APPENDIX G 

Evaluation of the 2002 GEOS-CHEM Global Chemistry Model 

Used for Boundary Conditions in the VISTAS Regional Haze Modeling 
 G-2 

Application of GEOS CHEM for 2002 Boundary Conditions for 

CMAQ Modeling of the VISTAS Region 

Ivar Tombach 

5 October 2007 

CMAQ modeling of regional haze in the VISTAS region for 2002 and 2018 was carried out on a 

grid of 12x12 km cells that covers the VISTAS states (AL, GA, FL, MS, SC, NC, TN, KY, VA, 

WV) and states adjacent to them. This grid is nested within a larger national CMAQ modeling 

grid of 36x36 km grid cells that covers the continental United States, portions of Canada and 

Mexico, and portions of the Atlantic and Pacific Oceans along the east and west coasts. 

Transport of pollution from the national domain into the VISTAS modeling domain is reflected 

in hourly-varying boundary conditions around the VISTAS domain, which are derived from the 

CMAQ simulations in the 36 km national domain. The national domain incorporates emissions 

from northern Mexico and southern Canada, and therefore explicitly calculates the effects of 

these international emissions on concentrations at the boundaries of the VISTAS modeling 

domain. 

Effects of emissions from more distant sources that are outside of the 36 km national domain 

were estimated using outputs from a global air pollution model. The Big Bend Regional Aerosol 

and Visibility Observational (BRAVO) study had found that the performance of national-scale 

modeling was enhanced by using monthly averages of outputs from a global air pollution model, 

GOCART, to generate SO2 and sulfate boundary conditions for its 36 km national modeling grid 

(Schichtel, et al., 2005), instead of using the EPA default concentrations that are normally used. 

VISTAS elected to use a similar approach, using the outputs of the GEOS-CHEM global air 

pollution model, which also contains a detailed representation of ozone-NOx-VOC-PM 

chemistry. 

Under EPRI sponsorship, Harvard University had already used the GEOS-CHEM model, with 

2°x2.5° horizontal resolution, to estimate natural conditions and transboundary influences on 

elemental and organic carbon aerosols in the United States in 1998 (Park et al., 2003). After 

some emission inventory and model improvements, similar information was developed for 

sulfate and nitrate aerosols in 2001 (Park et al., 2004). 

Under contract to VISTAS (which was acting on behalf of all the RPOs), Harvard then simulated 

global air pollution for the year 2002 (the VISTAS modeling year) with 4°x5° horizontal 

resolution and 3-hour temporal resolution (Jacob, 2005). The coarser grid scale was considered 

sufficient to provide boundary conditions outside of North America, largely over the oceans. 

Although the VISTAS simulation was generally similar to the 2001 EPRI simulation mentioned 

above, the VISTAS modeling incorporated the following substantive changes in addition to the 

change in grid scale: (1) use of U.S. anthropogenic emissions from the EPA NEI 1999 inventory, 

except for ammonia; (2) use of forest fire information specific to 2002; (3) inclusion of the 

secondary organic aerosol (SOA) formation mechanism from Chung and Seinfeld (2002); and 

(4) inclusion of prototype soil dust and sea salt simulations. An additional model improvement, 

which has since been implemented in the standard version of GEOS-CHEM, was the application 

of surface emissions and dry deposition to the entire mixed layer column diagnosed by GEOS 
G-3

(the Global Earth Observation System of NASA) rather than to just the surface layer of the 

model. Details of the emissions, meteorology, model configuration, and model performance 

evaluation for the VISTAS simulations are described in the report to VISTAS (Jacob, 2005). 

For North America, the VISTAS GEOS-CHEM modeling used monthly-average emissions, so 

the model results do not reflect diurnal or weekday-weekend variability in emissions, and also 

diminish the impacts of episodic emissions, such as the July 2002 wildfire in Ontario, Canada. 

The Mexican emissions were from the inventory of the BRAVO Study, while Canadian 

emissions for the year 2000 were used. Emissions from Caribbean islands were not included, 

which could impact model estimates at the Everglades. 

