Chlorine Chemistry on Polar Stratospheric Cloud Particles in the Arctic Winter

Abstract

Simultaneous in situ measurements of hydrochloric acid (HCl) and chlorine monoxide (ClO) in the Arctic winter vortex showed large HCl losses, of up to 1 part per billion by volume (ppbv), which were correlated with high ClO levels of up to 1.4 ppbv. Air parcel trajectory analysis identified that this conversion of inorganic chlorine occurred at air temperatures of less than 196 ± 4 kelvin. High ClO was always accompanied by loss of HCI mixing ratios equal to ½(ClO + 2Cl₂O₂). These data indicate that the heterogeneous reaction HCl + ClONO₂ → Cl₂ + HNO₃ on particles of polar stratospheric clouds establishes the chlorine partitioning, which, contrary to earlier notions, begins with an excess of ClONO₂, not HCl.