Je. Dibb et al., Aerosol chemical composition and distribution during the Pacific Exploratory Mission (PEM) Tropics, J GEO RES-A, 104(D5), 1999, pp. 5785-5800
Distributions of aerosol-associated soluble ions over much of the South Pac
ific were determined by sampling from the NASA DC-8 as part of the Pacific
Exploratory Mission (PEM) Tropics campaign. The mixing ratios of all ionic
species were surprisingly low throughout the free troposphere (2-12 km), de
spite the pervasive influence from biomass burning plumes advecting over th
e South Pacific from the west during PEM-Tropics. At the same time, the spe
cific activity of Be-7 frequently exceeded 1000 fCi m(-3) through much of t
he depth of the troposphere. These distributions indicate that the plumes m
ust have been efficiently scavenged by precipitation (removing the soluble
ions), but that the scavenging must have occurred far upwind of the DC-8 sa
mpling regions (otherwise Be-7 activities would also have been low). This i
nference is supported by large enhancements of HNO3 and carboxylic acids in
many of the plumes, as these soluble acidic gases would also be readily sc
avenged in any precipitation events. Decreasing mixing ratios of NH4+ with
altitude in all South Pacific regions sampled provide support for recent su
ggestions that oceanic emissions of NH3 constitute a significant source far
from continents. Our sampling below 2 km reaffirms the latitudinal pattern
in the methylsulfonate/non-sea-salt sulfate (MSA/nss SO4=) molar ratio est
ablished through surface-based and shipboard sampling, with values increasi
ng from <0.05 in the tropics to nearly 0.6 at 70 degrees S. However, we als
o found very high values of this ratio (0.2-0.5) at 10 km altitude above th
e intertropical, convergence zone near 10 degrees N, It appears that wet co
nvective pumping of dimethylsulfide from the tropical marine boundary layer
is responsible for the high values of the MSA/nss SO4= ratio in the tropic
al upper troposphere. This finding complicates use of this ratio to infer t
he zonal origin of biogenic S transported long distances.