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 10 ¿S to 60 ¿N. The largest nucleation rates were calculated for the data from the flights over the temperate latitudes (λ>30 ¿N). Within these latitudes, homogeneous nucleation rates averaged about 1--100 particles cm-3 s-1. Significantly smaller nucleation rates were calculated for the tropical (λ<20 ¿N) and subtropical (20 ¿N<λ<30 ¿N) regions. In the tropics, average nucleation rates in excess of 10 particles cm-3 s-1 were limited to the UT. In the subtropics, large average nucleation rates in excess of 1 particle cm-3 s-1 were obtained in the BL and in the UT, and average rates of about 10-1 particles cm-3 s-1 were obtained for the rest of the troposphere. 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-3 s-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. ¿ 1997 American Geophysical Union