Anthropogenic sources of chlorine and ozone formation in urban atmospheres

In this paper, we present ambient monitoring data from Houston, TX along wi th results from environmental chamber stud ies to suggest that molecular ch lorine (Cl-2), a photolytic source of chlorine atoms (Cl-.), may contribute significantly to ozone (O-3) formation in some urban environments. The amb ient data were collected during an ozone episode in August 1993 that involv ed an alkane-rich hydrocarbon plume passing over anthropogenic sources of C l-2. Two unusual observations were made about the plume a few hours after i t had passed over the Cl-2 sources: (1) a rapid loss of alkanes and (2) a l arge increase in ozone concentration. Neither of these observations could b e explained with models employing hydroxyl radical (OH.) chemistry (OH. are generally accepted to control oxidative chemistry in the daytime troposphe re). Environmental chamber experiments were performed to determine whether the addition of Cl-2 to a mixture of air, hydrocarbons, and nitrogen oxides (NOx) representative of conditions in the Houston area would yield similar results. The results of these chamber experiments indicated that Cl-2 enha nces O-3 production when alkanes dominate the hydrocarbon mixture, with a p ossible enhancement in ozone production of between 5 and 10 mol of ozone pr oduced per mol of Cl-2.