This paper reports on measurements made with a new system for real-time measurement of the chemical composition of ice nucleating aerosol particles. This system utilized a continuous flow diffusion chamber (CFDC) to activate natural aerosols as ice nuclei, a counterflow virtual impactor to separate nucleated crystals from other aerosols and a single particle mass spectrometer for measuring the ionic composition of ice crystal residuals from the CFDC. The focus of the first measurements with this system was the role of different natural aerosols in forming cirrus clouds. Therefore aerosols were processed as ice nuclei at temperatures between -30 and -65°C. Measurements were made in free tropospheric air sampled at a high altitude (~3200 m MSL) laboratory in November 2001 as a prelude to future aircraft deployment. Ancillary measurements were made of aerosol size distributions and meteorological parameters.

The results of this study have implications with regard to the long-range sources of natural IN, their relation to aerosol concentrations and properties, the contributions of anthropogenic sources to IN populations, and the interplay of heterogeneous and homogeneous ice nucleation as mechanisms for cirrus cloud formation. Preliminary findings will be discussed, including the fact that measurable concentrations of heterogeneous ice nuclei were present even in air of apparent upper tropospheric origin, mineral dust aerosols with and without associated sulfates and organic components represented more than 50% of heterogeneous IN, manmade aerosols contributed at least 10% to IN populations, and that particles of possible origin from biomass burning were also detected as low temperature IN.