The generation of aerosols by binary homogeneous nucleation

The rates of nucleation of liquid aerosols from the gaseous mixtures H₂SO₄ + H₂O and HNO₃ + H₂O at 25°C for various relative humidities (10 to 100%) and various activities of acid vapor are calculated using the Flood-Neumann-Döring-Reiss-Doyle theory of binary homogeneous nucleation. The activities of acid vapor needed for nucleation are 25 to 300 times smaller for H₂SO₄ + H₂O than for HNO₃ + H₂O. This is due to the much larger free energy of mixing in the liquid phase for H₂SO₄ + H₂O. Conversion from activities to actual pressures leads to concentrations of HNO₃ which are much too high to be found under normal atmospheric conditions. On the other hand, the concentrations of H₂SO₄ vapor needed to nucleate droplets in the H₂SO₄ + H₂O system are in the range 4(10^?5) to 1.3 (10^?2) ppm, a concentration which can result from photo-oxidation of SO₂ in the atmosphere. Calculations are made of the growth curves for H₂SO₄ + H₂O droplets (radius vs composition) at various relative humidities from the critical size radius up to a 1000 Å radius, corresponding to nuclei large enough to serve as condensation centers for heterogeneous nucleation. The limitations of binary homogeneous nucleation theory at extremely low concentrations of one of the components are discussed and it is shown that this theory becomes inapplicable if the actual vapor pressure of one component is below 10^?6 torr.