Size-dependent hygroscopicity of levoglucosan and D-glucose aerosol nanoparticles

Abstract

The interaction between water vapor and aerosol nanoparticles is important in atmospheric processes. Hygroscopicity of sub-10gnm organic nanoparticles and their concentration-dependent thermodynamic properties (e.g., water activity) in the highly supersaturated concentration range are, however, scarcely available. Here we investigate the size dependence of hygroscopicity of organics (i.e., levoglucosan, D-glucose) in dry particle diameter down to 6gnm using a nano-hygroscopicity tandem differential mobility analyzer (nano-HTDMA). Our results show that there is only weak size-dependent hygroscopic growth of both levoglucosan and D-glucose nanoparticles with diameters down to 20gnm. In the diameter range smaller than 20gnm (down to 6gnm), we observed strong size-dependent hygroscopic growth for D-glucose nanoparticles. The hygroscopic growth factors cannot be determined for levoglucosan below 20gnm due to its evaporation. In addition, we compare hygroscopicity measurements for levoglucosan and D-glucose nanoparticles with E-AIM (standard UNIFAC-functional group activity coefficients), the ideal solution theory, and differential Köhler analysis (DKA) predictions. The ideal solution theory describes the measured hygroscopic growth factors of levoglucosan with diameters down to 20gnm and D-glucose nanoparticles with diameters larger than 60gnm, while E-AIM (standard UNIFAC) can successfully predict the growth factors of D-glucose nanoparticles with diameters from 100 down to 6gnm at RH above 88g%-40g% (e.g., at RH above 88g% for 100gnm D-glucose, at RH above 40g% for 6gnm D-glucose). The use of the DKA method leads to good agreement with measured hygroscopic growth factors of D-glucose aerosol nanoparticles with diameters from 100 down to 6gnm. Predicted water activity for these aqueous organic solutions (i.e., levoglucosan, D-glucose) from different parameterization methods agrees well with observations in the low solute concentration range ( g20gmolgkg-1).