Abundance of biological ice nuclei at tropospheric cloud heights: results and perspectives from one year of observations

A crucial stage in the process of precipitation is the formation of ice nuclei, which provide a surface for the development of particles sufficiently big to fall on earth. Different substances present in the atmosphere enhance the aggregation of water molecules into ice structures, but particularly effective seem to be aerosols of biological origin, active at temperatures up to -2°C. Yet, the relevance of biological ice nucleation for cloud processes, such as initiating precipitation, remains ambiguous. Moreover, very little is known about abundance and .nucleation spectra of these IN at tropospheric cloud altitudes. The purpose of this project is then to understand the meteorological conditions and the environmental factors specifically associated with the presence of biological ice nuclei in precipitation. One full year of observations has been carried out at the High Altitude Research station of Jungfraujoch, in the Swiss Alps, 3580 m above sea level, as representative of tropospheric cloud heights. Fresh snow has been collected each month and analysed directly in site to estimate the concentration of nucleators active at temperatures higher than -12°C. A cooling bath apparatus with an innovative system of automatic recording of freezing events has been employed. Additional information has been provided through the recording of meteorological parameters associated with the snow sam pies, the determination of water stable isotopes (2H and 180) and of bacterial (direct epifluorescence microscope counting, live/dead staining) concentrations. The first results coming from one year's sampling campaign will be presented. The preliminary analysis of data suggests that the abundance of ice nuclei in snowfall is characterized not only by the presence of seasonal cycles over time, but also of geographical control on a spatial scale. This implies that air masses developed in different regions and different moments of the year can contain different amounts of biological particles. Future approaches will be then discussed, such as how to get better information from all the parameters monitored and how to obtain a better characterization of the samples collected, for instance a better identification of biological particles responsible for warm ice nucleation. New information will be then available to assess the real impact of biological aerosols in conditioning precipitation.