Size-dependent hygroscopicity of levoglucosan andD-glucose aerosol nanoparticles

The interaction between water vapor and aerosol nanoparticles is important in atmospheric processes. Hygroscopicity of sub-10 nm 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 6 nm 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 20 nm. In the diameter range smaller than 20 nm (down to 6 nm), we observed strong size-dependent hygroscopic growth for D-glucose nanoparticles. The hygroscopic growth factors cannot be determined for levoglucosan below 20 nm 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 Kohler analysis (DKA) predictions. The ideal solution theory describes the measured hygroscopic growth factors of levoglucosan with diameters down to 20 nm and D-glucose nanoparticles with diameters larger than 60 nm, while E-AIM (standard UNIFAC) can successfully predict the growth factors of D-glucose nanoparticles with diameters from 100 down to 6 nm at RH above 88 %-40% (e.g., at RH above 88% for 100 nm D-glucose, at RH above 40% for 6 nm 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 6 nm. 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 (< 20 mol kg(-1)) and starts to deviate from observations in the high solute concentration (> 20 mol kg(-1)).

Automated summary

This study investigates the hygroscopicity of different sized organic particles (levoglucosan and D-glucose) using a nano-HTDMA. The results show strong size-dependent hygroscopic growth for levoglucosan particles below 20 nm and above 6 nm, while it cannot be determined for D-glucose due to evaporation. The predicted water activity agrees well with observations at low solute concentrations but deviates at high solute concentrations.