HR: 0800h
AN: A31A-0817    [Abstracts]
TI: Nucleation of Aerosols at a Mountain Location
AU: * Shaw, G E
EM: gshaw@gi.alaska.edu
AF: Glenn E. Shaw, Geophysical Institute University of Alaska, Fairbanks, Ak 99775 United States
AB: In a situation when precursor gas concentration is high enough and pre-existing aerosol surface area low enough, large numbers of embryos (diameter 1 nm) can be produced by hetromolecular, homogeneous nucleation. This often occurs on short time scales, in "bursts", and is more difficult a process when dry conditions occur. We documented such occurrences in an investigation on the summit of Lt Lemmon in southern Arizona, at a mean pressure altitude of 723 mb. The area is dominated by the sub tropical Pacific High which brings clean, and quite dry mid troposphere air to the station in the evening, which arrives after long passages over the eastern Pacific ocean. Evidence was found for nucleation and growth of aerosols, which reached a peak number concentration and diameter of several tens of nanometers several hours after solar noon. The particle diameter growth rate, following nucleation, was typically 20 nm/hr but occurred earlier in the day during spring than in winter. We were able to determine from the Maxwellian growth of particles that the vapor concentration of percursor gases corresponded to ten to a hundred ppt. In contrast to the methodical everyday diurnal variation of nucleation and Aitken-mode particles, the accumulation mode particles (diameter approximately 100 nm) showed no systematic daily variation. This is surprising, since the mixing depth of the boundary layer was frequently, especially during the spring months, was above the mountain top observatory. We speculate that mountain meteorology, especially mountain induced thermal updrafts and valley thermal currents carried pollution aerosols from the nearby city of Tucson systematically away from the mountain. Mt Lemmon appears to be an excellent location for investigating background mid troposphere air masses.
DE: 0305 Aerosols and particles (0345, 4801, 4906)
SC: Atmospheric Sciences [A]
MN: Fall Meeting 2005