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

Abstract

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 NO x 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 x 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. NO x 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, NO x 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 NO x 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 NO x emission controls, especially from the neighbor counties and states, are also necessary to improve O 3 air quality in Southeast Texas. ► A new technique to determine the contributions of NO x sources to O 3 is developed. ► Natural gas, diesel and gasoline engines are the largest local NO x sources to O 3 . ► Contributions from upwind sources to O 3 can be as much as 50 ppb in rural areas.