Another Way To Form Atmospheric Chlorine

National Oceanic & Atmospheric Administration scientists report a new mechanism for the formation of molecular chlorine in the atmosphere: N₂O₅ can oxidize Cl^– to Cl₂ on acidic aerosols, the researchers say (Science, DOI: 10.1126/science.1158777). Molecular chlorine is a highly reactive species that affects atmospheric aerosol formation, ozone abundance, and the fate of trace gases such as mercury. Various mechanisms have been proposed to describe the conversion of Cl^– to Cl₂ in the atmosphere over the oceans. Researchers know that N₂O₅, which forms at night, interacts with sea salt to form nitryl chloride (ClNO₂). They also know that the hydrated surfaces of some aerosols behave more like concentrated solutions than solids. Using mass spectrometry in the lab, James M. Roberts and colleagues tested the reactivity of N₂O₅ with oxalic acid and ammonium sulfate aerosols that contained various halide ions. They also observed that at low atmospheric pH and low Cl^??? concentrations, both N₂O₅ and ClNO₂ efficiently produced Cl₂ in an oxalic acid/sodium chloride solution. The researchers note that understanding the implications of this chemistry in the troposphere and stratosphere will require further study.