Immersion ice nucleation of atmospherically relevant lipid particles†

Abstract

Ice nucleating particles (INPs) play a crucial role in freezing water droplets by acting as heterogeneous ice nuclei, influencing cloud phase state and climate dynamics. INPs from marine aerosol particles are particularly relevant. Saturated fatty alcohols and acids have been identified in sea spray aerosols (SSA). In this study, we employ a micro-Raman spectrometer integrated with an environmental cell to control relative humidity and temperature and measure the ice nucleation activity of individual lipid particles, including fatty alcohols and fatty acids of varying chain lengths. For fatty acids, we observe little IN activity for these lipid particles as they freeze close to the temperature found for homogeneous freezing. For fatty alcohols, we demonstrate that freezing temperatures depend on the carbon chain length, with longer chains leading to warmer ice nucleating temperatures. Although this result qualitatively agrees with existing literature, we observe that the ice nucleating temperatures of these lipid particles differ from the freezing temperatures measured for fatty alcohol monolayers at the air/water interface for large water droplets. To better understand these differences, we further investigate the effects of droplet size as well as phase state by theoretically determining the wet viscosity on freezing. Our results, taken together, suggest that for fatty alcohol particles, freezing occurs at the lipid particle/water interface. Overall, our findings highlight the influence of lipid chain length, droplet size, and phase state on ice nucleation for lipid particles.