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:  
HYPERLINK "http://cdiac.ornl.gov/oceans/co2rprt.html" 
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:

A reference scenario (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.  

An emissions reduction scenario in which this rulemaking is implemented,
reducing atmospheric CO2 concentrations by 2.9 ppm to 786 ppm [this
scenario assumes a mid-range 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:

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].

Choice of constants: Mehrbach et al. (1973), refit by Dickson and
Millero (1987) [Further discussion of this input is provided below] 

Choice of fCO2 or pCO2: pCO2  [pCO2 is the partial pressure of CO2 and
can be converted to fugacity (fCO2) if desired] 

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]. 

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)].  

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°C  (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.

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. 

 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-OAR-2009-0472)

 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 EPA-HQ-OAR-2009-0472)

 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.

