Development of CMAQ Boundary Conditions 

One of the necessary inputs for performing CMAQ simulations is the boundary condition 

(BC). The past CMAQ regional photochemical modeling covering episodes relied on the 

use of ‘clean’ BCs. Recently, it has been recognized that seasonal and annual simulations, 

the appropriate BCs can be developed from global models (EPA, 2004). With the 

development and progress 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), it is possible to generate BCs 

for CMAQ through appropriate interpolation of the global model outputs. 

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 

(VISTA) 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 was 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 

chemical species. We utilized and modified version of the GEOS-CHEM to CMAQ 

interface program developed by Prof. Daewon Byun of Huston University 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 is 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 VISTA domain. 

 

Spatial patterns of Ozone and SO4

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

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

smaller than VISTA domain, both exhibit similar maximum concentration level of about 

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

domain along the boundaries than VISTA (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 VISTA 

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

the OTC and VISTA domains. 

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 VISTA domain with 19 layers), at each one of 

the four boundaries. 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 east direction may 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. 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. The high SO4 in the 

East direction may be due to the outflow from the US continent. 

References 

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

http://www.epa.gov/air/interstateairquality/pdfs/GEOSCHEMforCMAQ_Description.pdf 
Table 1: The average ozone concentrations at each one of the four boundaries for 

the OTC and VISTA 36 km domains 

Boundary OTC (22 layer 

averaged) 

VISTA (19 

layer averaged) 

OTC (surface) VISTA (surface)

South 43.9 ppb 35.6 ppb 36.8 ppb 31.6 ppb 

East 52.2 ppb 46.4 ppb 40.5 ppb 40.7 ppb 

North 34.1 ppb 21.9 ppb 13.7 ppb 13.5 ppb 

West 40.0 ppb 32.9 ppb 25.8 ppb 24.9 ppb 

Table 2: The average SO4 concentrations at each one of the four boundaries for the 

OTC and VISTA 36 km domains 

Boundary OTC (22 layer 

averaged) 

VISTA (19 

layer averaged) 

OTC (surface) VISTA (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 
Figure 1: The lowest layer of ozone BC for OTC and VISTA 36 Km domain 

Figure 2: The top layer ozone BC for OTC and VISTA 36 Km domain 
Figure 3: The lowest layer SO4 BC for OTC and VISTA 36 Km domain 

Figure 4: Averaged vertical profiles from each direction of ozone BC 
Figure 5: Averaged vertical profiles from each direction of SO4 BC