Appendix M.2

Application of GEOS CHEM for 2002 Boundary Conditions for

CMAQ Modeling of the VISTAS Region 

Tennessee Department of Environment and Conservation 

Division of Air Pollution Control 

9th Floor L&C Annex 

401 Church Street 

Nashville, TN 37243

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Application of GEOS CHEM for 2002 Boundary Conditions for 

CMAQ Modeling of the VISTAS Region 

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 

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deposition to the entire mixed layer column diagnosed by GEOS (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 GEOSChem for the surface layer in 2002 were generally within a factor of two of regionalaverages (~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 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 

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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 1. As indicated 

by the colors along the boundary of the modeling 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 1. GEOS-CHEM projections of sulfate boundary conditions for the national CMAQ 

modeling grid on July 1, 2001

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 2-9. 

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%). 

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

and CASTNET monitoring networks. 

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

in the continental US. 

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

in the continental US. 

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 (%)

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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. 

 

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

(145 sites) and CASTNET monitoring networks. 

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

in the continental US. 

Figure 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 (%)

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Figure 8. GEOS-Chem 2002 Organic Carbon annual average performance compared to 

IMPROVE (145 sites) and CASTNET monitoring networks. 

Model performance for organic carbon matter and elemental carbon 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. 

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Figure 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. 

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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. 

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. 

Schichtel, B.A., Gebhart, K.A., Malm, W.C., Barna, M.G., Pitchford, M.L., Knipping, 

E.M., and Tombach, I.H. (2005). Reconciliation and Interpretation of Big Bend National 

Park Sulfur Source Apportionment: Results from the Big Bend Regional Aerosol and 

Visibility Observational Study – Part I. Journal of the Air & Waste Management Assoc.

Vol. 55, pp. 1709-1725. 

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