1 Draft V.0 April 28, 2005 

## Developm ent 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 C MAQ 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 d ynamic especiall y for grid modeling applications covering seasonal and annual simulations the use of global chemical transport models such as GEOS

- CHEM of Harvard Universit y ( http://www

- as.harvard.edu/chemistry /trop/geos/index.html ) and MOZART of National Center for Atmospheric Research (NCAR) ( http://www.acd.ucar.ed

u/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 applic ation 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 (VI STAS) modeling domain. GEOS

- CHEM simulation data for 2002 were obtained through the eff

orts 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 pr ovides 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 confiden ce 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 i n the vertical when compared with the VISTAS domain. Spatial patterns of Ozone and SO 4 Figure 1 display s ozone concentrations in layer 1 along the boundaries for OTC and VI STAS domains at hour 15 on August 13, 2002. Although the OTC domain is slightly s maller 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 lay er of the OTC domain is higher than VI STAS domain. Figure 3 shows similar display s for SO 4 at the boundary at the lowest layer of the OTC and VISTAS domains.2

Averaged Ozone and SO 4 concentrations at the boundarie s of the modeling domain Table 1 lists the averaged ozone concentrations at surface and for the whole boundary layer, (OTC domain with 22 lay ers, and VI STAS domain with 19 lay ers), at each one of the four boundaries for August 2002. In both cases the con centrations 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 SO 4 concentrations. The higher SO 4 boundary conditions at th e eastern boundary are suspected to be due to the outflow from the US continent. Averaged Ozone and SO 4 BC vertical profiles from each of the domain Figure 4 display s 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 display s the average vertical profile of SO 4 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 b y V ISTAS indicates good agreement.
References By un, D. W., Mo on, N. K., Jacob, D., and Park, R. (2004) Regional transport study of air pollutants with linked global tropospheric chemistry and regional air quality models. 2 nd ICAP Workshop, Research Triangle Park, NC http://www.cep.unc.edu/empd/projects/I CAP/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 By

un, 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/~dwby un/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) VI STAS (19 lay er avg) OTC (surface) VI STAS (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 SO 4 concentrations ( : g/m 3 ) at each one of the four boundaries for the OTC and VISTA 36 km domains during August 2002 Boundary OTC (22 layer avg) VI STAS (19 lay er avg) OTC (surface) VI STAS (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 0.534

Figure 1: The lowest layer ozone concentrations (ppm) along the boundaries for the OTC and VISTAS 36 km domain5

Figure 2: The top layer ozone concentrations (ppm) along the boundaries for the OTC and VISTAS 36 km domain6

Figure 3: The lowest layer sulfate concentrations ( : g/m 3 ) along the boundaries for the OTC and VISTAS 36 km domain7

Figure 4: Averaged ozone concentrations (ppb) in the vertical along each bo undary of the OTC and VISTAS 36 km domain8

Figure 5: Averaged sulfate concentrations ( : g/m 3 ) in the vertical along each boundary of the OTC and VISTAS 36 km domain