Size-Resolved Measurements of Ice Nucleating Particles at Six Locations in North American and One in Europe

Ice nucleating particles (INPs) are a small fraction of the total aerosol population capable of catalyzing ice formation under atmospheric conditions. The formation, albedo, and lifetime of mixed-phase and cirrus clouds may be influenced by the presence of such particles. Compared to ambient measurements of the total number concentration of INPs, relatively little data exists on the size distribution of INPs in the atmosphere. This size information may be useful in source identification, modeling the transport of INPs in the atmosphere, and for determining if common instrumentation captures the majority of the atmospheric INP population. Measured using the micro-orifice uniform deposit impactor-droplet freezing technique (MOUDI-DFT), we report immersion-mode INP number concentrations as a function of particle size at several ground-level sites across North America and Europe, including Arctic, alpine, coastal, marine, agricultural, and suburban environments. More than 91 % of INPs active at -15 °C were found to be supermicron in size and 62 % were in the coarse mode (> 2.5 µm). While these percentages decreased with decreasing temperature, supermicron INPs remained important with nearly half of those active at -25 °C belonging to the coarse mode.