Binary homogeneous nucleation as a mechanism for the formation of aerosols

Abstract

The rates of nucleation of liquid aerosols from the gaseous maxtures 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 necleation. 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 condentrations of nitric acid which are much too high to be found under normal atmospheric conditions. On the other hand, the concentrations of sulfuric acid 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 versus 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. A generalized version of the Kelvin equation which includes the composition dependence of the surface tension for a binary mixture is derived and applied in calculating these growth curves. Sign errors in the second derivatives of the free energy in Doyle's paper are corrected. The limitations of the 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.