Tuesday, 16 January 2007: 11:15 AM
Study of high pollution events in the Houston - Galveston area using CMAQ simulations with an extended version of SAPRC99 chemical mechanism
212A (Henry B. Gonzalez Convention Center)
Current photochemical models applied for the simulation of the Houston – Galveston air pollution can not reproduce many experimental results from the TexAQS 2000 campaign, such as Transient High Ozone Events, formaldehyde “spikes”, and unusual radical concentrations. Previous modeling studies employed a condensed chemical mechanism, which can only account for the overall contribution of lumped species classes. The model performance was usually evaluated based on a few species, such as ozone, carbon monoxide and nitrogen oxide. In order to provide more reliable model predictions for ozone, it is important to have additional precursor species and their secondary pollutants accurately characterized in the model. The aim of this study is to improve chemical representation in the chemical mechanism employed in an air quality model (AQM) to obtain new insights into the chemical processes that affect ozone production in the Houston - Galveston area. The SAPRC99 chemical mechanism was extended to account for the explicit representation of 26 additional individual organic species. These explicit species include some highly reactive alkenes, among them propene, butenes, and pentene, as well as slow reactive alkanes that are related to petrochemical activities and were found in significant concentrations in the Houston – Galveston area. Because of the increased chemical resolution in the model it was possible to take advantage of the measurements of individual organic species taken during TexAQS 2000 and 2006 to evaluate the model performance and the accuracy of the employed emission inventories. It was found that emissions of many individual alkanes and alkenes are currently underestimated with both regular and imputed Texas Emission Inventory, which was employed in the current air quality modeling. Based on these findings new strategies to improve model performance are suggested. The study concentrated on the importance of nighttime reactions of alkenes with ozone in the Houston industrial area and their impact on rapid increase in ozone concentration during the daytime. It is suggested that nighttime emission releases of some reactive hydrocarbons, such as internal alkenes, may be responsible for increased radical concentration, production of carbonyl species that are direct PAN precursors, increased formaldehyde plumes, and finally high peaks of ozone during the daytime.