Direct evidence for chlorine-enhanced urban ozone formation in Houston, Texas

Urban air pollution is characterized by high ozone levels, formed when volatile organic compounds (VOCs) are oxidized in the presence of nitrogen oxides (NOx). VOC and NOx 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.). C.l 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. 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 (similar to 16 ppb) predicted by regional photochemical models employing Cl. chemistry. These results suggest that reducing chlorine emissions should be considered in urban ozone management strategies. (C) 2003 Elsevier Science Ltd. All rights reserved.