Cloud droplet activation of saccharides and levoglucosan particles

This study is focused on the ability of water-soluble organic compounds from wood combustion to act as cloud condensation nuclei. In particular, we have studied glucose, fructose, and mannose (monosaccharides), lactose, maltose, and sucrose (disaccharides) and levoglucosan. Critical supersaturations for dry particle sizes in the range 40–150 nm were measured using a static thermal diffusion cloud condensation nucleus counter. For glucose and sucrose, critical supersaturations were calculated by applying Köhler theory in three different ways: using water activities from literature, using water activity calculated assuming a van’t Hoff factor of 1, and using osmolality-derived water activity values. Critical supersaturations for the other compounds were calculated using water activity calculated assuming a van’t Hoff factor of 1 and with osmolality-derived water activities (except for mannose and Levoglucosan). Calculated critical supersaturations agreed well with experimental data in all cases. For particles of the same size, the disaccharides (lactose, maltose and sucrose) were found to activate at a significantly higher critical supersaturations than the monosaccharides and levoglucosan. This is consistent with Köhler theory and is due to the higher molar mass of the disaccharides.