Chemical and mineralogical characterization of ice nucleating particles and ice particle residuals

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We investigate the abundance, chemical composition and mixing state of atmospheric ice nucleating particles (INP) experimentally, i.e. of such particles that allow the heterogeneous formation of ice in mixed-phase clouds. This information is inevitable to understand the mechanisms of precipitation in the atmosphere. Meteorological models use parameterizations of such data to predict the development of precipitation. INP are rare: only 1 in 10.000 to 1 in 100.000 of atmospheric particles is an INP. We identify and isolate INP from other constituents of air using physical methods, followed by photography, counting and mineralogical analysis of the single particles, using advanced electron microscopy. The INUIT research unit applies several different approaches to distinguish INP from the bulk of other atmospheric particles. The individual particles identified and separated by these different methods are then collected and analyzed by electron microscopy for their size, morphology, mixing state, and chemical composition. Two of the methods (INP counters FINCH and FRIDGE) identify the INP by exposing an aerosol sample to subzero temperatures and supersaturated water vapor, thus growing macroscopic ice crystals on the INP. Ice crystals and thus INP are then separated from aerosol, and INP analyzed. The INP counter FINCH grows ice crystals on ambient aerosol in a flow tube reactor, while the INP counter FRIDGE grows ice on atmospheric particles that were precipitated electrostatically on a silicon substrate. Two other methods, named Ice-CVI and ISI, attempt to isolate INP by first separating the ice crystals in a flow of accelerated cloud air from other constituents, based on their higher inertia, and then evaporate the ice, leaving the INP behind for subsequent collection and analysis. Results from all four methods are compared. Aerosols from various atmospheric environments (central Europe, Jungfraujoch, Mediterranean, desert dust) will be analyzed during field experiments. Model and standard aerosols will be investigated in lab experiments. The number concentration of INP in the deposition and immersion modes in ambient air at Taunus Observatory near Frankfurt will be analyzed routinely in years 1-3 and screened for a seasonality, potential source areas (desert, industry), and for the main factors or components (such as biological material, mineral dust) that determine its variability.