Ice Nucleation by Marine Aerosols Over the North Atlantic Ocean in Late Spring

Here we report the ice nucleating temperatures of marine aerosols sampled in the subarctic Atlantic Ocean during a phytoplankton bloom. Ice nucleation measurements were conducted on primary aerosol samples and phytoplankton isolated from seawater samples. Primary marine aerosol samples produced by a specialized aerosol generator (the Sea Sweep) catalyzed droplet freezing at temperatures between −33.4 °C and − 24.5 °C, with a mean freezing temperature of −28.5 °C, which was significantly warmer than the homogeneous freezing temperature of pure water in the atmosphere (−36 °C). Following a storm‐induced deep mixing event, ice nucleation activity was enhanced by two metrics: (1) the fraction of aerosols acting as ice nucleating particles (INPs) and (2) the nucleating temperatures, which were the warmest observed throughout the project. Seawater samples were collected from the ocean's surface and phytoplankton groups, including Synechococcus, picoeukaryotes, and nanoeukaryotes, were isolated into sodium chloride sheath fluid solution using a cell‐sorting flow cytometer. Marine aerosol containing Synechococcus, picoeukaryotes, and nanoeukaryotes serves as INP at temperatures significantly warmer than the homogeneous freezing temperature of pure water in the atmosphere. Samples containing whole organisms in 30 g L−1 NaCl had freezing temperatures between −33.8 and − 31.1 °C. Dilution of samples to representative atmospheric aerosol salt concentrations (as low as 3.75 g L−1 NaCl) raised freezing temperatures to as high as −22.1 °C. It follows that marine aerosols containing phytoplankton may have widespread influence on marine ice nucleation events by facilitating ice nucleation. Key Points Isolated small phytoplankton cells, including Synechococcus, picoeukaryotes, nanoeukaryotes catalyze atmospheric ice nucleation At seawater concentrations, salts in water droplets inhibit ice nucleation by biological components Following a storm‐driven mixing event, the mean ice‐nucleating temperature of primary aerosol rose from −28.0 ± 1.5 °C to −24.5 ± 0.9 °C