Ice-nucleating properties of glassy organic and organosulfate aerosol

The role of secondary organic aerosol (SOA) in atmospheric ice nucleation is not well understood, limiting accurate predictions of aerosol indirect effects in global climate simulations. This article details experiments performed to characterize the ice-nucleating properties of proxy SOA. Experimental techniques in conditioning aerosol to glass transition temperatures ( Tg) as low as -70°C using a pre-cooling unit are described. Ice nucleation measurements of proxy organosulfates (i.e., methyl, ethyl, and dodecyl sulfates) and citric acid were performed using the SPectrometer for ice nucleation (SPIN), operating at conditions relevant to upper-tropospheric cirrus temperatures ( -45 °C, -40 °C, -35°C) and ice saturation ratios ( 1.0<Sice<1.6). Methyl, ethyl, and dodecyl sulfates did not nucleate ice, despite dodecyl sulfate possessing a Tg higher than ambient temperature. Citric acid nucleated ice heterogeneously at -45 and -40°C ( 1.2<Sice<1.4) but required pre-cooling temperatures of -70°C, notably colder than the lowest published Tg. A kinetic flux model was used to numerically estimate water diffusion timescales to verify experimental observations and predict aerosol phase state. Diffusion modeling showed rapid liquefaction of glassy methyl and ethyl sulfates due to high hygroscopicity, preventing heterogeneous ice nucleation. The modeling results suggest that citric acid nucleated ice heterogeneously via deposition freezing or immersion freezing after surface liquefaction. We conclude that Tg alone is not sufficient for predicting heterogeneous ice formation for proxy SOA using the SPIN.