Ice Nucleating Properties of Organosulfate SOA Proxies

The role of secondary organic aerosols (SOA) as Ice Nucleating Particles (INPs) is poorly characterized despite their prevalence in the atmosphere. Understanding the contribution of SOA on ice nucleation is crucial in reducing uncertainty in global radiative budgets, understanding precipitation, and improving global climate models. We present results from a series of systematic experiments designed to characterize the ice nucleating properties of organosulfate SOA proxies. Organosulfate aerosols were generated by atomizing three commercially available sulfate salts in methanol and size selecting using a differential mobility analyzer (DMA). Ice nucleation measurements were performed using the Spectrometer for Ice Nucleation (SPIN) under conditions relevant to cirrus cloud formation (−46 < T < −34 °C; 1.0 < S_ice < 1.6). Dry glass transition temperatures were estimated using the Gordon-Taylor equation and ranged from -83 to 74 °C. Additionally, we detail a pre-cooling technique designed to precisely control relative humidity and sustain low temperatures that are applicable to the glass transition temperatures of some atmospherically relevant SOA. This technique allows for simultaneous cooling and sampling of SOAs to analyze both aerosol phase state and ice nucleation. For glassy organosulfate proxies, we observed heterogenous ice nucleation within the deposition nucleation regime and freezing consistent with the homogenous freezing threshold otherwise. This result further corroborates that aerosol phase state is a crucial factor when considering the heterogeneous ice nucleation properties of SOA.