Atmospheric chlorine chemistry in southeast Texas: Impacts on ozone and particulate matter formation and control

Recent evidence has demonstrated that chlorine radical chemistry can enhance tropospheric hydrocarbon oxidation and has the potential to enhance ozone formation in urban atmospheres. In order to assess these effects quantitatively, an August--September, 2000 photochemical episode in southeast Texas was simulated using the Comprehensive Air Quality Model, with extensions (CAMx). During this episode, ambient measurements of a unique marker of atmospheric chlorine chemistry, 1-chloro-3-methyl-3-butene-2-one (CMBO) were made and model performance was assessed by comparing modeled and observed CMBO mixing ratios. The model predicted ambient CMBO mixing ratios within the uncertainty limits of the emissions inventory, so the model was used to assess the impacts of chlorine chemistry on ozone formation. Based on the current emissions inventory, chlorine chemistry has the potential to enhance 8-hour averaged ozone mixing ratios by more than 20 ppb, and 1-hour averaged mixing ratios by more than 70 ppb. These enhancements occur largely in morning hours, and the impacts of chlorine chemistry on daily peak ozone concentrations are typically under 10 ppb. Chlorine emissions also influenced changes in ozone concentrations due to hydrocarbon and NOx emission controls, and contributed formation of particulate matter through the production of HCl.