Revisiting ice nucleation from precipitation samples

An emerging and unsolved question is the sensitivity of cloud processes, precipitation, and climate to the atmospheric ice nucleus spectrum. This work revisits estimation of atmospheric ice-nucleating particle concentration derived from cloud water and precipitation samples representing a wide range of geographical locations, seasons, storm systems, precipitation types, instruments, concentrations, and temperatures. Concentrations of ice-nucleating particles are shown to vary over 10 orders of magnitude. High variability is observed in the -5 degrees C to -12 degrees C range which is suggested to be biologically derived nuclei whose life cycle is associated with intermittent source and efficient sink processes. The highest ever observed nucleus concentrations at -8 degrees C are 3 orders of magnitude lower than observed ice crystal concentrations in tropical cumuli at the same temperature. The observed upper and lower limits of the nucleus spectrum provide a possible constraint on minimum enhancement factors for secondary ice formation processes.