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 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 (H₂SO₄) concentrations, low relative humidity and low particle surface area concentrations. The data point to H₂SO₄ as a principle nucleation precursor species with typical daytime concentrations between 10⁶ and 10⁷ molecules cm^-3. Particle production was observed at H₂SO₄ concentrations that are well below predicted values for binary nucleation of H₂O and H₂SO₄, 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 H₂SO₄ and its associated water. This suggests that species in addition to H₂SO₄ were also making large contributions to ultrafine particle growth. Finally, calculated steady-state H₂SO₄ concentrations were found to be in good agreement with measured values if the mass accommodation coefficient for H₂SO₄ on aerosol surfaces was assumed equal to ∼1.