NEW PARTICLE FORMATION AT A REMOTE CONTINENTAL SITE - ASSESSING THE CONTRIBUTIONS OF SO2 AND ORGANIC PRECURSORS

Ultrafine aerosols, with diameters less than 10 nm, nucleate from gas phase species. The composition of newly formed ultrafine atmospheric a erosols is not known with certainty; new particles have variously been conjectured to be sulfates, organic compounds, and sulfate/organic mi xtures. The 1993 Tropospheric OH Photochemistry Experiment at Idaho Hi ll, Colorado, provided an opportunity to examine the question of which class of compounds, i.e., sulfates or organics, make the major contri bution to new particle formation in the unpolluted troposphere. This s tudy compared the production rates of sulfuric acid (from the oxidatio n of sulfur dioxide) and oxidized organic compounds to gauge their rel ative contributions to the formation of ultrafine particles. Potential organic precursor species examined in this study were the naturally o ccurring terpenes alpha- and beta pinene, and the anthropogenic hydroc arbons toluene, m-xylene, ethyl benzene, 1,2,4 trimethyl benzene, and methylcyclohexane. The calculated production of oxidized organics appe ared well correlated with total particle surface area and volume, sugg esting that at least some of the organic compounds formed in gas phase reactions condensed upon the preexisting aerosol. New particle format ion was found to be more highly associated with elevated production of gas phase sulfuric acid (via the SO2-OH reaction) than with productio n of oxidized organic products, although data from one day, during whi ch sulfuric acid production and total aerosol surface area were both l ower than usual, provided evidence for the involvement of terpene spec ies in new particle formation. The results suggest that for this conti nental site, sulfuric acid was probably responsible for most of the ob served new ultrafine particle formation. Low-volatility organic compou nds may have caused particle formation under the right conditions, but were more likely to condense upon preexisting particles.