Antarctic Stratospheric Chemistry of Chlorine Nitrate, Hydrogen Chloride, and Ice: Release of Active Chlorine

Abstract

The reaction rate between atmospheric hydrogen chloride (HCl) and chlorine nitrate (ClONO 2 ) is greatly enhanced in the presence of ice particles; HCl dissolves readily into ice, and the collisional reaction probability for ClONO 2 on the surface of ice with HCl in the mole fraction range from ∼0.003 to 0.010 is in the range from ∼0.05 to 0.1 for temperatures near 200 K. Chlorine (Cl 2 ) is released into the gas phase on a time scale of at most a few milliseconds, whereas nitric acid (HNO 3 ), the other product, remains in the condensed phase. This reaction could play an important role in explaining the observed depletion of ozone over Antarctica; it releases photolytically active chlorine from its most abundant reservoir species, and it promotes the formation of HNO 3 and thus removes nitrogen dioxide (NO 2 ) from the gas phase. Hence it establishes the necessary conditions for the efficient catalytic destruction of ozone by halogenated free radicals. In the absence of HCl, ClONO 2 also reacts irreversibly with ice with a collision efficiency of ∼0.02 at 200 K; the product hypochlorous acid (HOCI) is released to the gas phase on a time scale of minutes.