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Draft V.0 April 28, 2005

Development of CMAQ Boundary Conditions 

Bureau of Air Quality Analysis and Research

Division of Air Resources

New York State Department of Environmental Conservation

Albany, NY 12233

One of the inputs needed for performing CMAQ simulation is the concentration field 

along the boundaries commonly referred to as boundary conditions (BCs). In prior 

regional photochemical modeling simulations covering extended episodes often utilized 

‘clean’ BCs. Recognizing that pollutant influx arising from the upwind regions are 

varying and dynamic especially for grid modeling applications covering seasonal and 

annual simulations the use of global chemical transport models such as GEOS-CHEM of 

Harvard University (http://www-as.harvard.edu/chemistry/trop/geos/index.html) and 

MOZART of National Center for Atmospheric Research (NCAR) 

(http://www.acd.ucar.edu/science/gctm/mozart/index.php) Byun et al (2004) has 

proposed developing BCs which had been applied in some of the current modeling 

exercises (EPA, 2004, 2005). 

In this note we describe the development of the BCs for the OTC Modeling Committee 

application of CMAQ at 36 km grid spacing and perform a limited comparison with the 

BCs reported for Visibility Improvement State and Tribal Association of the Southeast’s 

(VISTAS) modeling domain. GEOS-CHEM simulation data for 2002 were obtained 

through the efforts of the Northeast Consortium of Air Use Management (NESCAUM) in 

December 2004. The GEOS-CHEM simulation data were at the spatial resolution of 4 ̊ 

by 5 ̊ in the horizontal and 20 layers in the vertical extending from surface to about 100 

mb. The model provides information for about 50 chemical gas and particulate matter 

species. We utilized a modified version of the GEOS-CHEM to CMAQ interface 

program (Moon and Byun, 2004) so as to match the OTC 36 km modeling domain. To 

provide an added level of confidence in the development of the BCs, a comparison was 

performed with the BCs reported for the VISTAS 36 km modeling domain. It should be 

noted that the two domains differ in their definition with the OTC domain being smaller 

horizontally and has more layers in the vertical when compared with the VISTAS domain.

Spatial patterns of Ozone and SO4

Figure 1 displays ozone concentrations in layer 1 along the boundaries for OTC and 

VISTAS domains at hour 15 on August 13, 2002. Although the OTC domain is slightly

smaller than VISTAS domain, both exhibit similar maximum level of concentration of 

about 70 ppb. The OTC domain had slightly higher peak ozone (168 ppb) at the top of the 

domain along the boundaries than VISTAS (see Figure 2), which has a maximum of 139 

ppb. This is not unexpected since the top layer of the OTC domain is higher than 

VISTAS domain. Figure 3 shows similar displays for SO4 at the boundary at the lowest 

layer of the OTC and VISTAS domains.
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Averaged Ozone and SO4 concentrations at the boundaries of the modeling domain

Table 1 lists the averaged ozone concentrations at surface and for the whole boundary

layer, (OTC domain with 22 layers, and VISTAS domain with 19 layers), at each one of 

the four boundaries for August 2002. In both cases the concentrations exhibit similar 

pattern, with higher concentrations at south and east boundaries and low concentration in 

the north and west boundaries. Table 2 is similar to Table 1, except it lists SO4

concentrations. The higher SO4 boundary conditions at the eastern boundary are 

suspected to be due to the outflow from the US continent.

Averaged Ozone and SO4 BC vertical profiles from each of the domain

Figure 4 displays the average vertical profile of ozone concentrations at each of four 

boundaries for August 2002. As to be expected, GEOS-CHEM yields increasing ozone 

concentrations with increasing altitude and this is reflected in Figure 4. Figure 5 displays 

the average vertical profile of SO4 concentration at each of the four boundaries. 

Conclusions

Boundary conditions based upon (GEOS-CHEM) were generated for the period of May

through September 2002 for use in the 36 km CMAQ modeling simulations. Comparison 

of the BCs with those developed by VISTAS indicates good agreement.

References

Byun, D. W., Moon, N. K., Jacob, D., and Park, R. (2004) Regional transport study of air 

pollutants with linked global tropospheric chemistry and regional air quality models. 2nd

ICAP Workshop, Research Triangle Park, NC

http://www.cep.unc.edu/empd/projects/ICAP/2004wkshp_pres.html.

EPA (2004): Use of GEOS-CHEM for CMAQ Boundary Conditions, 

http://www.epa.gov/air/interstateairquality/pdfs/GEOSCHEMforCMAQ_Description.pdf

EPA (2005): Technical support document for the final clean air mercury rule: Air quality

modeling. USEPA, Research Triangle Park, NC

http://www.epa.gov/camr/

Moon N. K., and Byun, D. W. (2004) A simple User’s Guide for “geos2cmaq” code: 

Linking CMAQ with GEOS-CHEM, Version 1.0,” Interim report from Institute for 

Multidimensional Air Quality studies (IMAQS) University of Houston, Houston TX

http://www.math.unh.edu/~dwbyun/Meetings/icap/.
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Table 1: The average ozone concentrations (ppb) at each one of the four boundaries 

for the OTC and VISTAS 36 km domains during August 2002

Boundary OTC (22 layer avg) VISTAS (19 layer avg) OTC (surface) VISTAS (surface)

South 43.9 35.6 36.8 31.6

East 52.2 46.4 40.5 40.7

North 34.1 21.9 13.7 13.5

West 40.0 32.9 25.8 24.9

Table 2: The average SO4 concentrations (g/m3) at each one of the four boundaries 

for the OTC and VISTA 36 km domains during August 2002

Boundary OTC (22 layer avg) VISTAS (19 layer avg) OTC (surface) VISTAS (surface)

South 0.54 0.51 0.70 0.56

East 1.50 1.67 2.04 1.88

North 0.45 0.54 0.63 0.64

West 0.42 0.47 0.53 0.53
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Figure 1: The lowest layer ozone concentrations (ppm) along the boundaries for the 

OTC and VISTAS 36 km domain
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Figure 2: The top layer ozone concentrations (ppm) along the boundaries for the 

OTC and VISTAS 36 km domain
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Figure 3: The lowest layer sulfate concentrations (g/m3) along the boundaries for 

the OTC and VISTAS 36 km domain
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Figure 4: Averaged ozone concentrations (ppb) in the vertical along each boundary 

of the OTC and VISTAS 36 km domain
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Figure 5: Averaged sulfate concentrations (g/m3) in the vertical along each 

boundary of the OTC and VISTAS 36 km domain