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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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Policy Relevant Background (PRB) ozone concentrations are defined by the United States (U.S.) Environmental Protection Agency (EPA) as those concentrations that would occur in the U.S. in the absence of anthropogenic emissions in continental North America (i.e., the U.S, Canada, and Mexico). Estimates of PRB ozone have had an important role historically in the EPA's human health and welfare risk analyses used in establishing National Ambient Air Quality Standards (NAAQS). The margin of safety for the protection of public health in the ozone rulemaking process has been established from human health risks calculated based on PRB ozone estimates. Sensitivity analyses conducted by the EPA have illustrated that changing estimates of PRB ozone concentrations have a progressively greater impact on estimates of mortality risk as more stringent standards are considered. As defined by the EPA, PRB ozone is a model construct, but it is informed by measurements at relatively remote monitoring sites (RRMS). This review examines the current understanding of PRB ozone, based on both model predictions and measurements at RRMS, and provides recommendations for improving the definition and determination of PRB ozone.