Is Ice Formation by Sea Spray Particles at Cirrus Temperatures Controlled by Crystalline Salts?

The ice nucleating ability of sea spray aerosol (SSA) particles has been explored in recent years due to the abundance of SSAs in the atmosphere. The role of SSAs in ice nucleation extends to cirrus clouds, due to processes that loft SSAs 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 SSAs and its primary constituent sodium chloride (NaCl) at temperatures <235 K. Results show that, above ∼220 K, the majority of NaCl and SSA particles fully deliquesce and freeze via homogeneous nucleation at or below water relative humidities, RH_w, of ∼95%. However, below 220 K, the onset RH_w of freezing for NaCl and SSA particles is much lower, at ∼75%, where strong heterogeneous freezing of 10% of the aerosol population occurs. Similar heterogeneous freezing behavior for NaCl and SSA particles, occurring near their predicted deliquescence RH_w, points toward SSA freezing at the lowest temperatures being controlled by the crystalline salts. Finally, the calculations of ice nucleation active surface site densities show that particle size does not dictate the efficiency of freezing for NaCl and SSA particles. These results indicate SSAs 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.