The rate of water vapor evaporation from ice substrates in the presence of HCl and HBr: implications for the lifetime of atmospheric ice particles
[摘要] Using amultidiagnostic approach the rate Rev [ molec cm-3 s-1]or flux Jev [ molec cm-2 s-1] ofevaporation of H2O and its corresponding rate constant forcondensation, kcond [s-1 ], on a 1 µm thick icefilm have been studied in the temperature range 190 to 240 K as well as inthe presence of small amounts of HCl and HBr that left the vapor pressureof H2O on ice unchanged. The resulting Arrhenius expressionsfor pure ice are Jev = 1.6 · 10 28 ± 1 ·exp(- 10.3 ± 1.2/ RT)[ molec cm-2 s-1], kcond = 1.7 · 10 - 2 ± 1 · exp(+ 1.6 ±1.5/ RT ) [s -1], in the presence of a HCl mole fraction in therange 3.2 · 10 - 5 - 6.4 · 10 - 3 : Jev= 6.4 · 10 26 ± 1 · exp(- 9.7 ± 1.2/ RT)[molec cm-2 s-1] , kcond = 2.8 · 10 -2 ± 1 · exp ( + 1.5 ± 1.6 /RT)[s -1], and a HBrmole fraction smaller than 6.4 · 10 - 3 : Jev = 7.4· 10 25 ± 1 · exp ( - 9.1 ± 1.2 /RT)[ molec cm-2s-1] , kcond = 7.1 · 10 - 5 ± 1 · exp(+ 2.6 ± 1.5/ RT) [s -1]. The small negative activation energyfor H2O condensation on ice points to a precursor mechanism.The corresponding enthalpy of sublimation is DHsubl= Eev - Econd = 11.9 ± 2.7 kcal mol-1 , DHsubl= 11.2 ± 2.8 kcal mol-1, and DHsubl= 11.7 ± 2.8 kcal mol-1 whose values are identical withinexperimental uncertainty to the accepted literature value of 12.3 kcal mol-1. Interferometric data at 633 nm and FTIR absorption spectra intransmission support the kinetic results. The data are consistent with asignificant lifetime enhancement for HCl- and HBr-contaminated iceparticles by a factor of 3–6 and 10–20, respectively, for submonolayercoverages of HX once the fraction of the ice not contaminated by HX hasevaporated.
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[效力级别] [学科分类] 大气科学
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