Atmospheric particles acting as Ice Forming Nuclei in different size ranges

The work presents the results of an experimental campaign performed at a rural site (S. Pietro Capofiume, near Bologna July 2007) concerning measurements of ice nuclei in different size classes of aerosol: PM1, PM2.5, PM10 and total suspended particles (TSP). Simultaneous measurements of particle number concentrations were also performed. Aerosol in the PM1 fraction contributes about 50% of the measured ice nuclei number concentration, and in the PM10 fraction contributes about 70-90%. Consequently, the dominant fraction of aerosol that can be activated as ice nuclei involves particles with aerodynamic diameter less than 10 ?m. A positive correlation is observed between higher supersaturation with respect to ice and water (Sice and Sw, respectively) values, and ice nuclei number concentration. The variations of Sw from 2 to 10% and Sice from 20 to 32% (Tair=-15 °C) determine an approximately threefold increase (from 110 to 337 m-3) in the ice nuclei average number concentration. There is no correlation between ice nuclei measured in the different size ranges, either with the particle number concentration measured with the counter spectrometer (d>0.3 ?m) or with the condensation nuclei counter.