The impacts of trans-boundary transport of aerosols on the regional air quality in the United States

We use an approach of linking a regional air quality model, CMAQ, with a global chemistry transport model, GEOS-CHEM, to quantify influences of trans-boundary transport of aerosols on regional air quality in the United States. The current CMAQ model uses a constant lateral background condition which does not allow us to account for these influences. We present our model simulation for May 1998 when severe biomass burning occurred in Mexico and Central America due to an ENSO-related drought. In this simulation, we link the GEOS-CHEM output to CMAQ lateral boundary values through interpolation with coordinate transformation and species mapping between the chemistry mechanisms. We also investigate the effects of willdland fires occurred both within and outside the United States focusing on aerosol concentrations and visibility degradation. We evaluate our daily and monthly mean aerosol concentrations from CMAQ simulation with Interagency Monitoring of Protected Visual Environments (IMPROVE) network data. The CMAQ simulation using GEOS-CHEM as a lateral boundary condition brings out appreciable improvement of carbonaceous aerosol concentrations, especially in the southern United States, demonstrating successful effects of trans-boundary transport of aerosols. The correlation coefficient between simulated and observed monthly mean organic carbon concentrations is 0.81 with the GEOS-CHEM linkage, much higher than 0.39 with default constant lateral conditions. We also conduct several sensitivity analyses to assess the relative contributions of aerosols from various sources and to explore current and future aerosol concentrations in the United States. Reliance on this result is to contribute useful establishment of aerosol emission control strategies to meet the goal of the U. S. Environmental Protection Agency Regional Haze Rule.