UNITED STATES ENVIRONMENTAL PROTECTION AGENCY WASHINGTON, D.C. 20460

OFFICE OF AIR AND RADIATION

MEMORANDUM

TO: Docket EPA-HQ-OAR-2009-0472

FROM: Jason Samenow Climate Change Division, Office of Atmospheric Programs

SUBJECT: Documentation for Computing Changes in oceani c pH resulting from EPA’s Light Duty Vehicle Greenhous e Gas Emission Standards

Background

To calculate the changes in oceanic pH resulti ng arising from the change in the atmospheric
concentration of carbon dioxide (CO2) resulting from EPA’ s Light-Duty Vehicle Greenhouse Gas Emission Standards, EPA’s Office of Atmos pheric Programs used the Program Developed
for CO 2 System Calculations (CO2SYS). The program was developed by Ernie Lewis at Brookhaven National Laboratory and Doug Wallace at the Insitut fuer Meereskunde in Germany, supported by the U.S. Department of Energy, Offi ce of Biological and E nvironmental Research,
under Contract No. DE-ACO2-76CH00016.

The program is written and compiled using Microsoft QuickBASIC and runs under DOS on almost any personal computer pro cessor. It can also be run from a Microsoft Excel spreadsheet
or in MATLAB. It is downloadable in these various formats at Oak Ridge National Laboratory’s (ORNL) Center for Carbon Dioxide Informati on and Analysis Center (CDIAC) Web site: http://cdiac.ornl.gov/oceans/co2rprt.html . This CDIAC Web site al so contains full model
documentation and references.

The program CO2SYS performs calculations re lating parameters of the carbon dioxide (CO 2 ) system in seawater and freshwater. The program us es two of the four meas urable parameters of
the CO 2 system [total alkalinity (TA), total inorganic CO 2 (TC), pH, and either fugacity (fCO 2 ) or partial pressure of CO 2 (pCO 2 )] to calculate the other two parameters at a set of input conditions (temperature and pressure) and a set of output conditions chosen by the user.

Methodology and Results

Summary

We ran the DOS version (Lewis and Wallace, 1 998) of the program to compute pH under two
emissions scenarios as follows:1) A reference scenario 1 (i.e., business-as-usual, no ac tion) in which atmospheric CO 2 concentrations (also known as partial pressure) in 2100 reached 788.9 parts per million (ppm) [this scenario assumes a mid-range climate sensitivity of 3°C] in 2100. 2) An emissions reduction scenario 2 in which this rulemaking is implemented, reducing atmospheric CO 2 concentrations by 2.9 ppm to 786 ppm [this scenario assumes a midrange climate sensitivity of 3°C] in 2100.

In order to determine the change in pH resulti ng from the emissions reduction, we subtracted the reference scenario pH from the emission reducti on scenario pH. We found that emissions reductions result in a 0.0014 increase in pH. Th e values for pH under th e two scenarios varied according to the inputs used. For one set of seawat er parameters (described below), the reference scenario (788.9 ppm) pH was 7.7716 and the em issions reduction scenario (786 ppm) was
7.7730.

Details

Running the CO2SYS program required the input of a number of variables and constants when we ran each scenario (i.e., both the reference and emissions reduc tion). Our choices for these inputs are described below.

Upon starting the program, the user is asked for a number of inputs. The inputs we selected were as follows, with our justification fo r these inputs provided in brackets:

1) Input mode: Single-input [This means that the program calculates pH for one set of input variables at a time, instead of a batch of variables. It does not affect results]. 2) Choice of constants: Mehrbach et al. (1973), refit by Dickson and Mi llero (1987) [Further discussion of this input is provided below] 3) Choice of fCO 2 or pCO 2 : pCO 2 [pCO 2 is the partial pressure of CO2 and can be converted to fugacity (fCO2) if desired] 4) Choice of KSO4: Dickson (1990) [Lewis and Wallace (1998) recommend using the equation of Dickson (1990) for this dissociat ion constant. The model also allows the use of the equation of Khoo et al. (1977). Switc hing this parameter to Khoo et al. (1977) instead of Dickson (1990) had no effect on the difference in pH between the reference and emissions scenario]. 5) Choice of pH scale: Total scale [The model a llows pH outputs to be provided on the total scale, the seawater scale, the free scale, and the National Bureau of Standards (NBS) scale. The various pH scales can be interr elated using equations provided by Lewis and Wallace (1998)].

