Formation of chemically active chlorine compounds by reactions of atmospheric NaCl particles with gaseous N₂O₅ and ClONO₂

Abstract

Participate NaCl and NaBr are known^1,2 to react with NO₂ in air at concentrations of a few parts per 10⁶ (p.p.m.) to form the corresponding nitrosyl halides XNO (X = Cl, Br), a process that may contribute to the photochemistry of polluted marine areas and remote regions such as the Arctic^3,4. Here we report experimental evidence that gaseous chlorine nitrate (ClONO₂) and dinitrogen pentoxide (N₂O₅) at p.p.m. concentrations also react with NaCl particles at 298 K, to form Cl₂ and ClNO₂ respectively. Thus, the chloride ion in a neutral salt can be transformed into gaseous, photochemically active forms of chlorine at 298 K in a manner analogous to that reported recently for the reactions of HCl in ice with ClONO₂ and N₂O₅ at 185–200 K (refs 5–8). This work suggests that reactions of N₂O₅ with NaCl in tropospheric marine areas, and possibly in cold climates where chloride salts are used on roads in the winter, should be considered in assessing the atmospheric chemistry of these regions. Furthermore, if the reactions of ClONO₂ and N₂O₅ with NaCl occur also at stratospheric temperatures and if NaCl is present in significant concentrations, these reactions may provide another mechanism for recycling ionic halide ions into gaseous species and hence should be considered in models of stratospheric ozone depletion.