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AMENDED VERSION FOR DISTRIBUTION

Determination of Total Alkalinity, pH and pCO2 in Seawater

Oceanology International 2014, Ocean Observing Systems London, 11 March 2014

Steffen Aßmann, Peer Fietzek, Carsten Frank, Wilhelm Petersen, Arne Körtzinger, Peter Linke


Outline

Dr. Steffen Aßmann, [email protected] 2

• Introduction Ocean Acidification Autonomous Measurements

• Carbonate System Parameters Cross Calculations

• Measuring Systems pCO2 pH Total Alkalinity (TA)

• Field Data

• Summary


Introduction


Temporal Variability


• Rising CO2 concentration in the atmosphere

• Increasing pCO2 and decreasing pH in the surface ocean

• High temporal variability

• Measured at one spot in the earth

• Good for monitoring trends

Doney et al., 2005


Spatial Variability


• Most data from voluntary observing ships (VOS)

• Averaged over decades

• Only pCO2 measured

• No access to the carbonate chemistry

• More parameters are

needed

Takahashi et al., 2005


Ocean Observation


• Most data from voluntary observing ships (VOS)

• Averaged over decades

• Only pCO2 measured

• No access to the carbonate chemistry

• More parameters are needed

Takahashi et al., 2005


Carbonate System


Ocean Observation


Bicarbonate

Carbonate

Carbonic Acid Primary Production Respiration

Air Sea Exchange

Calcification

Dissolution

Dissolved CO2

Atmospheric CO2

Kleypas et al., 2005

Four measureable

parameters:

• pH (hydrog. ion concentration)

• pCO2 (partial pressure of CO2)

• DIC (dissol. inorganic carbon)

• TA (total alkalinity)

AMENDED VERSION FOR DISTRIBUTION Calculation of the Carbonate Parameters


Input ∆pH ∆TA

(µmol kg-1) ∆DIC

(µmol kg-1) ∆pCO2 (µatm)

Value 8.045 2300 2000 397 Error ± 0.002 ± 3 ± 2 ± 2 pH – TA ± 3.8 ± 2.1 pH – DIC ± 2.7 ± 1.8 pH – pCO2 ± 21 ± 18 pCO2 – DIC ± 0.0025 ± 3.4 pCO2 – TA ± 0.0026 ± 3.2 TA – DIC ± 0.0062 ± 5.7

Millero et al., 2007


Availability of Autonomous Sensors


pCO2

• Complex surface water systems are available with high accuracy (e.g. GO)

• Autonomous sensors for in-situ measurements (e.g. CONTROS)

pH • Surface water systems are available

with high accuracy and precision (e.g. Aßmann et al., 2011)

• Sensors for in-situ application in

shallow waters (e.g. Sunburst, ISFET)

No commercial sensors for TA or DIC available. These parameters allow optimal carbonate system characterization.


CO2 in Seawater


CO2 Sensor – Principle


CO2

H2O

Communication Port

Titanium Housing

NDIR Detector

Digital Signal Processor

Zeroing Membrane

(= “Gas Inlet” and “Gas Outlet”)


Specification for CO2 Sensor


Mean Offset to Reference - 0.6 µatm

RMSE ± 3.7 µatm

Resolution < 1 µatm

Measurement Frequency 1 Hz

Measurement Range 200 … 1000 µatm* *dependent on calibration, other ranges possible

Specification was found in the field after evaluation of more than 60,000 data points in comparison to a reference system from General Oceanics (GO).

Fietzek et al., 2014


Field Data pCO2


• Deployments near Cape Verde Islands November 2010 – June 2011 (4 deployments)

• 1st deployment: ≈8 weeks (9.11.-5.1., 57days) and 44 collected profiles in upper 200 m (total: 123 profiles; 156 days)

• Sensor runtime is ≈3 h every 30 h

• Total runtime of the sensor was ≈88 h; (+22 h the sensor kept running in the lab); battery capacity was 52 Ah

• Redundant data storage system on float and sensor high resolution

• Data files sent via satellite after each profile (5 dbar steps); two-way satellite communication

Measurements on a Profiling Float


Photographs: Courtesy of Dr. B. Fiedler


Measurements on a Profiling Float


Fiedler et al., 2013


pH Determination Spectrophotometry


pH System – Principle


• FIA system using an indicator dye m-cresol purple

• Determination of the concentration of the indicator acid (HI-) / base (I2-) due to different absorption spectra using a CCD spectrometer

• Calculation of the pH value using Henderson–Hasselbalch equation pH = p𝐾𝑎 + log10

I2−

HI−

Aßmann et al., 2011


v

Perturbation Correction


0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.98.095

8.100

8.105

8.110

8.115

8.120

pHA(487.6nm)

1σresiduals = 0.0011σintercept = 0.0002



Specification for pH


For use in seawater

Accuracy ± 0.003 (with CRM*)

Offset to Reference ≈ 0.01

Precision ± 0.0007

Meas. Interval 1 min

Meas. Range 7.4 … 8.4 *certified reference material

Salinities = 20 … 40 psu Temperatures = 0 … 35 °C



Field Data pH


Field Data


North Sea

Aßmann et al., 2011 Date 2010


Total Alkalinity Determination Closed-Cell Titration

AMENDED VERSION FOR DISTRIBUTION TA System – Principle Closed-Cell Titraion


• FIA system applying an acidic titration with HCl

• Addition of the indicator dye bromocresol green

• Determination of the concentration of the indicator acid (HI-) / base (I2-) due to different absorption spectra using a CCD spectrometer

• Calculation of the pH value using Henderson–Hasselbalch equation

pH = p𝐾𝑎 + log10I2−

HI−

AMENDED VERSION FOR DISTRIBUTION Specification for TA Closed-Cell Titration


For use in seawater

Average Offset to Reference + 3 µmol/kg Precision ± 15 µmol/kg Meas. Interval 5 min

Meas. Range 1000 … 3000 µmol/kg* *highly variable due to dynamic titrant dosing All values were found in the field during a 2-week campaign in the North Sea

Salinities = 20 … 40 psu Temperatures = 0 … 35 °C


Total Alkalinity Determination Open-Cell Titration

AMENDED VERSION FOR DISTRIBUTION Preliminary Results Open-Cell Titration


Δ = +10 µmol/kg σ = ±1.1 µmol/kg

Time

TA (µ

mol

/kg)

Time pC

O2 (

µatm

)

63 %

00:00:30

Short term precision Degassing

AMENDED VERSION FOR DISTRIBUTION Open-Cell Titration Figure of Merit


• Only one Titration point needed

• Shorter measuring cycles (<5 min)

• Low reagent consumption • Acid (HCl) <100 µL • Indicator (BCG) <100 µL • Sample <10 mL

• No perturbation of the

equilibrium constants due to low pH values and high pCO2


Summary


• Knowledge about the carbonate system is mostly gained from bottled data and a view time series stations.

• There is a need for autonomous, continuously measuring sensors providing the parameters of biogeochemical cycles, especially for the carbonate system: • understanding and monitoring Ocean Acidification, • giving a picture of the carbonate system in coastal areas.

• New systems have been developed for pCO2 / pH / TA

measurements providing adequate quality for a characterization of the carbonate system and experience ongoing optimization.

Summary



We look forward to welcoming you to our booth in the German Pavilion at booth no. H300 and booth no. H250

Thank You


Determination of Total Alkalinity, pH and pCO in Seawateroceanrep.geomar.de/24651/1/Steffen.pdf ·...

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