Complex organic and inorganics enriched particles have been generally detected in the atmosphere. These particles could take up water and form ice crystals. Currently, the ice nucleation of mixed organic/inorganic particles is not well understood. Here, we investigated the freezing temperatures of aqueous droplets containing poly(ethylene glycol)-400 (PEG400), 3,3-dimethylglutaric acid (DMG) and ammonium sulfate (AS) at different organic-to-inorganic dry mass ratios (OIRs) as representative organic/inorganic model systems using optical microscopy and cryo-cooling stage. The results showed that the freezing temperatures were consistent with the water-activity-based homogeneous freezing temperatures without crystalline solids in aqueous droplets. It indicted that these droplets likely form ice through homogeneous nucleation irrespective of OIR. We found a substantial temperature dependence of water activity for aqueous PEG400-AS droplets. The freezing of ice occurred in the first cycle at lower temperature than in the second cycle in all aqueous DMG-AS droplets due to the crystallized solids acting as heterogeneous ice nuclei. Our work showed that the surface area of crystalline solids presented in aqueous DMG-AS droplets could significantly change the heterogeneous freezing temperature. The ice nucleation induced by organic/inorganic particles may be very important in the formation of upper tropospheric clouds.

Key words: ice nucleation, organic/inorganic particle, aqueous droplet, crystallized solid, surface area