The surface meteorological data inputs to GEOS-CHEM had 3-hr temporal resolution and the 

upper air meteorological inputs had 6-hr resolution, with a horizontal resolution of 1°x1° or 

better and 48 layers vertically. 

The output of the GEOS-CHEM modeling, as delivered to VISTAS, contained concentrations of 

all visibility-related components of particulate matter with 3-hr temporal resolution and 4°x5°

horizontal resolution over multiple layers of the atmosphere. 

Even though the coarse spatial resolution degraded the performance of the model over North 

America, relative to results from prior modeling for other years using finer grids, the 

concentrations simulated by GEOS-CHEM maintained the synoptic-scale structure and did not 

have a continental-scale bias. At a finer scale, the concentrations simulated by GEOS-CHEM for 

the surface layer in 2002 were generally within a factor of two of regional-averages (~500 km 

scale) of measurements of the various chemical components of particulate matter in the United 

States and, except for organic carbon, satisfactorily simulated the seasonal variability in the 

observations. 

In the meantime, Harvard University had also carried out new finer-grid simulations for 2001 for 

EPRI, using a 1°x1° grid over North America, nested in a coarser grid that covered the globe. 

The final version of these simulations included modifications to the modeling grid to improve 

delineation between domestic US emissions and those of foreign sources at the national 

boundaries, and revision (reduction) of the Canadian and Mexican sulfur emissions. These 

GEOS-CHEM 1°x1° grid scale simulations for 2001 and their results are described in the paper 

by Park et al. (2006). Over appropriate geographic and temporal scales, the results of these 

simulations for 2001were similar to those of the 2002 VISTAS simulations, which gave 

confidence in the VISTAS GEOS-CHEM results. 

The concentration fields that resulted from the GEOS-CHEM simulations were not directly 

usable as boundary conditions for the 36 km grid scale CMAQ simulations. GEOS-CHEM and 

CMAQ use different horizontal and vertical grid scales and different map projections, and have 

different labels for chemical constituents. The GEOS-CHEM outputs were processed by Dr. 

Daewon Byun at the University of Houston to adjust for these discrepancies and to develop 

boundary conditions for the 36 km CMAQ modeling. Since the CMAQ modeling has temporal 

resolution of one hour, the boundary conditions were given for every hour. However, since the 

GEOS-CHEM outputs had 3-hr resolution, the hourly boundary conditions were the same for 

three consecutive hours. An example of the resulting boundary conditions for sulfate at the 

surface is shown in Figure G-1. As indicated by the colors along the boundary of the modeling 
G-4

domain, GEOS-CHEM projects higher sulfate values for the southern boundary, indicating 

emissions from Mexico, and the eastern boundary, indicating recirculation of emissions from the 

eastern US. 

Average Sulfate Boundary Conditions 

GEOS-CHEM for July 1, 2001 

Figure G-1. GEOS-CHEM projections of sulfate boundary conditions for the national CMAQ 

modeling grid on July 1, 2001.
G-5

Model performance for the 2002 4x4 degree annual GEOS-CHEM run was compared to 

observations at 145 IMPROVE sites and CASTNET sites across the continental US. Example 

plots from the model performance evaluation are illustrated in Figures G-2 through G-9. 

Additional information is provided in the final GEOS-CHEM report (Jacobs, 2005). 

Performance at the boundaries is driven by accuracy of international emission estimates; 

performance at US continental monitors is driven by US emissions. Given the large grid size, 

projections are approximations that show the correct order magnitude. Performance for 2002 

boundary conditions is acceptable compared to observations and an improvement over the 

CMAQ default single annual value for all boundaries. Sulfate projections reproduce observed 

values fairly well, particularly for boundary conditions. Sulfate is slightly over predicted for 

winter months. Summer sulfate levels are under predicted, particularly on high sulfate days, but 

mean fraction bias for monthly average sulfate is within performance criteria set for CMAQ (+/- 

30%). 
G-6

Figure G-2. GEOS-CHEM 2002 SO4 annual performance compared to IMPROVE (145 sites) 

and CASTNET monitoring networks. 
G-7

Figure G-3. GEOS-CHEM 2002 Monthly SO4 Performance compared to 145 IMPROVE sites 

in the continental US. 

