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 oceanic pH resulting from EPA’s 

Light Duty Vehicle Greenhouse Gas Emission Standards

Background

To calculate the changes in oceanic pH resulting 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 Atmospheric Programs used the Program Developed 

for CO2 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, Office of Biological and Environmental 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 processor. 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 Information and Analysis Center (CDIAC) Web site: 

http://cdiac.ornl.gov/oceans/co2rprt.html. This CDIAC Web site also contains full model 

documentation and references.

The program CO2SYS performs calculations relating parameters of the carbon dioxide (CO2) 

system in seawater and freshwater. The program uses two of the four measurable parameters of 

the CO2 system [total alkalinity (TA), total inorganic CO2 (TC), pH, and either fugacity (fCO2) 

or partial pressure of CO2 (pCO2)] 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, 1998) of the program to compute pH under two 

emissions scenarios as follows:
1) A reference scenario1

(i.e., business-as-usual, no action) in which atmospheric CO2 

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 scenario2in which this rulemaking is implemented, reducing 

atmospheric CO2 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 resulting from the emissions reduction, we subtracted the 

reference scenario pH from the emission reduction scenario pH. We found that emissions 

reductions result in a 0.0014 increase in pH. The values for pH under the two scenarios varied 

according to the inputs used. For one set of seawater parameters (described below), the reference 

scenario (788.9 ppm) pH was 7.7716 and the emissions 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 reduction). 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 for 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 Millero (1987) [Further 

discussion of this input is provided below] 

3) Choice of fCO2 or pCO2: pCO2 [pCO2 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 dissociation constant. The model also allows the use 

of the equation of Khoo et al. (1977). Switching 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 allows 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 interrelated 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 Pacific Northwest National Laboratory. (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 the Pacific Northwest National Laboratory. (See Docket EPAHQ-OAR-2009-0472)
The program provides seven choices of constants for saltwater that are needed for the 

calculations. We calculated pH values using all seven of these choices and found that in all cases 

the emissions reductions result in a 0.0014 increase in pH, with the exception of Roy et al. 

(1993) which calculated the emissions reductions as resulting in a 0.0013 increase in pH.

The user is then prompted to choose which variables to calculate given user-specified input

variables. We chose to input total alkalinity (TA) and partial pressure of CO2 (pCO2) in order 

for the program to output total inorganic CO2 (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 sensitive 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 Dickson (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°C3(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 dissociation 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 comparison of the equilibrium constants for the 

dissociation of carbonic acid 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, bottled 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, bottled on January 4, 2005). Certified by Andrew Dickson, Scripps 

Institution of Oceanography. July 12, 2005.

 

3 Even though the temperature changed between the reference 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 November 13, 2009). Certified by Andrew Dickson, 

Scripps Institution of Oceanography. 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 CO2 System Calculations.

ORNL/CDIAC-105. Carbon Dioxide Information 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 atmospheric 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 dissociation constants of carbonic acid in seawater 

at salinities 5 to 45 and temperatures 0 to 45 deg C. Marine Chemistry 44:249-267. 

------- Erratum. 1994. Marine Chemistry 45:337. 

--------Erratum. 1996. Marine Chemistry 52:183.