Anthropogenic sources of chlorine and ozone formation in urban atmospheres

In this paper, we present ambient monitoring data from Houston, TX along with results from environmental chamber stud ies to suggest that molecular chlorine (Cl-2), a photolytic source of chlorine atoms (Cl-.), may contribute significantly to ozone (O-3) formation in some urban environments. The ambient data were collected during an ozone episode in August 1993 that involved an alkane-rich hydrocarbon plume passing over anthropogenic sources of Cl-2. Two unusual observations were made about the plume a few hours after it had passed over the Cl-2 sources: (1) a rapid loss of alkanes and (2) a large increase in ozone concentration. Neither of these observations could be explained with models employing hydroxyl radical (OH.) chemistry (OH. are generally accepted to control oxidative chemistry in the daytime troposphere). 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 enhances O-3 production when alkanes dominate the hydrocarbon mixture, with a possible enhancement in ozone production of between 5 and 10 mol of ozone produced per mol of Cl-2.