Research Articles

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

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Science  27 Nov 1987:
Vol. 238, Issue 4831, pp. 1253-1257
DOI: 10.1126/science.238.4831.1253


The reaction rate between atmospheric hydrogen chloride (HCl) and chlorine nitrate (ClONO2) is greatly enhanced in the presence of ice particles; HCl dissolves readily into ice, and the collisional reaction probability for ClONO2 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 (Cl2) is released into the gas phase on a time scale of at most a few milliseconds, whereas nitric acid (HNO3), 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 HNO3 and thus removes nitrogen dioxide (NO2) 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, ClONO2 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.

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