Aerosol Chemical Composition in Asian Continental Outflow During TRACE-P: Comparison to PEM-West B.

We present the spatial distributions of aerosol-associated soluble ions, and the radionuclide tracers Be-7 and Pb-210, determined by bulk filter sampling from the NASA DC-8 during the TRACE-P airborne campaign over the western north Pacific in spring of 2001. Mean mixing ratios of ions dominated by anthropogenic sources (sulfate, ammonium, nitrate) are enhanced by at least a factor of two in samples collected below 6 km altitude within several hundred km of the Asian continent, compared to our results obtained using the same sampling system and analytical procedures during the PEM-West B campaign in the same region during spring 1994. Calcium (a tracer of dust) was enhanced more than 5-fold in this near-Asia region during TRACE-P. Mixing ratios of all four ions decreased with distance from the Asian continent in both campaigns, but within the boundary layer over the remote Pacific average values were also enhanced roughly 2-fold during TRACE-P. We do not feel these differences in mixing ratios reflect increased emissions of dust and pollutants from Asia over the seven year period, because sea salt (indicated by sodium and chloride) and Be-7 are also at least 2-fold higher in both regions in the TRACE-P data set. Measurements of numerous gas phase pollutants (presented in other papers in this session) generally show no significant increases between the two campaigns. Lead-210 concentrations are higher near Asia than further east, but show only modest differences between the two campaigns. The apparent secular trend in dust and aerosol-associated pollutants over the western Pacific can be attributed to a combination of: 1) more efficient precipitation scavenging during 1994 (removing aerosol-associated ions and soluble precursors, but not the Rn-222 precursor of Pb-210), 2) spatially heterogeneous distribution of aerosols, and 3) the limited spatial and temporal data coverage inherent to airborne sampling.