CLIMATIC CONTEXT OF THE FIRST AEROSOL CHARACTERIZATION EXPERIMENT (ACE-1) - A METEOROLOGICAL AND CHEMICAL OVERVIEW

During the intensive field operations period (November 15 to December 14, 1995) of the First Aerosol Characterization Experiment (ACE 1) col d front activity was generally above average, resulting in below avera ge temperatures, pressures, and rainfall. The principal cause was the presence for much of the experiment of a long wave trough. This trough was mobile, traversing the ACE area during the project, with some war m anomalies evident in the areas under the influence of the long wave ridges. There is evidence of greater convective activity than normal p ossibly leading to a slightly deeper than average mixing layer. A grea ter west to northwesterly component to the air flow than average durin g November appears to have led to higher than average concentrations o f radon and particles in the clean, marine or ''baseline'' sector at C ape Grim (190 degrees to 280 degrees). This is likely to have resulted from inclusion of continental air from western parts of the Australia n mainland in the baseline sector winds. Although aerosol-bound sulfur species were generally near their normal concentrations across the AC E 1 area, the overall pattern including atmospheric dimethylsulfide su ggests slightly higher than usual sulfur species levels in the souther n part of the region and lower concentrations in the northern part dur ing November. This could be related to changes in marine biogenic prod uctivity, air-sea exchange, or atmospheric removal. In December, the c hanging long wave pattern brought an increase in south and southwester ly flow over the entire region. The baseline sector became less affect ed by continental species, but it appears that the colder conditions b rought by this pattern have led to lower than usual atmospheric concen trations of biogenic species, as the region went into one of the colde st summers on record.