Aerosol hygroscopicity at high (99 to 100%) relative humidities
The hygroscopicity of an aerosol strongly influ- ences its effects on climate and, for smaller particles, atmo- spheric lifetime. While many aerosol hygroscopicity mea- surements have been made at lower relative humidities (RH) and under cloud formation conditions (RH>100%), rela- tively few have been made at high RH (99 to 100%), where the Kelvin (curvature) effect is comparable to the Raoult (so- lute) effect. We measured the size of droplets at high RH that had formed on particles composed of one of seven com- pounds with dry diameters between 0.1 and 0.5 µm. We re- port the hygroscopicity of these compounds using a parame- terization of theKelvin term, in addition to a standard param- eterization (κ) of the Raoult term. For inorganic compounds, hygroscopicity could reliably be predicted using water ac- tivity data (measured in macroscopic solutions) and assum- ing a surface tension of pure water. In contrast, most organ- ics exhibited a slight to mild increase in hygroscopicity with droplet diameter. This trend was strongest for sodium do- decyl sulfate (SDS), the most surface-active compound stud- ied. 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 reduc- tion. Furthermore, we found no evidence that micellization limits SDS activity in micron-sized solution droplets, as ob- served in macroscopic solutions. We conclude that while the high-RH hygroscopicity of inorganic compounds can be reli- ably predicted using readily available data, surface-activity parameters obtained from macroscopic solutions with or- ganic solutes may be inappropriate for calculations involving micron-sized droplets.