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The hygroscopicity of an aerosol strongly influences its effects on climate and, for smaller particles, atmospheric lifetime. While many aerosol hygroscopicity measurements have been made at lower relative humidities (RH) and under cloud formation conditions (RH>100%), relatively few have been made at high RH (99 to 100%), where the Kelvin (curvature) effect is comparable to the Raoult (solute) effect. We measured the size of droplets at high RH that had formed on particles composed of one of seven compounds with dry diameters between 0.1 and 0.5 μm. We report the hygroscopicity of these compounds using a parameterization of the Kelvin term, in addition to a standard parameterization (κ) of the Raoult term. For inorganic compounds, hygroscopicity could reliably be predicted using water activity data (measured in macroscopic solutions) and assuming a surface tension of pure water. In contrast, most organics exhibited a slight to mild increase in hygroscopicity with droplet diameter. This trend was strongest for sodium dodecyl sulfate (SDS), the most surface-active compound studied. The results suggest that, for single-component aerosols at high RH, partitioning of solute to the particle-air interface reduces particle hygroscopicity by reducing the bulk solute concentration. This partitioning effect is more important than the increase in hygroscopicity due to surface tension reduction. Furthermore, we found no evidence that micellization limits SDS activity in micron-sized solution droplets, as observed in macroscopic solutions. We conclude that while the high-RH hygroscopicity of inorganic compounds can be reliably predicted using readily available data, surface-activity parameters obtained from macroscopic solutions with organic solutes may be inappropriate for calculations involving micron-sized droplets.

The influence of aerosols on cloud properties remains one of the largest sources of uncertainty in estimates of the anthropogenic component of climate change. Here we report the rate of cloud droplet formation on particles sampled at a site near the California coast that is typically above the marine boundary layer. We observed persistent bimodal diameter spectra which are better explained by kinetic limitations than by differences in equilibrium properties. The slowly-growing mode contained 10-25% of the total cloud condensation nuclei (CCN) and had apparent mass accommodation coefficients (a) 10-30 times smaller than mat measured for ammonium sulfate. Cloud parcel modeling suggests that most of these slowly-growing CCN will not form cloud droplets. The relatively small and narrow size distribution of the low-a droplets suggest that a condensed film is a more likely cause of these limitations than slow dissolution. Copyright 2009 by the American Geophysical Union.

The ability of airborne particles to take up water may be enhanced by surface-active components, but the importance of this effect is controversial because direct measurement of the surface tension of microscopic droplets has not been possible. Here we infer droplet surface tension from water uptake measurements of mixed organic-inorganic particles at relative humidities just below saturation (99.3-99.9%). The surface tension of droplets formed on particles composed of NaCl and-pinene ozonolysis products was reduced by 50-75%, but only when enough organic material was present to form a film on the droplet surface at least 0.8nm thick. This study suggests that if atmospheric particles are predominantly (≳80%) composed of surface-active material, their influence on cloud properties and thus climate could be enhanced, and their atmospheric lifetimes could be reduced. © 2012. American Geophysical Union. All Rights Reserved.