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 (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 H20 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 H2SO 4 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.

Abstract. Particle formation in the binary H2SO4-H20 vapor system was studied at 295 K in a series of experiments employing a flow reactor. The concentration of H2SO4 was detected by chemical ionization mass spectrometry, and an ultrafine particle condensation nucleus counter was used to count the newly nucleated particles. Results yield a particle formation rate that is approximately proportional to [H2SO4] raised to the eighth power and to [H20] raised to the fifth power. The power dependencies measured here are significantly different from those determined in previous experimental work, and furthermore, the water dependence is markedly different from that predicted from current theories. The effect of adding ammonia vapor to the binary system was investigated; concentrations of NH3 in the many tens of parts per trillion by volume range were observed to promote dramatically the rate of particle nucleation.

[1] Correlations between concentrations of newly formed particles and sulfuric acid vapor were analyzed for twenty one nucleation events measured in diverse continental and marine atmospheric environments. A simple power law model for formation rates of 1 nm particles, J 1 = K Á [H 2 SO 4 ] P , where P and K are least squares parameters, was tested for each environment. We found that, to within experimental uncertainty, P = 2. Constraining P to 2, the prefactor K kinetic ranges from 10 À14 to 10 À11 cm 3 s À1 . According to the nucleation theorem, an exponent value of 2 indicates that the critical cluster contains two sulfuric acid molecules. Existing nucleation rate expressions based on classical nucleation theory predict significantly larger values of P. The prefactor values vary with environment and are 1 to 4 orders of magnitude below the hard-sphere collision limit. These results provide a simple parameterization for atmospheric new particle formation that could be used in global climate models.

Measurements of gas phase sulfuric and methane sulfonic acid (MSA) have been performed using a relatively new atmospheric pressure selected ion chemical ionization mass spectrometric technique at two field sites. Both gas phase acids are photooxidation products, and their concentrations are seen to qualitatively follow solar flux. While sulfuric acid concentrations typically decline in conjunction with declining solar radiation, they sometimes level off in the mid 105–106 molecules cm−3 range after dark, even in relatively clean air. The reason for this quasi‐stable nighttime sulfuric acid concentration is not well understood but may be a result of a steady state exchange of sulfuric acid between particles and the gas phase. Measurements of OH, H2SO4, and SO2 concentrations in conjunction with aerosol number and size distribution also made possible the independent calculation of gas phase sulfuric acid production and loss rates. Calculated production and loss rates are seen to agree well in relatively clean air during the daylight hours. At night, however, the sulfuric acid concentrations and its calculated loss rate often have a nonzero value. In more polluted air masses, calculated gas phase sulfuric acid losses significantly exceed calculated production if H2SO4/aerosol reaction probabilities of 1.0 or 0.5 are assumed.