Radical Budget and Ozone Production in Houston, TX during SHARP 2009

The chemistry of atmospheric radicals, especially the hydroxyl radical (OH) and hydroperoxyl radical (HO2), together called HOx, is deeply involved in the formation of the secondary pollutants ozone and fine particles. Radical precursors such as nitrous acid (HONO) and formaldehyde (HCHO) significantly affects HOx budget in urban environments like Houston. These chemical processes connect surface emissions, both human and natural, to local and regional pollution, and climate change. Using the data collected during the Study of Houston Atmospheric Radical Precursors (SHARP) in Houston, TX in spring 2009, we test our understanding of photochemistry through the analysis of the radical budget and ozone production. A numerical box model was used to simulate the oxidation processes and observed OH and HO2 during this study. Using the model results, we calculate the radical budget and analyze the sensitivity of ozone production to nitrogen oxides (NOx) and volatile organic compounds (VOCs). The radical budget shows that the photolysis of HONO and the photolysis of HCHO were significant HOx sources in this urban environment. We also compare the observed OH reactivity and ozone production rate to the model calculations. In general, ozone production rate was VOC limited in the morning and NOx limited in the afternoon. This relationship results from the ratio of VOCs to NOx in Houston. Results from this study provide additional support for regulatory actions to reduce reactive VOCs in Houston in order to reduce ozone and other secondary pollutants.