Measurements of the concentration and composition of nuclei for cirrus formation

This article addresses the need for new data on indirect effects of natural and anthropogenic aerosol particles on atmospheric ice clouds. Simultaneous measurements of the concentration and composition of tropospheric aerosol particles capable of initiating ice in cold (cirrus) clouds are reported. Measurements support that cirrus formation occurs both by heterogeneous nucleation by insoluble particles and homogeneous (spontaneous) freezing of particles containing solutions. Heterogeneous ice nuclei concentrations in the cirrus regime depend on temperature, relative humidity, and the concentrations and physical and chemical properties of aerosol particles. The cirrus-active concentrations of heterogeneous nuclei measured in November over the western U.S. were