Laboratory-measured nucleation rates of sulfuric acid and water binary homogeneous nucleation from the SO2 + OH reaction

We have measured H2SO4‐H2O nucleation rates (J) at atmospheric pressure, 288 K and 10–55 % relative humidity (RH). H2SO4 was produced from the SO2 + OH → HSO3 reaction and was measured with a chemical ionization mass spectrometer (CIMS). The CIMS‐measured [H2SO4] that is required to produce the unit J ranged from 107–108 cm−3, but if wall loss is taken into account the threshold [H2SO4] was at the 108–109 cm−3 range, much higher than the atmospheric conditions. The measured J was proportional to [H2SO4] with the second to tenth powers and proportional to RH with the eleventh to fifteenth powers. The measured aerosol sizes were proportional to log RH, showing a weak dependence of aerosol growth on RH. But the RH effects were more pronounced at higher [H2SO4], indicating that RH is more important for aerosol growth in the H2SO4 rich environment.