The deposition ice nucleation and immersion freezing potential of amorphous secondary organic aerosol: Pathways for ice and mixed-phase cloud formation

Secondary organic aerosol (SOA) generated from the oxidation of organic gases are ubiquitous in the atmosphere, but their interaction with water vapor and their ice cloud formation potential at low temperatures remains highly uncertain. We report on onset conditions of water uptake and ice nucleation by amorphous SOA particles generated from the oxidation of naphthalene with OH radicals. Water uptake above 230K was governed by the oxidation level of the SOA particles expressed as oxygen-to-carbon (O/C) ratio, followed by moisture-induced phase transitions and immersion freezing. For temperatures from 200 to 230K, SOA particles nucleated ice via deposition mode from supersaturated water vapor independent of O/C ratio at relative humidity with respect to ice (RH_ice) ∼10-15% below homogeneous ice nucleation limits. The glass transition temperature (T_g) for the amorphous SOA particles was derived as a function of two parameters: (1) relative humidity (RH) with respect to water and (2) oxidation level of the SOA. The data show that particle phase and viscosity govern the particles' response to temperature and RH and provide a straightforward interpretation for the observed different heterogeneous ice nucleation pathways and water uptake by the laboratory-generated SOA and field-collected particles. Since SOA particles undergo glass transitions, these observations suggest that atmospheric SOA are potentially important for ice cloud formation and climate.