AIR-MASS CHARACTERISTICS, AEROSOL-PARTICLE NUMBER CONCENTRATIONS, ANDNUMBER SIZE DISTRIBUTIONS AT MACQUARIE ISLAND DURING THE FIRST AEROSOL CHARACTERIZATION EXPERIMENT (ACE-1)

During the First Aerosol Characterization Experiment (ACE 1), continuo us measurements were made of the particle number size distribution (be tween 18 and 540 nm diameter (D-p)) and total particle number concentr ation (D-p >3 nm and D-p >12 nm) on Macquarie Island, Tasmania (54 deg rees 30'S, 158 degrees 57'E, 7 m above sea level). Periodic real-time measurements of dimethyl sulfide were also made. Sampled air masses we re separated into clean marine and those influenced by Tasmania or Ant arctica. Observations were compared to those from a southern hemispher e midlatitude site (Cape Grim) and to sites on the Antarctic continent . It was found that the average total number concentration observed du ring clean marine conditions, 675 cm(-3), was about 21% higher than va lues observed at Cape Grim during ACE 1 and was similar to the high en d of the historical range of number concentrations reported by Gras [1 995] for Cape Grim during the same time of year. During both clean mar ine and influenced conditions, the Aitken and accumulation modes domin ate the number size distribution, with a Young Aitken mode observed le ss often. The number size distribution between 18 and 540 nm exhibited two and three modes 75% and 25% of the time, respectively, during cle an marine conditions, more consistent with previous observations at Ca pe Grim than with those from coastal Antarctica. The typical bimodal n umber distribution at Macquarie Island exhibited average modal diamete rs of 33 and 113 nm during clean marine conditions, corresponding to t he 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 influe nced periods when the size distribution exhibits an Aitken mode near 5 0 nm and an accumulation mode near 128 nm. The Young Aitken mode appea red most often during or immediately after periods of precipitation as sociated with both warm and cold fronts, when the Aitken and accumulat ion mode number concentrations were depleted. Evidence for possible cl oud processing of aerosol was found during two Antarctic influenced pe riods. Variability in observed aerosol characteristics was found to co incide with changes in air mass source region as indicated by back tra jectories and frontal passages.