HETEROGENEOUS PHOTOCHEMICAL SOURCES OF ATOMIC CL IN THE TROPOSPHERE

The reaction of N2O5 with wet NaCl aerosol is observed to produce ClNO2 at high yields. The sticking coefficient of N2O5 on NaCl aerosol is determined in a smog chamber (in an inserted teflon bag) at 292 K, and the rate constant for the consecutive chemical reaction leading to formation of ClNO2 is determined at various relative humidities between 71 and 92%. A preliminary evaluation of the experiments yields a sticking coefficient of 0.03(+O.02/0.01), similar to that reported for pure water droplets. At the deliquescence point of NaCl, approximately 50% of the collisions lead to the formation of ClNO2. This portion decreases slightly with increasing humidity (i.e. liquid water content of the NaCl aerosol). The process is expected to be an efficient sink of N2O5 in the troposphere. Model calculations indicate a significant contribution of atomic Cl to global marine tropospheric chemistry at higher latitudes of the northern hemisphere. Phosgene, chloroacetone, and 1,1-dichloroacetone are observed as products from a heterogeneous chlorination in the aerosol smog chamber.