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What controls the ice nucleating ability of sea spray aerosols at cirrus temperatures?
  • Ryan Patnaude
Ryan Patnaude
Colorado State University

Corresponding Author:patnaude.ryan@gmail.com

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Cirrus clouds are frequently observed in the upper troposphere and have a strong impact on Earth’s radiative balance. However, significant uncertainties remain regarding the role aerosols play in cirrus cloud formation and evolution. Due to their abundance in the atmosphere sea spray aerosols (SSA) have recently been identified as potential ice nucleating particles in cirrus clouds. Using a continuous flow diffusion chamber (CFDC), the ice nucleating ability of size-selected SSA and sodium chloride (NaCl) particles is investigated at temperatures < 235 K. Above ~220 K, the majority of SSA and NaCl particles fully deliquesce and freeze via homogeneous nucleation near water relative humidities RHw of ~95%. Below 220 K, the onset freezing RHw is much lower at ~75% for SSA and NaCl, and strong heterogeneous freezing of 10% of the aerosol population is observed. The similar heterogeneous freezing behavior between SSA and NaCl, which occurs near their expected deliquescence RHw indicates the freezing of SSA at the lowest temperatures is controlled by the inorganic salt components. Lastly, ice nucleation active surface site densities are calculated and show that particle size does not dictate the heterogeneous freezing efficiency for SSA and NaCl. This study demonstrates SSA as a potentially significant source of ice nucleating particles in cirrus clouds, and may contribute further climate impacts if SSA emissions and transport change in the future.