Soot particles in the free troposphere over Australia

Aircraft observations were carried out in the middle troposphere at 5–6 km altitude over Australia in the PACE-II (Pacific Atmospheric Chemistry Experiment) campaign in October 1994. Concentrations of particles with radii larger than 4 nm and 0.15 μm were measured with a condensation nucleus (CN) counter and an optical counter, respectively. Individual aerosol particles were collected on electron microscopic grids using a jet impactor and subsequently examined using a transmission electron microscope coupled with an energy-dispersive X-ray (EDX) analyzer.High concentrations of CN (⩾4 nm) and large particles (0.15–0.5 μm radius) were found over the areas in northern Australia between Alice Springs and Darwin. Samples collected in this region had a large fraction of submicrometer soot-containing particles that had both K and S with a median S/(S+K) weight ratio of 0.4. The hygroscopic properties of particles with radii between 0.03 and 0.8 μm were examined with the dialysis (extraction) of water-soluble material. The sample collected in the area with high particle concentrations showed that the abundance of internally mixed soot particles was 88% of particles in the radius range of 0.1–0.35 μm. Approximately 50% of these mixed particles with radii ⩾0.2 μm were covered with semi-transparent material (probably organic material). Present research suggested that these particles were aged biomass burning particles originating from northern Australia and had been modified by chemical reactions and probably by coagulation during long-range transport. These soot particles were found to be present as internally mixed particles, with a soluble fraction that would make them efficient cloud condensation nuclei (CCN).