Interactions of organosulfates with water vapor under sub- and supersaturated conditions

Organosulfates (OSs) are important constituents of secondary organic aerosols, but their hygroscopic properties and cloud condensation nucleation (CCN) activities have not been well understood. In this work we employed three complementary techniques to characterize interactions of several OSs with water vapor under sub- and supersaturated conditions. A vapor sorption analyzer was used to measure mass changes in OS samples with relative humidity (RH, 0 %-90 %); among the 11 organosulfates examined, only sodium methyl sulfate (methyl-OS), sodium ethyl sulfate (ethyl-OS), sodium octyl sulfate (octyl-OS) and potassium hydroxyacetone sulfate were found to deliquesce as RH increased, and their mass growth factors at 90 % RH were determined to be 3.65 +/- 0.06, 3.58 +/- 0.02, 1.59 +/- 0.01 and 2.20 +/- 0.03. Hygroscopic growth of methyl-, ethyl- and octyl-OS aerosols was also studied using a humidity tandem differential mobility analyzer (H-TDMA); continuous hygroscopic growth was observed, and their growth factors at 90 % RH were determined to be 1.83 +/- 0.03, 1.79 +/- 0.02 and 1.21 +/- 0.02. We further investigated CCN activities of methyl-, ethyl- and octyl-OS aerosols, and their single hygroscopicity parameters (kappa(cnn)) were determined to be 0.459 +/- 0.021, 0.397 +/- 0.010 and 0.206 +/- 0.008. For methyl- and ethyl-OS aerosols, kappa(cnn) values agree reasonably well with those derived from H-TDMA measurements (kappa(gf)) with relative differences being < 25 %, whereas kappa(cnn) was found to be similar to 2.4 times larger than kappa(gf) for octyl-OS, likely due to both the solubility limit and surface tension reduction