelometers. Aerosol Science 34:1445-1463. 8.Sheridan PJ, et al. (2005) The Reno Aerosol Optics Study: An Evaluation of Aerosol Absorption Measurement Methods. Aerosol Sci. Tech. 39(1):1. 9.Bond TC, Anderson TL, & Campbell D (1999) Calibration and Intercomparison of Filter-Based Measurements of Visible Light Absorption by Aerosols. Aerosol Sci. Tech. 30(6):582. 10.Ogren JA (2010) Comment on - Calibration and Intercomparison of Filter-Based Measurements of Visible Light Absorption by Aerosols. Aerosol Science and Technology 44(8):589 - 591. 11.Petzold A & Schönlinner M (2004) Multi-angle absorption photometry - a new method for the measurement of aerosol light absorption and atmospheric black carbon. Journal of Aerosol Science 35(4):421-441. 12.Kondo Y, et al. (2009) Stabilization of the Mass Absorption Cross Section of Black Carbon for Filter-Based Absorption Photometry by the Use of a Heated Inlet. Aerosol Sci. Tech. 43(8):DOI: 10.1080/02786820902889879. 13.Kondo Y, et al. (2011) Consistency and Traceability of Black Carbon Measurements Made by Laser-Induced Incandescence, Thermal-Optical Transmittance, and Filter-Based Photo-Absorption Techniques. Aerosol Sci. Tech. 45(2):295-312. 14.Miyazaki Y, et al. (2008) Performance of a newly designed continuous soot monitoring system (COSMOS). Journal of Environmental Monitoring 10(10):1195-1201. 15.Nakayama T, et al. (2010) Size-Dependent Correction Factors for Absorption Measurements using Filter-based Photometers: PSAP and COSMOS. J. Aerosol. Sci. 41(4):333-343.

Sample Pre-Treatment �  Well controlled sample pre-treatment is as

important as measurement choice.

�  Control of: �  Dilution, RH, Pressure, Temperature

�  Removal of: �  Semi-volatiles �  NO2 (if PAS)

will minimise biases in ALL measurement methods.

�  Sample heating to 150 – 300 °C removes most semi-volatile components. �  But different temperatures remove different amounts

�  Needs to be standardised.

�  Convergence of BC absorption after sample pre-treatment with heating.

Why do we need an accurate measure of absorption?

�  BC absorption and mass are the units used for regulation, control, climate/health impacts.

�  BC absorption can be converted to BC mass by using a single conversion factor. �  Accuracy of this factor is debatable but so long as it is known, interchange

is possible.

�  Filter-based absorption (MAAP, PSAP, Aethelometer, COSMOS) have variable accuracies even with thousands of hours of laboratory corrections

�  FSN: Is an un-calibrated precise unitless number, but �  Cannot be compared to other measurements �  Cannot be used for regulatory, air quality, climate work. �  Cannot know what the true contribution of BC is.

�  It does have the potential to join the ranks of other filter-based methods.

Take Home Message

�  Precise and accurate BC absorption/mass measurement are available if some simple standardisations are made by the community.

�  Research, industry and regulators have contributions to make, but cannot exchange data unless they work within these standards.