Laboratory studies of sticking coefficients and heterogeneous reactions important in the Antarctic stratosphere

Abstract

Laboratory studies of sticking coefficients of H2O, HCl, Cl2 and HNO3 on ice and heterogeneous reactions of ClONO2 with ice or HCl/ice have been performed in a fast flow reactor. A quadrupole mass spectrometer with electron impact ionization was used as a detector. Measured sticking coefficients are: 0.3 (+0.7. −0.1) for H2O, 0.4 (+0.6, −0.2) for HCl, <1.0 × 10−4 for Cl2, and 0.3 (+0.7, −0.1) for HNO3 at 200 K. The reaction probability of ClONO2 on ice was found to be 0.06 (±0.03) while HOCl was observed as a sole product in the gas phase. With HCl present in ice, the reaction probability of ClONO2 is greatly enhanced, approaching 0.27 (+0.73, −0.13) while molecular chlorine was found to be the major product in the gas phase. Another reaction product was nitric acid which remained in the solid phase. Since the polar stratospheric clouds contain ice particles or possibly HCl/ice particles on the surface, the present results should be a major factor in producing the observed springtime ozone depletion in the Antarctic stratosphere.

Evaluated kinetic and photochemical data for atmospheric chemistry: Volume V – heterogeneous reactions on solid substrates

“…Despite the use of larger HCl mole fractions, the uptake coefficients quoted by are significantly lower than the values of Leu et al Both Leu et al (1988) and detected ClNO 2 as product, but did not quantify the yield. found that small amounts of HCl increased the N 2 O 5 uptake coefficient slightly, the results in qualitative agreement with those of Leu (1988). At high concentrations of HCl (which induced surface melting) the uptake coefficient increased as did the yield of ClNO 2 .…”

“…There is substantial disagreement between the Kenner et al (1993) and studies on one hand, and the Leu (1988) and studies on the other hand. One of the reasons could simply lie in the ice preparation method leading to highly porous films in case of vapour deposition from the gas phase.…”

“…Leu (1988) combined the two experimental values of γ on HCl-ice with his own data on a pure ice surface to show a distinct trend in γ with HCl mole fraction. As the surface concentration of HCl is unknown in these experiments, the dependence of γ on [HCl] (gas phase) cannot be extracted from the dataset.…”

“…However, used a film frozen from solution and found low reactivity consistent with . The Leu (1988) with HBr at molecular densities of (5.6-560)×10 10 molecule cm −3 . The temperature dependence confirmed the strong negative effect observed by .…”

Influence of the ice growth rate on the incorporation of gaseous HCl

“…We note that this value is very different from what would be expected if surface processes only were operative, and mass accommodation coefficients α could be used to predict X kin . The parameter α HCl has been measured in the laboratory around −80 • C, and values between 0.1 and 1 have been found (Leu et al, 1988;Hanson and Ravishankara, 1992). Similarly, α H 2 O is in the range 0.1 to 1 (Haynes et al, 1992 and references therein), so that α HCl /α H 2 O is expected to be in the range 0.1 to 10.…”