Impact of transboundary transport of carbonaceous aerosols on the regional air quality in the United States: A case study of the South American wildland fire of May 1998

Abstract

The present work is an attempt to improve the performance of a regional air quality model by means of linking it with a global chemistry transport model in order to provide initial and lateral boundary conditions. The current Community Multiscale Air Quality (CMAQ) model uses a set of constant lateral background condition profiles of the pollutant species, without reflecting temporal and spatial variations at the boundaries. A modeling study of a severe biomass burning event during May 1998 in Mexico and Central America, attributable to an ENSO-related drought, is presented. In this study, the GEOS-Chem global model output is incorporated as the CMAQ lateral boundary and initial values through an interpolation reconciling the differences in the vertical and horizontal coordinates and the chemical species representations of the two models. Simulated daily and monthly mean aerosol concentrations were evaluated by comparing these predicted concentrations with observational data from the Interagency Monitoring of Protected Visual Environments (IMPROVE) surface network. CMAQ, using the GEOS-Chem output to provide the lateral boundary conditions, improves model simulation of carbonaceous aerosols, such as the elemental carbons (EC) and organic carbons (OC). The square of correlation coefficients between overall simulated versus observed monthly mean concentration of EC was 0.36 and OC was 0.72, demonstrating successful simulations of transboundary transport of aerosols. Model sensitivity simulations were performed to assess the carbonaceous emissions in the U.S. EPA's National Emissions Inventory for 1999 (NEI99) and to explore long-range transport and local contributions of wild fire emissions as potential sources for correcting the low bias of EC simulations in CMAQ. In the simulation without the boundary influx from wildfire, the analysis shows a general net export of EC and OC from the continental United States of America, while in the simulations with the GEOS-Chem linkage, the imported fluxes of the EC and primary OC are balanced by the export. The net fluxes of EC and OC through the southern Mexico region are equivalent to 53.7% of EC emission and 51.5% of OC emission amounts over the entire domain, which indicate that the transboundary fluxes are crucial for the proper simulation of EC and OC concentrations in this event. Secondary OC formations from gas phase precursors are equivalent to 175% of the NEI99 OC emissions over the model domain, or 55% of the OC emissions when the wildfire emission is included. Visibility degradation due to the carbonaceous aerosols from the Mexican fire resulted in an increase of the aerosol extinction coefficient by 40% over the background level in the southern United States during the May 1998 episode. Copyright 2007 by the American Geophysical Union.