Low Temperature Ice Nucleation of Sea Spray and Secondary Marine Aerosols under Cirrus Cloud Conditions

Sea spray aerosols (SSA) represent one of the most abundant aerosol types on a global scale and have been observed at all altitudes including the upper troposphere. SSA has been explored in recent years as a source of ice nucleating particles (INPs) in cirrus clouds due to the ubiquity of cirrus clouds and the uncertainties in their radiative forcing. This study expands upon previous works on low temperature ice nucleation of SSA by investigating the effects of atmospheric aging of SSA and the ice nucleating activity of newly formed secondary marine aerosols (SMA) using an oxidation flow reactor. Polydisperse aerosol distributions were generated from a Marine Aerosol Reference Tank (MART) filled with 120 L of real or artificial seawater and their subsequent freezing was measured using a Continuous Flow Diffusion Chamber (CFDC). Results show that for both primary SSA (pSSA), and the aged SSA and SMA (aSSA+SMA) at temperatures > 220 K, 1 % of the particles freeze via homogeneous nucleation. However, below 220 K, heterogeneous nucleation occurs for both pSSA and aSSA+SMA at much lower relative humidities (RHs), where up to 1 % of the aerosol population freezes between 75–80 % RH. Similarities between freezing behaviors of the pSSA and aSSA+SMA at all temperatures suggest atmospheric aging has little effect on the heterogeneous freezing behavior of SSA at these cirrus temperatures and remains dominated by the crystalline salts. Occurrence of 1 % frozen fraction of SMA, generated in the absence of primary SSA, was observed at/near water saturation below 220 K, suggesting it is not an effective INP at cirrus temperatures, similar to findings in the literature of other organic aerosols. Thus, any SMA coatings on the pSSA are also unlikely to modify the ice nucleation behavior of pSSA. These results demonstrate the ability of lofted primary sea spray particles to remain an effective ice nucleator at cirrus temperatures, even after atmospheric aging.