Modeling and Direct Sensitivity Analysis of Biogenic Emissions Impacts on Regional Ozone Formation in the Mexico-U.S. Border Area

ABSTRACT

A spatially and temporally resolved biogenic hydrocarbon and nitrogen oxides (NO_x) emissions inventory has been developed for a region along the Mexico-U.S. border area. Average daily biogenic non-methane organic gases (NMOG) emissions for the 1700 × 1000 km² domain were estimated at 23,800 metric tons/day (62% from Mexico and 38% from the United States), and biogenic NO_x was estimated at 1230 metric tons/day (54% from Mexico and 46% from the United States) for the July 1820, 1993, ozone episode. The biogenic NMOG represented 74% of the total NMOG emissions, and biogenic NO_x was 14% of the total NO_x. The CIT photochemical airshed model was used to assess how biogenic emissions impact air quality. Predicted ground-level ozone increased by 510 ppb in most rural areas, 10-20 ppb near urban centers, and 20-30 ppb immediately downwind of the urban centers compared to simulations in which only anthropogenic emissions were used. A sensitivity analysis of predicted ozone concentration to emissions was performed using the decoupled direct method for three dimensional air quality models (DDM-3D). The highest positive sensitivity of ground-level ozone concentration to biogenic volatile organic compound (VOC) emissions (i.e., increasing biogenic VOC emissions results in increasing ozone concentrations) was predicted to be in locations with high NO_x levels, (i.e., the urban areas). One urban center—Houston—was predicted to have a slight negative sensitivity to biogenic NO emissions (i.e., increasing biogenic NO emissions results in decreasing local ozone concentrations). The highest sensitivities of ozone concentrations to on-road mobile source VOC emissions, all positive, were mainly in the urban areas. The highest sensitivities of ozone concentrations to on-road mobile source NO_x emissions were predicted in both urban (either positive or negative sensitivities) and rural (positive sensitivities) locations.