Air mass characteristics, aerosol particle number concentrations, and number size distributions at Macquarie Island during the First Aerosol Characterization Experiment (ACE 1)

Abstract

During the First Aerosol Characterization Experiment (ACE 1), continuous measurements were made of the particle number size distribution (between 18 and 540 nm diameter (Dp )) and total particle number concentration (Dp > 3 nm and Dp > 12 nm) on Macquarie Island, Tasmania (54°30'S, 158°57'E, 7 m above sea level). Periodic real-time measurements of dimethyl sulfide were also made. Sampled air masses were separated into clean marine and those influenced by Tasmania or Antarctica. Observations were compared to those from a southern hemisphere midlatitude site (Cape Grim) and to sites on the Antarctic continent. It was found that the average total number concentration observed during clean marine conditions, 675 cm-3, was about 21% higher than values observed at Cape Grim during ACE 1 and was similar to the high end of the historical range of number concentrations reported by Gras [1995] for Cape Grim during the same time of year. During both clean marine and influenced conditions, the Aitken and accumulation modes dominate the number size distribution, with a Young Aitken mode observed less often. The number size distribution between 18 and 540 nm exhibited two and three modes 75% and 25% of the time, respectively, during clean marine conditions, more consistent with previous observations at Cape Grim than with those from coastal Antarctica. The typical bimodal number distribution at Macquarie Island exhibited average modal diameters of 33 and 113 nm during clean marine conditions, corresponding to the smaller Aitken mode and larger accumulation mode, respectively. The 50 to 70 nm diameter range corresponds to the minimum in the bimodal size distribution at Macquarie Island, except for continentally influenced periods when the size distribution exhibits an Aitken mode near 50 nm and an accumulation mode near 128. nm. The Young Aitken mode appeared most often during or immediately after periods of precipitation associated with both warm and cold fronts, when the Aitken and accumulation mode number concentrations were depleted. Evidence for possible cloud processing of aerosol was found during two Antarctic influenced periods. Variability in observed aerosol characteristics was found to coincide with changes in air mass source region as indicated by back trajectories and frontal passages. Copyright 1998 by the American Geophysical Union.