Sources and Composition of Southern Ocean Ice Nucleating Particles during CAPRICORN-2 and SOCRATES

Mixed phase clouds, which contain both liquid droplets and ice crystals, are ubiquitous in polar regions and have a large influence on Earth's energy budget. The initiation of ice in mixed phase clouds is dependent on aerosols known as ice nucleating particles (INPs), which can have an outsized effect on cloud phase and related climate feedbacks due to their relative scarcity. INPs from different sources vary in ice nucleating ability, contributing to regional or seasonal differences in cloud phase. While mineral and soil dusts are the dominant INP type in most areas due to their generally effective ice nucleating ability and large emission rates, the Southern Ocean is thought to be the largest area where marine INPs dominate the overall budget, based on modeling studies and observed low INP concentrations. Measurements from the CAPRICORN-2 (Clouds, Aerosols, Precipitation Radiation and atmospherIc Composition Over the southeRN ocean II) ship campaign and SOCRATES (Southern Ocean Cloud Radiation Aerosol Transport Experimental Study) aircraft campaign represent some of the very few measurements of INP composition from this region, which are necessary for validating this hypothesis. A combination of ice nucleating particle number and composition observations, aerosol trace metal measurements, and air mass back trajectories suggest particle sources in the Southern Ocean could be divided into three roughly 10° latitude bands during austral summer 2018. North of 50 °S, the main influences were from Australian and long-range transported air masses, and between 50-60 °S long range transport dominated, with significant atmospheric processing of particles observed. Surprisingly, observations south of 60 °S were characterized by relatively high INP and aerosol iron concentrations and low iron solubilities, with back trajectories indicating significant Antarctic influence. This prompted analysis of the potential emission of INPs from local sources in Antarctica, including INP number and composition measurements on previously collected soil samples from Shackleton Glacier and the McMurdo Dry Valleys. Highly active INPs with warm onset temperatures (~-5 °C) were observed in some samples, as well as mineral INPs, and support the possibility of INP emission from ice-free regions of Antarctica.