Changes in hygroscopic growth of atmospheric submicrometer particles during air mass subsidence events in remote marine environments.

Sea-level measurements of the hygroscopic growth of submicrometer aerosol particles were performed during the ACE-1 field experiment in the South Tasmanian Sea and on a north-south transect across the Pacific Ocean preceeding the ACE-1 intensive campaign. The measurements were made on board the NOAA research ship R/V Discoverer from Oct. 15 until Dec. 14 1995. A Hygroscopic Tandem Differential Mobility Analyser OI-TDMA) was used to measure the hygroscopic diameter growth of submicrometer aerosol particles, when taken from a dry state to a humid state with a relative humidity of 90 %. A substantial lowering of the hygroscopic growth factors was seen during events with air mass subsidence, resulting also in increased sea-level particle concentrations in the ultrafine (<20 nm diameter) and lower (< ca. 50 nm) Aitken mode size ranges. The free troposphere is a probable source for these particles. The hygroscopic growth factors observed during these events were much lower than what is expected from particles consisting of pure sulphuric acid or ammonium sulphate. This implies that hygroscopically inactive material constituted a substantial volume fraction of the aerosol particles in the subsiding air.