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The HOCl heterogeneous reaction on frozen natural (NSS) and recrystallized (RSS) sea salt, KCl and NaCl solutions was studied using a low pressure flow reactor in order to measure the uptake coefficient γ and products of reaction. The HOCl sample used in these experiments always contained up to 25% Cl 2 O which was also studied separately as a pure gas in order to understand the heterogeneous chemistry of both gases. By performing HOCl uptake on frozen NSS solution at 200 K and a gas-phase residence time of (1.6±0.6) s we obtained a steady state uptake coefficient γ HOCl on NSS  = (2.5±0.7)×10 -3  and γ Cl2O on NSS  = (2.8±0.8)×10 -3 . On frozen KCl solution at 200 K we obtain γ HOCl on KCl  = (2.8±1.3)×10 -3 , identical to NSS, and γ Cl2O on KCl  = (4.6±0.8)×10 -4 . The main product formed during the uptake on frozen NSS solution is Cl 2  which is sustained for at least one hour. In contrast, only a transient Cl 2  flow (pulse) decreasing on the time scale of 100 s was observed on frozen KCl (NaCl) solution. 25±10 % of the HOCl taken up on all chloride-containing frozen substrates at 200 K react to produce Cl 2  at high HOCl concentration (4.5×10 11  molecule cm -3 ) and at a residence time of 1.6 s in comparison with twice that for Cl 2 O. For smaller concentrations such as [HOCl] = 3.7×10 10  molecule cm -3  and/or a shorter residence time (0.137±0.004s), HOCl uptake did not generate Cl 2  in contrast to Cl 2 O. A single Br 2  burst event was monitored when a Cl 2 O or HOCl/Cl 2 O mixture is taken up on fresh frozen NSS solution during the first uptake at 200 K. Further Cl 2 O or HOCl/Cl 2 O uptake on the same sample, even after annealing at 240 K does not show an additional Br 2  pulse. This Br 2  release may be significant in the autocatalytic ozone destruction mechanism in the troposphere during polar sunrise. Some of the atmospheric implications of the present results are highlighted with emphasis on the preequilibrium Cl 2 O(ads) + H 2 O(ice) ↔ 2 HOCl(ads) between adsorbed HOCl and Cl 2 O, with the latter being the gateway to reactive uptake of HOCl at low temperatures.

Heterogeneous Chemistry of Cl 2 O and HOCl on Frozen Natural Sea Salt, Recrystallized Sea Salt, KCl and NaCl Solutions at 200 and 215 K