HETEROGENEOUS REACTIONS OF CLONO2, HCL, AND HOCL ON LIQUID SULFURIC-ACID SURFACES

The heterogeneous reactions of ClONO2 + H2O --> HNO3 + HOCl (1), ClONO2 +/- HCl --> Cl-2 + HNO3 (2), and HOCl +/- HCl --> Cl-2 + H2O (3) on liquid sulfuric acid surfaces have been studied using a fast flow reactor coupled to a quadrupole mass spectrometer. The main objectives of the study are to investigate (a) the temperature dependence of these reactions at a fixed H(2)0 partial pressure typical of the lower stratosphere (that is, by changing temperature at a constant water partial pressure, the H2SO4 content of the surfaces is also changed), (b) the relative importance or competition between reactions 1 and 2, and (c) the effect of HNO3 on the reaction probabilities due to the formation of a H2SO4/HNO3/H2O ternary system. The measurements show that all the reactions depend markedly on temperature at a fixed H2(O) partial pressure: they proceed efficiently at temperatures near 200 K and much slower at temperatures near 220 K. The reaction probability (gamma(1)) for ClONO2 hydrolysis approaches 0.01 at temperatures below 200 K, whereas the values for gamma(2) and gamma 3 are on the order of a few tenths at 200 K. Although detailed mechanisms for these reactions are still unknown, the present data indicate that the competition between ClONO2 hydrolysis and ClONO2 reaction with HCl may depend on temperature (or H2SO4 wt %): in the presence of gaseous HCl at stratospheric concentrations, reaction 2 is dominant at lower temperatures (<200 K), but reaction 1 becomes important at temperatures above 210 K. Furthermore, reaction probability measurements performed on the H2SO4/HNO3/ H2O ternary solutions do not exhibit noticeable deviation from those performed on the H2SO4/H2O binary system, suggesting little effect of HNO3 in sulfate aerosols on the ClONO2 and HOCl reactions with HCl. The results reveal that significant reductions in the chlorine-containing reservoir species (such as ClONO2 and HCl) can take place on stratospheric sulfate aerosols at high latitudes in winter and early spring, even at temperatures too warm for polar stratospheric clouds (PSCs) to form or in regions where nucleation of PSCs is sparse. This is particularly true under elevated sulfuric acid loading, such as that after the eruption of Mt. Pinatubo. Comparisons between our results and those presently available have also been made.