Laboratory‐measured nucleation rates of sulfuric acid and water binary homogeneous nucleation from the SO 2 + OH reaction

We have measured H 2 SO 4 ‐H 2 O nucleation rates ( J ) at atmospheric pressure, 288 K and 10–55 % relative humidity (RH). H 2 SO 4 was produced from the SO 2 + OH → HSO 3 reaction and was measured with a chemical ionization mass spectrometer (CIMS). The CIMS‐measured [H 2 SO 4 ] that is required to produce the unit J ranged from 10 7 –10 8 cm −3 , but if wall loss is taken into account the threshold [H 2 SO 4 ] was at the 10 8 –10 9 cm −3 range, much higher than the atmospheric conditions. The measured J was proportional to [H 2 SO 4 ] 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 [H 2 SO 4 ], indicating that RH is more important for aerosol growth in the H 2 SO 4 rich environment.