Urban air pollution is characterized by high ozone levels, formed when volatile organic compounds (VOCs) are oxidized in the presence of nitrogen oxides (NO x ). VOC and NO x emissions controls have traditionally been implemented to reduce urban ozone formation, however, a separate chemical species implicated in ozone formation in Houston, TX and possibly other urban areas is the chlorine radical (Cl Á ). Cl Á enhances tropospheric VOC oxidation, but is not included in models used to develop air quality attainment plans. We present results of a three-fold approach to elucidate the importance of Cl Á in urban ozone formation: (1) the first direct evidence of chlorine chemistry in the urban troposphere, (2) enhanced ozone formation (>75 parts per 10 9 (ppb/h) observed when small amounts of chlorine (Cl 2 ) are injected into captive ambient air, and (3) enhanced ozone formation (B16 ppb) predicted by regional photochemical models employing Cl Á chemistry. These results suggest that reducing chlorine emissions should be considered in urban ozone management strategies. r

Chlorine chemistry has been incorporated into the carbon bond IV mechanism and employed in a regional photochemical model (the Comprehensive Air Quality Model with Extensions (CAMx)) for preliminary use in assessing the regional impact of chlorine on ozone formation in Houston, Texas. Mechanisms employed in regional photochemical models do not currently account for chlorine chemistry. However, when chlorine chemistry is accounted for, predicted ozone levels are enhanced by up to 16 ppbv in the Houston area, with the greatest enhancement predicted for morning hours after sunrise. Thirteen reactions have been added to the chemical mechanism used by CAMx to describe chlorine chemistry in the urban atmosphere. The reactions include photolysis of chlorine radical (Cl·) precursors, Cl·+ hydrocarbon reactions, and Cl·+ ozone reactions. The hydrocarbon reactions include the reactions of Cl· with isoprene and 1,3‐butadiene that yield unique reaction products, or marker species. The development of this mechanism is presented along with a discussion of the initial set of predictions of chlorine‐based ozone enhancement in the Houston area. Of significant interest is that methane may be activated by chlorine to contribute significantly to the predicted ozone enhancement in the Houston area. Such behavior suggests that the impact of chlorine chemistry would be proportional to the availability of Cl· precursor. In urban areas with anthropogenic sources of chlorine radical precursors, chlorine radical chemistry may be important to more accurately predict ozone formation.

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Recent evidence has demonstrated that chlorine radical chemistry can enhance tropospheric hydrocarbon oxidation and has the potential to enhance ozone formation in urban atmospheres. 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-3butene-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 associated with 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 emissions have the potential to enhance 1-h-averaged ozone mixing ratios by 70 ppb, in very localized areas, during morning hours. Over wider areas, and at times of day when peak ozone concentrations are observed, the impacts of chlorine emissions on ozone concentrations are typically less than 10 ppb. Chlorine emissions also influenced changes in ozone concentrations due to hydrocarbon and NOx emission controls.

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