Modeling and direct sensitivity analysis of biogenic emissions impacts on regional ozone formation in the Mexico-US border area

A spatially and temporally resolved biogenic hydrocarbon and nitrogen oxide s (NOx) emissions inventory has been developed for a region along the Mexic o-U.S. border area. Average daily biogenic non-methane organic gases (NMOG) emissions for the 1700x1000 km(2) domain were estimated at 23,800 metric t ons/day (62% from Mexico and 38% from the United States), and biogenic NOx was estimated at 1230 metric tons/day (54% from Mexico and 46% from the Uni ted States) for the July 18-20, 1993, ozone episode. The biogenic NMOG repr esented 74% of the total NMOG emissions, and biogenic NOx was 14% of the to tal NOx. The CIT photochemical airshed model was used to assess how biogeni c emissions impact air quality. Predicted ground-level ozone increased by 5 -10 ppb in most rural areas, 10-20 ppb near urban centers, and 20-30 ppb im mediately downwind of the urban centers compared to simulations in which on ly anthropogenic emissions were used. A sensitivity analysis of predicted o zone concentration to emissions was performed using the decoupled direct me thod for three dimensional air quality models (DDM-3D),The highest positive sensitivity of ground-level ozone concentration to biogenic volatile organ ic compound (VOC) emissions (i.e., increasing biogenic VOC emissions result s in increasing ozone concentrations) was predicted to be in locations with high NOx levels, (i.e., the urban areas). One urban center-Houston-was pre dicted to have a slight negative sensitivity to biogenic NO emissions (i.e. , increasing biogenic NO emissions results in decreasing local ozone concen trations). The highest sensitivities of ozone concentrations to on-road mob ile source VOC emissions, all positive, were mainly in the urban areas. The highest sensitivities of ozone concentrations to on-road mobile source NOx emissions were predicted in both urban (either positive or negative sensit ivities) and rural (positive sensitivities) locations.