Hysteresis in water content of ultrafine glassy organic aerosols: Evidence from laboratory and modelling study

Water content of aerosol particles is important for atmospheric impacts, such as radiative effects and chemical reactivity. Traditionally, crystalline inorganic aerosol particles such as NaCl were known to experience hysteresis in water content, meaning that hygroscopic growth depends on exposure history to water vapor. On the contrary, past laboratory studies for organic aerosol reported absence of hysteresis, especially for ultrafine size range. Here, we show that water contents for ultrafine organic aerosol particles have hysteresis at sub-0 °C. Hygroscopic growth of monodisperse ultrafine particles (diameter = 40, 100, and 200 nm) of sucrose and glucose were investigated for the temperature range of -21 °C to +23 °C. Hygroscopic growth of these particles did not exhibit any hysteresis process at +23 °C, consistent with literature. However, hygroscopic growth of these particles was different for hydration and dehydration experiments at sub-0 °C, demonstrating the occurrence of hysteresis. The lowest relative humidity (RH), at which the two modes of experiments provided the same water content, was defined as merge RH. Merge RH was approximately the same as that for the glass transition point, demonstrating that water diffusion in a highly viscous matrix of organic aerosols is the key for the occurrence of hysteresis. Employment of a kinetic multilayer model provided quantitative prediction of merge RH as a function of temperature, particle size, and residence time. Considering the temperature and RH range of Earth’s atmosphere, we hypothesize that hysteresis in organic aerosol ubiquitously occur in the upper troposphere.