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The heterogeneous reaction of HOCl + HCl → Cl 2 + H 2 O on ice and nitric acid trihydrate: Reaction probabilities and stratospheric implications
Author(s) -
Abbatt J. P. D.,
Molina M. J.
Publication year - 1992
Publication title -
geophysical research letters
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.007
H-Index - 273
eISSN - 1944-8007
pISSN - 0094-8276
DOI - 10.1029/92gl00373
Subject(s) - chemistry , nitric acid , hypochlorous acid , stratosphere , water vapor , analytical chemistry (journal) , ozone , polar , inorganic chemistry , meteorology , organic chemistry , physics , astronomy
The heterogeneous reaction HOCl + HCl → Cl 2 (g) + H 2 O(s) has been observed to proceed readily on cold water‐ice and nitric acid trihydrate (NAT) surfaces. For reactant partial pressures in the 10 −7 –10 −5 torr range, the reaction probability (γ) on water‐ice is found to be 0.16 ± 0.10 at 202 K, and 0.24 +0.5/−0.15 at 195 K, as measured by the decay of HOCl vapor when the HCl partial pressure is above that of HOCl. On NAT, γ is found to have a strong dependence on the H 2 O vapor pressure of the NAT crystals: at 202 K, reaction probabilities of 0.17 are measured for H 2 O vapor pressures approaching those of ice (H 2 O‐rich NAT), whereas γ drops to 0.002 if the H 2 O pressure is lower by a factor of five (HNO 3 ‐rich NAT). The yield of Cl 2 (g) relative to HOCl(g) loss is measured to be 0.87 ± 0.20. These results suggest that the heterogeneous process ClONO 2 + HCl → Cl 2 (g) + HNO 3 (s), which occurs on polar stratospheric cloud surfaces, may be comprised of two steps: ClONO 2 + H 2 O(s) → HOCl + HNO 3 (s), followed by HOCl + HCl → Cl 2 (g) + H 2 O(s). Also, the HOCl + HCl reaction may be directly of importance in the polar stratosphere providing an additional path for chlorine activation.