1School of Earth and Atmospheric Sciences, Georgia Institute of Technology, Atlanta
2NASA Langley Research Center, Hampton, Virginia
3NASA Ames Research Center, Moffet Field, Califotnia
4Department of Chemistry, Drexel University, Philadelphia, Pennsylvania
5Institute for the Study of Earth, Ocean and Space, University of New Hampshire, Durham
6Nicholas School of the Environment, Duke University, Durham, North Carolina
Abstract. Aircraft observations during the Pacific Exploratory Mission in the western Pacific Ocean, phase B (PEM-West B), taken in February-March 1994, have been used to constrain a numerical model that calculates local concentrations of gaseous H2SO4, rates of homogeneous nucleation, and concentrations of newly formed, nanometer-sized particles. The data was selected from 13 flights over the western Pacific Ocean that covered an altitude range from the boundary layer (BL) to the upper troposphere (UT) and latitudes from 100S to 600N. The largest nucleation rates were calculated for the data from the flights over the temperate latitudes (L>300N). Within these latitudes, homogeneous nucleation rates averaged about 1-100 particles cm-3s-1. Significantly smaller nucleation rates were calculated for the tropical (L<200N)and subtropical (200N - 300N) regions. The relatively large nucleation rates calculated for the temperate latitudes could be largely attributed to the cold temperatures encountered in this region during the PEM-West B flights. For the data from the tropical and subtropical flights, little or no homogeneous nucleation was calculated for the average conditions encountered in the BL and midtroposphere (MT). Instead, significant nucleation was limited either to the UT or to several small-scale events. These enhanced nucleation events were generally characterized by spikes in relative humidity and low aerosol surface density. However, the strongest nucleation events, with homogeneous nucleation rates of about 10 particles cm-3s-1, were associated with high concentrations of SO2, most likely as a result of pollution from the Asian continent. Our results imply that in regions in which homogeneous nucleation is dominated by small-scale fluctuations, approaches that attempt to infer nucleation rates using average or typical conditions will grossly underestimate the actual average rate of nucleation.
Copyright 1997 by the American Geophysical Union.
Global Tropospheric Experiment (GTE)