MEASUREMENTS OF NEW PARTICLE FORMATION AND ULTRAFINE PARTICLE GROWTH-RATES AT A CLEAN CONTINENTAL SITE

Simultaneous measurements of aerosol particles and their expected gas phase precursors were made at Idaho Hill, Colorado, a remote continent al site. This study used apparatus and techniques similar to those emp loyed in an earlier study at the Mauna Loa Observatory, Hawaii [Weber et al., 1995]. New particle formation, identified by the presence of u ltrafine particles (nominally 3 to 4 nm diameter), was commonly observ ed in downslope (westerly) air and was correlated with high sulfuric a cid (H2SO4) concentrations, low relative humidity and low particle sur face area concentrations. The data point to H2SO4 as a principle nucle ation precursor species with typical daytime concentrations between 10 (6) and 10(7) molecules cm(-3). Particle production was observed at H2 SO4 concentrations that are well below predicted values for binary nuc leation of H2O and H2SO4. suggesting that another species participated . Particle growth rates were estimated from the data with two independ ent approaches and in both cases were similar to 5 to 10 times higher than can be explained by condensation of H2SO4 and its associated wate r. This suggests that species in addition to H2SO4 were also making la rge contributions to ultrafine particle growth. Finally, calculated st eady-state H2SO4 concentrations were found to be in good agreement wit h measured values if the mass accommodation coefficient for H2SO4 on a erosol surfaces was assumed equal to similar to 1.