Unexpectedly high concentrations of molecular chlorine in coastal air

Abstract

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 are dominated by photochemically produced OH radicals; at night and in polluted environments, NO₃ radicals are an important oxidant¹. Ozone can contribute to the oxidation of atmospheric species during both day and night¹. In recent years, laboratory investigations^2,3,4, modelling studies^5,6,7, measured Cl deficits in marine aerosols⁸ and species-nonspecific observations^9,10,11 of gaseous inorganic chlorine compounds other than HCl have suggested that reactive halogen species may contribute significantly to—or even locally dominate—the oxidative capacity of the lower marine troposphere. Here we report night-time observations of molecular chlorine concentrations at a North American coastal site during onshore wind flow conditions that cannot be explained using known chlorine chemistry. The measured Cl₂ mixing ratios range from <10 to 150 parts per 10¹² (p.p.t.), exceeding those predicted⁵ for marine air by more than an order of magnitude. Using the observed chlorine concentrations and a simple photochemical box model, we estimate that a hitherto unrecognized chlorine source must exist that produces up to 330 p.p.t. Cl₂ per day. The model also indicates that early-morning photolysis of molecular chlorine can yield sufficiently high concentrations of chlorine atoms to render the oxidation of common gaseous compounds by this species 100 times faster than the analogous oxidation reactions involving the OH radical, thus emphasizing the locally significant effect of chlorine atoms on the concentrations and lifetimes of atmospheric trace species in both the remote marine boundary layer and coastal urban areas.