Ice-Nucleating Efficiency of Aerosol Particles and Possible Sources at Two Coastal Marine Sites

Ice-nucleating particles (INPs) are a small subset of aerosol particles that can trigger heterogeneous ice nucleation in the atmosphere. These particles can influence climate and precipitation by modifying the formation of ice clouds and mixed-phase clouds. We report here the ice-nucleating efficiency of aerosol particles at two coastal marine sites. These efficiencies were determined from size-resolved measurements of INP concentrations in the immersion freezing mode, and from size distributions of aerosol number and surface area. At both sites, the ice-nucleating efficiency on a per number basis (expressed as the fraction of aerosol particles acting as INPs) was strongly dependent on the size. For particles with diameters of 0.2 μm, approximately 1 in 106 particles acted as an INP, while for particles with diameters of 8 μm, approximately 1 in 10 particles acted as an INP. The ice-nucleating efficiency on a per surface area basis (expressed as the surface active site density, ns) was also dependent on the size, with larger particles being more efficient. The measurements were compared with previously reported results and modeling results to determine or rule out possible sources of INPs at these two coastal marine sites.