1 For more information on the reference scenario, please refer to technical report prepared by the Joint Global Change Research Institute, which is part of the Pacifi c Northwest National Laborato ry. (See Docket EPA-HQ-OAR2009-0472)

2 For more information on the emissions reduction scenario, please refer to technical report prepared by the Joint Global Change Research Institute, which is part of th e Pacific Northwest National Laboratory. (See Docket EPAHQ-OAR-2009-0472)The program provides seven choices of consta nts for saltwater that are needed for the calculations. We calculated pH valu es using all seven of these choi ces and found that in all cases
the emissions reductions result in a 0.0014 increa se in pH, with the exception of Roy et al. (1993) which calculated the emissions reducti ons as resulting in a 0.0013 increase in pH.

The user is then prompted to choose which vari ables to calculate give n user-specified input variables. We chose to input total alkalinity (TA) and partial pressure of CO2 (pCO 2 ) in order for the program to output total inorganic CO 2 (TC) and pH. This was option 3 in the program’s

menu.

Additional inputs were then requested. For both scenarios we ran the model using a variety of input values to test whether the model was sens itive to these inputs. We found that the model was not sensitive to these input values. The input values came from certified reference materials of sterilized natural sea water (Dickson, 2003, 2005, 2010). The inputs were as follows (The
three values are for Dicks on (2003, 2005, 2010) respectively):

• Salinity: 35.2, 31.6, 33.4

• Total phosphate: 0.41, 0.33, 0.54 umol/kg;

• Total silicate: 2.0, 2.6, 1.5 umol/kg

• Input/output temperature: 27°C, 27°C , 27°C 3 (characteristic tropical ocean temperature)

• Input/output pressure: 0,0,0 dbar (surface pressure)

• Total alkalinity (TA): 2338, 2114, 2232 umol/kg

• Partial Pressure (pCO2): 788.9 ppm (for the reference scenario) and 786 ppm (for the emissions reduction scenario)

After all of this information was input into the model, it calculated the ocean pH for each set of input values.

References

A. G. Dickson. 1990. Thermodynamics of the dissoc iation of boric acid in synthetic sea water
from 273.15 to 318.15 K. Deep-Sea Res. 37 , 755-766.

Dickson, A. G. and F. J. Millero. 1987. A co mparison of the equilibrium constants for the
dissociation of carbonic aci d in seawater media. Deep-Sea Res. 34 , 1733-1743. (Corrigenda. Deep-Sea Res. 36 , 983).

Dickson, A. G. 2003. Certificate of Analysis – Reference material for oceanic CO2 measurements (Batch #62, bottl ed on August 21, 2003). Certified by Andrew Dickson, Scripps Institution of Oceanography. November 21, 2003.

Dickson, A. G. 2005. Certificate of Analysis – Reference material for oceanic CO2 measurements (Batch #69, bottl ed on January 4, 2005). Certif ied by Andrew Dickson, Scripps Institution of Oceanography. July 12, 2005.

3 Even though the temperature changed between the referen ce and emissions reduction scenarios, we did not alter it as the model (and hence the pH) was not sensitive to the magnitude of temperature change computed.Dickson, A. G. 2009. Certificate of Analysis – Reference material for oceanic CO2 measurements (Batch #100, bottled on Nove mber 13, 2009). Certified by Andrew Dickson, Scripps Institution of Oc eanography. February 10, 2010.

Hansson, I. (1973) A new set of pH-scales and standard buffers for seawater. Deep Sea
Research

, 20 : 479-491.

Khoo, K.H., R.W. Ramette, C.H. Culberson, and R. G. Bates. 1977. Determination of hydrogen

ion concentrations in seawater from 5 to 40 ° C: Standard potentials at salinities from 20 to 45 ‰. Analytical Chemistry 49(1): 29-34.

Lewis, E., and D. W. R. Wallace. 1998. Program Developed for CO 2 System Calculations.
ORNL/CDIAC-105. Carbon Dioxide Informati on Analysis Center, Oak Ridge National Laboratory, U.S. Department of Energy, Oak Ridge, Tennessee.

Mehrbach, C., C. H. Culberson, J. E. Hawley, and R. N. Pytkowicz. 1973. Measurement of the apparent dissociation constants of carbonic acid in seawater at atmos pheric pressure. Limnology and Oceanography 18 :897-907.

Roy, R. N., L. N. Roy, K. M. Vogel, C. Porter-Moore, T. Pearson, C. E. Good, F. J. Millero, and D. M. Campbell. 1993. The diss ociation constants of carbonic acid in seawater
at salinities 5 to 45 and te mperatures 0 to 45 deg C. Marine Chemistry 44:249-267. ------- Erratum. 1994. Marine Chemistry 45:337. --------Erratum. 1996. Marine Chemistry 52:183.