Abstract

Simultaneous measurements of aerosol particles and their expected gas phase precursors were made at Idaho Hill, Colorado, a remote continental site. This study used apparatus and techniques similar to those employed in an earlier study at the Mauna Loa Observatory, Hawaii [ Weber et al. , 1995]. New particle formation, identified by the presence of ultrafine particles (nominally 3 to 4 nm diameter), was commonly observed in downslope (westerly) air and was correlated with high sulfuric acid (H 2 SO 4 ) concentrations, low relative humidity and low particle surface area concentrations. The data point to H 2 SO 4 as a principle nucleation precursor species with typical daytime concentrations between 10 6 and 10 7 molecules cm −3 . Particle production was observed at H 2 SO 4 concentrations that are well below predicted values for binary nucleation of H 2 O and H 2 SO 4 , suggesting that another species participated. Particle growth rates were estimated from the data with two independent approaches and in both cases were ∼5 to 10 times higher than can be explained by condensation of H 2 SO 4 and its associated water. This suggests that species in addition to H 2 S0 4 were also making large contributions to ultrafine particle growth. Finally, calculated steady‐state H 2 SO 4 concentrations were found to be in good agreement with measured values if the mass accommodation coefficient for H 2 SO 4 on aerosol surfaces was assumed equal to ∼1.