Contributions of local and regional sources of NOx to ozone concentrations in Southeast Texas

The Community Multi-scale Air Quality (CMAQ) model with a modified SAPRC-99 photochemical mechanism was used to investigate the contributions of local and upwind NOx sources to O-3 concentrations in Southeast Texas during the 2000 Texas Air Quality Study (TexAQS 2000) from August 25 to September 5, 2000. Contributions from eight different local NO source types and eight different source regions to the 8-h average daytime O-3 concentrations from 1100 to 1800 CST (referred to as AD O-3 hereafter) are determined. Both diesel engines and highway gasoline vehicles account for 25 ppb of AD O-3 in the urban Houston area. NOx from natural gas combustion produces 35 ppb of AD O-3 in the industrial area of Houston. Contributions from industrial sources and coal combustion to AD O-3 have comparatively less broad spatial distribution with maximum values of 14 ppb and 20 ppb, respectively. Although the local sources are the most important sources, upwind sources have non-negligible influences (20-50%) on AD O-3 in the entire domain, with a maximum of 50 ppb in rural and coastal areas and 20 ppb in urban and industrial areas. To probe the origins of upwind sources contributions, NOx emissions in the entire eastern United States are divided into eight different regions and their contributions to O-3 concentrations in the Houston-Galveston-Brazoria (HGB) and Beaumont-Port Arthur (BPA) areas are determined. Among the various NOx source regions resolved in this study, other Texas counties near the HGB and BPA areas and southeastern states are the most important non-local sources of O-3. Under favorable transport conditions, emissions from neighbor states and northeastern states could also contribute to non-negligible O-3 concentrations (7-15%) in the HGB and BPA areas. This indicates that in addition to reduce local emissions, regional NOx emission controls, especially from the neighbor counties and states, are also necessary to improve O-3 air quality in Southeast Texas. (C) 2011 Elsevier Ltd. All rights reserved.