Modeling analysis of the impact of anthropogenic emission sources on ozone concentration over selected urban areas in Texas

Due to several regional–scale high ozone episodes that impacted the urban areas in south and central Texas over the past decade, the Texas Commission on Environmental Quality (TCEQ) has designated several urban regions as near non–attainment areas. A regional photochemical modeling experiment was set up to simulate a high ozone episode of September 11–14, 2002 in order to evaluate the impact of various anthropogenic emissions sources on ozone concentrations. The base case simulation showed reasonable model performance by capturing the peaks and the diurnal variability of observed ozone concentrations within the modeling domain. A comprehensive impact assessment of anthropogenic emissions from various source categories to the 8–hour ozone concentration was evaluated for each of the urban areas within the study region. Through a source apportionment analysis of emissions influencing the 8– hour ozone concentrations, NOx and VOC limited areas were identified. The model results showed that the net effect of all anthropogenic emissions was approximately 8.4 ppb in Victoria, 8.8 ppb in Corpus Christi, and 31.2 and 34.1 ppb in Austin and San Antonio, respectively. Impact of major emissions source categories differed regionally with ozone concentrations in Austin and San Antonio mainly influenced by mobile sources, while Corpus Christi and Victoria were largely impacted by long–range transport of ozone. On a local scale, Corpus Christi was also impacted by non–road sources, while Victoria was influenced by point sources. Ozone sensitivity analysis showed higher sensitivity towards VOC within the urban cores of Austin, San Antonio, and Corpus Christi, while the overall modeling regions showed higher NOx sensitivity. This would indicate that both NOx and VOC emissions reduction plans need to be developed and implemented for the mitigation of regional and urban ozone. The results also revealed that biogenic emissions played an important role in the urban regions of south and central Texas.