Figure G-4. GEOS-CHEM 2002 Monthly SO4 Performance compared to 145 IMPROVE sites 

in the continental US. 

GEOS-CHEM nitrate performance is acceptable for boundary conditions. Nitrate is over 

predicted at some sites on some winter days; overall nitrate is under predicted in all months, and 

particularly in summer months. Since summer nitrate values are generally low, this under 

prediction does not affect overall model performance for CMAQ. 

Monthly Average Sulfate

0

2

4

6

8

10

12

14

0 2 4 6 8 10 12 14

IMPROVE (μg/m3)

GEOS-CHEM (

μg/m

3

)

January

February

March

April

May

June

July

August

September

October

November

December

Sulfate Model Performance (IMPROVE)

-40

-30

-20

-10

0

10

20

30

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Mean Fractional Bias (%)
G-8

 

Figure G-5. GEOS-CHEM 2002 NO3 annual average performance compared to IMPROVE 

(145 sites) and CASTNET monitoring networks. 
G-9

Figure G-6. GEOS-CHEM 2002 Monthly NO3 Performance compared to 145 IMPROVE sites 

in the continental US. 

Figure G-7. GEOS-CHEM 2002 Monthly NO3 Performance compared to 145 IMPROVE sites 

in the continental US. 

Monthly Average Nitrate

0

1

2

3

4

5

6

7

8

012345678

IMPROVE (μg/m3)

GEOS-CHEM (μg/m3

)

January

February

March

April

May

June

July

August

September

October

November

December

Nitrate Model Performance (IMPROVE)

-180

-160

-140

-120

-100

-80

-60

-40

-20

0

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Mean Fractional Bias (%)
G-10

Figure G-8. GEOS-CHEM 2002 Organic Matter Carbon (OMC) annual average performance 

compared to IMPROVE (145 sites) and CASTNET monitoring networks. 

Model performance for organic matter carbon (OMC) and elemental carbon (EC) are acceptable. 

Performance at the boundaries is driven by quality of the fire inventory for international 

emissions. While a new biomass burning inventory was developed as part of this study, monthly 

average fire emissions do not accurately capture the impacts from episodic fires. 
G-11

Figure G-9. GEOS-CHEM 2002 Elemental Carbon annual average performance compared to 

IMPROVE (145 sites) and CASTNET monitoring networks. 

GEOS-CHEM 3-hr average outputs were used as inputs to CMAQ modeling for 2002 and 2018. 

Since VISTAS did not have information to project future changes in international emissions, 

VISTAS used the same boundary conditions in 2018 as developed for 2002. 

References 

Chung, S. H. and Seinfeld, J.H. (2002), Global distribution and climate forcing of carbonaceous 

aerosols, Journal of Geophysical Research, Vol. 107 (D19), 4407, doi:10.1029/2001JD001397. 

Jacob, D.J. (2005), Documentation and Evaluation of the GEOS-CHEM Simulation for 2002 

Provided to the VISTAS Group. Final report, Harvard University. June 24. 

Park. R.J., Jacob, D.J., Chin, M., and Martin, R.V. (2003). Sources of carbonaceous aerosols 

over the United States and implications for natural visibility. Journal of Geophysical Research, 

Vol. 108 (D12), 4355, doi:10.1029/2002JD003190. 
G-12

Park. R.J., Jacob, D.J., Field, B.D., Yantosca, R.M., and Chin, M. (2004). Natural and 

transboundary pollution influences on sulfate-nitrate-ammonium aerosols in the United States: 

Implications for policy. Journal of Geophysical Research, Vol. 109, D15204, 

doi:10.1029/2003JD004473. 

Park. R.J., Jacob, D.J., Kumar, N., and Yantosca, R.M. (2006). Regional visibility statistics in 

the United States: Natural and transboundary pollution influences, and implications for the 

Regional Haze Rule. Atmospheric Environment, Vol. 40, pp. 5405-5423. 

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Tombach, I.H. (2005). Reconciliation and Interpretation of Big Bend National Park Sulfur 

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