Atmospheric Ageing of Inorganic Sea Spray Aerosol: Implications for Hygroscopicity and Cloud Activation Potential

Sea spray aerosol (SSA) contributes significantly to natural aerosol particle concentrations globally, in marine areas even dominantly. The potential changes of the omnipresent inorganic fraction of SSA due to atmospheric ageing is largely unexplored. We demonstrate that ageing of liquid NaCl and artificial sea salt aerosol by exposure to ozone and UV light leads to a substantial decrease in hygroscopicity and cloud activation potential. The results point towards surface reactions that are more crucial for small particles and the formation of salt structures with water bound within the aerosols, termed hydrates. Our findings suggest an increased formation of hydrate forming salts during ageing and the presence of hydrates in dried SSA. Field observations indicate a reduced hygroscopic growth of sub-micrometre SSA in the marine atmosphere compared to pure NaCl which is typically attributed to organic matter or sulphates. Aged inorganic sea salt offers an additional explanation for reduced hygroscopicity and cloud activation potential.