The contribution of chlorine radicals to tropospheric ozone formation in southeastern Texas
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Ground-level ozone is produced by well-understood reactions between volatile organic compounds (VOCs) and hydroxyl radicals in the presence of nitrogen oxides (NOx). It has been suggested that chlorine radicals (Cl·) could also contribute to ground-level ozone formation. Previous experiments involving purified air supported this hypothesis. However, experimental verification of this phenomenon in ambient air had never been observed. In this work, experiments were performed to investigate the ability for Cl· to enhance ground-level ozone formation under the following conditions: a) Release of Cl· precursor into captive ambient air, b) release of Cl· simultaneously with other ozone precursors (i.e., VOCs and NOx) into captive ambient air, and c) release of Cl· into captive aged air masses. These conditions were chosen to simulate conditions that Cl· precursor would encounter near co-located sources of viii VOC and NOx. The addition of chlorine resulted in rapid enhancement of ozone formation most significantly (>75 ppb/hr) during morning hours when photochemical reactivity is otherwise low. To assess the regional impact of chlorine radicals on ozone formation in Houston, TX, the Carbon Bond IV chemical mechanism was modified to include thirteen reactions involving chlorine radicals. The reactions included photolysis of Cl· precursors, reactions between Cl· and VOCs, and reactions between ozone and Cl·. The VOC reactions include the reactions of Cl· with isoprene and 1,3- butadiene that yield unique reaction products, or marker species. This modified chemical mechanism was then employed within a numerical model that accounted for chemical as well as physical processes (regional photochemical model). The Comprehensive Air Quality Model with Extensions (CAMx) was thus used to predict the impact of chlorine radicals on regional ozone formation in and around Houston, TX. When the modified chemical mechanism (that included chlorine radical reactions) was employed, ozone levels were enhanced by up to 16 ppb in the Houston area, with the greatest enhancement predicted for morning hours after sunrise. Of significant interest is that methane may be activated by chlorine radicals to contribute significantly to the predicted ozone enhancement in the Houston area. Such behavior suggests that the impact of chlorine radicals would be proportional to the availability of Cl· precursor. In urban areas with anthropogenic sources of Cl· precursors, Cl· reactions may need to be considered to more accurately predict ozone formation.