Toward a climatology of immersion mode ice nuclei present in ambient air and rain water

Ice nuclei (IN) play an important role in the hydrological cycle due to their necessary presence to initiate precipitation within many types of clouds. Their atmospheric concentration, both in ambient air and within precipitation, is poorly understood. In particular there is ambiguity on whether ground level ambient IN concentrations have an impact on the concentration of IN within rain clouds (or vice versa). Here we present a new method to measure ice nuclei spectra (defined as IN concentration vs. supercooling temperature) from an immersion mode droplet freezing assay instrument. Ice nuclei were collected from ambient air using a swirling aerosol collector and from precipitation that had scavenged IN from the atmospheric column. Raw freezing spectra were used to probe the freezing activity of both abundant and rare IN contained in sample liquids by analysis of drops that had varying degrees of preconcentration and size (~50 to ~650 µm). Extreme value statistics is used to collapse the raw freezing data into a single ice nuclei spectrum that spans ~6 orders of magnitude in IN concentration. Data from this method are presented for precipitation samples from various storm types (e.g. local convective vs. frontal vs. tropical storm) as well as the hourly evolution of ice nuclei spectra through the passage of individual storm systems. Specific focus is given to a two week intensive campaign coinciding spatially and temporally with the Southern Oxidant & Aerosol Study (SOAS). For a selected number of samples, relative concentrations of biological and non-biological ambient aerosol and particles within the collected rain water are measured by the use of a UV fluorescence Wideband Integrated Bioaerosol Sensor (WIBS-4A) and correlated with the IN spectrum. Our results show that “warm” ice nuclei that require minimal supercooling to induce droplet freezing (T > -15 deg C) show the most variability pointing to spatially and temporally confined sources or preferential washout via nucleation scavenging. These results can be used to direct future studies of the regional and global transport of ice nuclei throughout the atmosphere and better constrain the ice nuclei concentrations necessary to initiate precipitation within certain clouds.