UNEXPECTEDLY HIGH-CONCENTRATIONS OF MOLECULAR CHLORINE IN COASTAL AIR

The fate of many atmospheric trace species, including pollutants such as nitrogen oxides and some volatile organic compounds, is controlled by oxidation reactions, In the daytime troposphere, these reactions ar e dominated by photochemically produced OH radicals; at night and in p olluted environments, NO3 radicals are an important oxidant(1). Ozone can contribute to the oxidation of atmospheric species during both day and night(1), In recent years, laboratory investigations(2-4), modell ing studies(5-7), measured Cl deficits in marine aerosols(8) and speci es-nonspecific observations(9-11) of gaseous inorganic chlorine compou nds other than HCl have suggested that reactive halogen species may co ntribute significantly to-or even locally dominate-the oxidative capac ity of the lower marine troposphere. Here we report nighttime observat ions of molecular chlorine concentrations at a North American coastal site during onshore wind flow conditions that cannot be explained usin g known chlorine chemistry. The measured Cl-2 mixing ratios range from <10 to 150 parts per 10(12) (p.p.t.), exceeding those predicted(5) fo r marine air by more than an order of magnitude. Using the observed ch lorine concentrations and a simple photochemical box model, we estimat e that a hitherto unrecognized chlorine source must exist that produce s up to 330 p.p,t, Cl-2 per day. The model also indicates that early-m orning photolysis of molecular chlorine can yield sufficiently high co ncentrations of chlorine atoms to render the oxidation of common gaseo us compounds by this species 100 times faster than the analogous oxida tion reactions involving the OH radical, thus emphasizing the locally significant effect of chlorine atoms on the concentrations and lifetim es of atmospheric trace species in both the remote marine boundary lay er and coastal urban areas.