Ice nucleation in sulfuric acid/organic aerosols: implications for cirrus cloud formation

[摘要] Using an aerosol flow tube apparatus, we have studied the effects ofaliphatic aldehydes (C₃ to C₁₀) and ketones (C₃ and C₉)on ice nucleation in sulfuric acid aerosols. Mixed aerosols were prepared bycombining an organic vapor flow with a flow of sulfuric acid aerosols over asmall mixing time (~60 s) at room temperature. No acid-catalyzedreactions were observed under these conditions, and physical uptake wasresponsible for the organic content of the sulfuric acid aerosols. In theseexperiments, aerosol organic content, determined by a Mie scatteringanalysis, was found to vary with the partial pressure of organic, the flowtube temperature, and the identity of the organic compound. The physicalproperties of the organic compounds (primarily the solubility and meltingpoint) were found to play a dominant role in determining the inferred modeof nucleation (homogenous or heterogeneous) and the specific freezingtemperatures observed. Overall, very soluble, low-melting organics, such asacetone and propanal, caused a decrease in aerosol ice nucleationtemperatures when compared with aqueous sulfuric acid aerosol. In contrast,sulfuric acid particles exposed to organic compounds of eight carbons andgreater, of much lower solubility and higher melting temperatures, nucleateice at temperatures above aqueous sulfuric acid aerosols. Organic compoundsof intermediate carbon chain length, C₄-C₇, (of intermediatesolubility and melting temperatures) nucleated ice at the same temperatureas aqueous sulfuric acid aerosols. Interpretations and implications of theseresults for cirrus cloud formation are discussed.

[学科分类] 大气科学