Evidence for the widespread presence of liquid-phase particles during the 1999–2000 Arctic winter

In situ Multiangle Spectrometer Probe (MASP) particle measurements have been analyzed to determine the typical behavior of sulfate particles during the SAGE III Ozone Loss and Validation Experiment (SOLVE) campaign. The study has explored variations in the total particle concentration measured by MASP. A new analysis method has been developed in which increases of the MASP concentration can be interpreted as growth of small particles (those which are smaller than 0.2 mm in radius at midlatitudes). The method also allows all of the MASP measurements made during the SOLVE campaign to be incorporated in a single analysis. At all levels of the stratosphere, the total MASP concentration (and therefore aerosol growth) varies continuously with temperature. This behavior is well-reproduced by assuming that the sulfate aerosols are liquid solutions, but cannot be reproduced if the aerosol is assumed to be frozen. At sufficiently cold temperatures, larger increases in the MASP concentration are consistently seen; the observed onset temperature for this growth is in good agreement with model expectations for liquid ternary solutions. Liquid-like behavior is apparent for all measurements made in the Arctic during SOLVE, both inside and outside the vortex, and even at the coldest temperatures sampled during the campaign. At the levels with the coldest measured temperatures, which cause maximum particle sizes and thus the greatest total MASP concentrations, 90% of the particles grow as liquids. Therefore, the freezing that occurred during the 1999–2000 Arctic winter was selective, with most of the particles remaining liquid even in the presence of a small number of frozen particles.