Laboratory-Measured Sulfuric Acid and Water Homogeneous Nucleation Rates from the SO2 + OH Reaction

Sulfuric acid and water binary homogeneous nucleation is the most important atmospheric nucleation system, but the nucleation mechanisms are poorly understood. Here, we report laboratory-measured sulfuric acid and water binary homogeneous nucleation rates at the atmospheric pressure, 288 K and 10 - 55 % relative humidity. The SO2 + OH reaction was used to produce sulfuric acid vapor. Residual sulfuric acid concentrations measured with a chemical ionization mass spectrometer (CIMS) ranged from 3e6 to 2e9 per cc, but the wall loss factors of sulfuric acid were substantial, ranging from 10 to 1,000 depending on nucleation time. Therefore, actual sulfuric acid concentrations in the nucleation region were one or three orders of magnitude higher with wall loss factors taken into account. The measured aerosol nucleation rates were proportional to sulfuric acid concentration with the second to tenth powers and proportional to RH with the eleventh to fifteenth powers. These power dependences became weaker at higher RH or higher sulfuric acid concentrations, indicating that the sizes of critical clusters are smaller at higher supersaturation ratios of aerosol precursors. The measured aerosol sizes were larger at higher sulfuric acid concentrations and higher RH, consistent with the predictions of homogeneous nucleation theories. The measured aerosol sizes were proportional to log RH, showing a weak dependence of aerosol growth on RH. However, the RH effects were more pronounced at higher sulfuric acid concentrations, indicating that RH effects on aerosol growth are more important in the polluted atmosphere. Furthermore, sulfuric acid concentrations required to produce the unit nucleation rate were on the order of e9 to e10 per cc, inconsistent with Berndt et al. [Science, 2005; GRL, 2006] where only e7 per cc sulfuric acid concentrations are required to produce the unit nucleation rates, although both our and Berndt et al. studies used the SO2 + OH reaction to form sulfuric acid vapor. Because wall loss factor is a sensitive function of nucleation time, considering Berndt et al. used extraordinary long nucleation times (290 s), it is likely that their sulfuric acid concentrations were strongly underestimated. Different power dependences of nucleation rate on sulfuric acid concentration measured from atmospheric observations (linear) and from laboratory binary homogeneous nucleation studies [This study, Ball et al., JGR, 1999; Berndt et al., 2005, Science; Zhang et al., Science, 2004; Berndt et al., GRL, 2006] (often fifth to eighth powers) also suggest that ternary species (such as ammonia and organics) play important roles in aerosol nucleation in the atmosphere.