Abstract

In contemplating the potential indirect effect of various natural and anthropogenic aerosol particles on clouds that extend to below the freezing level, it is understood from previous studies that it is vital to document the transport, transformations, and cloud activation behaviors of desert dus... Show moreIn contemplating the potential indirect effect of various natural and anthropogenic aerosol particles on clouds that extend to below the freezing level, it is understood from previous studies that it is vital to document the transport, transformations, and cloud activation behaviors of desert dust and certain carbonaceous particles in the atmosphere. The Asian continent provides large sources for both of these basic particle species to the atmosphere, with dust production peaking in the spring. Here we report first measurements of the ice nucleation behavior of aerosols exiting the Asian continent at various stages and levels of transport across the Pacific Ocean. These measurements were made during the Pacific Dust Experiment (PACDEX), a project that utilized the special capabilities of the NSF HIAPER aircraft for extended deployment over intercontinental scales. The study occurred during the months of April and May 2007. We will describe ice nuclei (IN) measurement results and relate these to other measurements of aerosol properties, including the aerosol size distribution and single particle compositions. IN concentrations varied over the full range ever measured in the free troposphere, and in general proportion with the concentrations of larger aerosol particles, presumably indicating the strong influence of dust particle plumes in the free troposphere. Aerosol processing during transport was associated with apparent negative impacts on IN concentrations. Conclusions regarding the potential role of carbonaceous particles as ice nuclei will require more careful analyses due to frequent mixing with dust particles. We will also discuss some preliminary results regarding aerosol-cold cloud interactions using additional measurements of the ice nucleation ability of residues of cloud particles sampled with a counterflow virtual impactor inlet. Show less