The potential for atmospheric mixing processes to enhance the binary nucleation rate

Abstract

The formation of sulfate aerosol particles due to atmospheric mixing processes is investigated using a classical model for binary nucleation. The nucleation rate is seen to be enhanced when two air parcels with different temperature and relative humidity mix with each other. This is due to the curvature on the vapor pressure diagram, and the whole process is more enhanced in the binary H₂SO₄ ‐ H₂O system when compared with the unary case. If the differences are, for example, 8 K and 60%, the nucleation rate can increase by 2 to 3 orders of magnitude if they are mixed. A brief survey of atmospheric situations that could favor this process is included. The negative feedback effects of coagulation and condensation on the new aerosol particles are found to decelerate the particle formation, but not to prevent it. The damping effect of condensation on preexisting aerosols is examined. It is found that the mixing process is most likely to be important at background conditions, which are the most difficult for explaining nucleation. The mixing effect is also compared with the effect of fluctuations in temperature and relative humidity. In some cases the mixing effect seems to be the most important of the effects. The differences in the effect of mixing on nucleation rate and the limitations of its validity are given for different initial conditions. A simple parameterization of the effect of atmospheric mixing on the binary homogeneous nucleation rate of H₂SO₄ and H₂O is also given.