Soluble ion chemistry of the atmospheric aerosol and SO2 concentrations over the eastern North Atlantic during ACE-2

Abstract

ACE-2, the second Aerosol Characterization Experiment of the International Global Atmospheric Chemistry Project (IGAC), was conducted in the area between Portugal, Tenerife and Madeira from 15 June to 25 July 1997. We determined the concentration of SO2 and the soluble ion composition of the atmospheric aerosol in 113 samples collected by aircraft. Comparison between aircraft and ground-based samples collected from the same or similar airmasses showed good agreement (better than 40%) for the fine fraction of the aerosol, but suggests that, for the coarse fraction, the sampling efficiency of the aircraft inlet is only about 35%. During periods when trajectory analysis suggested no recent contact of the airmass with Europe or North America, SO2 and aerosol ions were at levels comparable to those found over remote ocean regions. The composition of airmasses originating from Europe showed signatures characteristic of the source regions and suggested rapid oxidation of SO2 during transport over the ocean. The first Lagrangian experiment was conducted in an unpolluted airmass and showed the physical and chemical evolution of a marine boundary layer traversing over increasingly warmer ocean waters. The sulfur cycle in this airmass could be explained based on the emission of DMS from the sea surface. In three other Lagrangian experiments, we investigated the evolution of boundary layers with increasing age since having left the European continent. SO2 was removed rapidly with lifetimes on the order of half a day in cloud-topped boundary layers. The production of nss-sulfate ceased after SO2 had declined to background levels, and the aerosol approached a nearly constant composition, with concentrations dominated by physical removal and dilution processes. Aerosol nitrate concentrations suggested that gaseous HNO3 was taken up by seasalt aerosol and subsequently removed by dry deposition.