Atmospheric Chemistry and Physics Aerosol hygroscopicity at high (99 to 100%) relative humidities

Abstract. 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 the Kelvin 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