Individual Particles Types and Size Distributions from Lagrangian Experiment "B" of the Southern Hemisphere Aerosol Characterization Experiment (ACE-1)

J. R. Anderson and P. R. Buseck, Depts. of Chemistry and Biochemistry and Geology, Arizona State Univ., Tempe, AZ 85287-1604, email:

Aerosol-particle samples from ACE-1 Lagrangian Experiment "B" south of Tasmania, 6-8 December 1995, were analyzed using automated scanning electron microscope. A clear area of post-frontal air was tracked and sampled by four flights of the NCAR C-130. Individual-particle results from the boundary layer show a slowly evolving aerosol dominated by sea-salt particles in the size range 0.1 to 1.0 microns. In the lower boundary layer, sampled at altitudes of 100' and 500', submicron sea salt comprises 75 to 90% by count of the particles larger than 0.1 microns, with a peak in the size distribution between 0.35 and 0.45 microns.

Most of the n.s.s sulfate is in the form of completely or partially reacted sea salt. Two reaction end-members are present, with Na:S of about 1:1 and about 2:1. These are interpreted to be sodium methanesulfonate and sodium sulfate, respectively. Non-volatile sulfate particles (without sea-salt cations) are in very low relative abundance and, in the lower boundary layer, uniformly decrease from 0.6 to 0.1% relative number concentration over the experiment period. The decrease in abundance is accompanied by a decrease in mean diameter from 0.8 to 0.3 microns. This suggests that the larger particles were progressively removed and not replaced. Nonvolatile sulfate particles are relatively more abundant in the free troposphere, but still are only a minor component.

Mineral particles and a distinct group of anthropogenic Cr-bearing particles are present in low relative abundance at all levels sampled, but have highest concentrations in the free troposphere. Small numbers of similar Cr-bearing particles are present in many other ACE-1 samples, both from the NCAR C-130 and the station at Cape Grim, Tasmania, but not in blanks. These are somewhat surprising in an otherwise clean marine aerosol and may represent long-range transport in the free troposphere from South Africa.