[1] The formation of secondary organic aerosol (SOA) in an anthropogenic-influenced region in the southeastern United States is investigated by a comparison with urban plumes in the northeast. The analysis is based on measurements of fine-particle organic compounds soluble in water (WSOC) as a measure of secondary organic aerosol. Aircraft measurements over a large area of northern Georgia, including the Atlanta metropolitan region, and in plumes from New York City and surrounding urban regions in the northeast show that fine-particle WSOC are spatially correlated with vehicle emission tracers (e.g., CO), yet the measurements indicate that vehicles do not directly emit significant particulate WSOC. In addition to being correlated, WSOC concentrations were in similar proportions to anthropogenic tracers in both regions, despite biogenic volatile organic compounds (VOCs) that were on average 10-100 times higher over northern Georgia. In contrast, radiocarbon analysis on WSOC extracted from integrated filters deployed in Atlanta suggests that roughly 70-80% of the carbon in summertime WSOC is modern. If both findings are valid, the combined results indicate that in northern Georgia, fine-particle WSOC was secondary and formed through a process that involves mainly modern biogenic VOCs but which is strongly linked to an anthropogenic component that may largely control the mass of SOA formed. Independent of the radiocarbon results, a strong association between SOA and anthropogenic sources has implications for control strategies in urban regions with large biogenic VOC emissions.Citation: Weber, R. J., et al. (2007), A study of secondary organic aerosol formation in the anthropogenic-influenced southeastern United States,

[1] The Particle Analysis by Laser Mass Spectrometry (PALMS) instrument has measured the composition of single particles during a number of airborne and ground-based campaigns. In the regions studied, 30% to over 80% of the aerosol mass in the free troposphere was carbonaceous material. Most of this carbonaceous material was probably organic. Although there were variations in their amounts, over 90% of accumulation mode particles away from local sources were internal mixtures of sulfates and carbonaceous material. Within this internal mixing, there was variation in the pattern of carbonaceous peaks in the spectra, especially in peaks related to organic acids. Particles with a biomass burning signature were a significant fraction of accumulation mode particles even far from fires. The accumulation mode near the ocean surface off the coasts of California and New England had significant numbers of carbonaceous-sulfate particles whereas at Cape Grim, Tasmania, the organic-sulfate particles were apparently all smaller than 160 nm. Three kinds of nitrate were evident: on mineral particles, on carbonaceous-sulfate particles when the sulfate was fully neutralized, and at temperatures below about 198 K. Most mineral particles showed evidence of the uptake of nitrates and chloride. A peak that probably represents protonated pyridine appears in some spectra in the free troposphere.

Nitrogen oxides in the lower troposphere catalyze the photochemical production of ozone (O3) pollution during the day but react to form nitric acid, oxidize hydrocarbons, and remove O3 at night. A key nocturnal reaction is the heterogeneous hydrolysis of dinitrogen pentoxide, N2O5. We report aircraft measurements of NO3 and N2O5, which show that the N2O5 uptake coefficient, g(N2O5), on aerosol particles is highly variable and depends strongly on aerosol composition, particularly sulfate content. The results have implications for the quantification of regional-scale O3 production and suggest a stronger interaction between anthropogenic sulfur and nitrogen oxide emissions than previously recognized.