Is ice formation by sea spray particles at cirrus temperatures controlled by crystalline salts?

The ice nucleating ability of sea spray aerosols (SSA) has been explored in recent years due to the abundance of SSA in the atmosphere. The role of SSA in ice nucleation extends to cirrus clouds, due to processes that loft SSA to the upper troposphere. This is of special relevance because of the frequent occurrence of cirrus in the atmosphere, their role in the Earth’s radiative balance, and uncertainties regarding how aerosols may affect their formation and evolution. In this study, a continuous flow diffusion chamber (CFDC) is used to investigate the ice nucleating ability of size-selected particle distributions of SSA and its primary constituent sodium chloride (NaCl) at temperatures < 235 K. Results show that above ~220 K, the majority of SSA and NaCl particles fully deliquesce and freeze via homogeneous nucleation at or below water relative humidities, RHw, of ~ 95%. However, below 220 K, the onset RHw of freezing for NaCl and SSA is much lower, at ~75%, where strong heterogeneous freezing of 10% of the aerosol population occurs. Similar heterogeneous freezing behavior for NaCl and SSA aerosols, occurring near their predicted deliquescence RHw, points towards SSA freezing at the lowest temperatures being controlled by the crystalline salts. Finally, calculations of ice nucleation active surface site densities show that particle size does not dictate the efficiency of freezing for NaCl and SSA. These results indicate SSA as a potentially significant source of ice nucleating particles at cirrus temperatures, with the ability to contribute to cirrus-mediated climate impacts if sea spray emission and transport scenarios change in the future.