Post-fog nucleation of H₂SO₄H₂O particles in smog

Homogenous nucleation of sulfuric acid and water associated with urban fogs was examined. We approached the problem by analyzing the factors determining the gas-phase sulfuric acid concentration, and thereby identifying the ambient conditions where new particle production is expected to occur. We showed that H₂SO₄(g) concentration is in steady state with respect to its production and depletion via condensation in urban environment. Based on this, a simple formula was derived for estimating the nucleation probability from temperature, fine dry aerosol mass, and SO₂(g) concentration and its oxidation rate during the post-fog conditions. The major uncertainty is associated with the value of the accomodation coefficient for sulfuric acid condensation on water. Nucleation was shown to be most favorable around 90% relative humidity after the dissipation of fog. We calculated a very high nucleation probability associated with fog observed in the San Joaquin Valley of California. In the South Coast Air Basin of Los Angeles, post-fog nucleation was predicted to occur frequently in Hawthorne and Long Beach, and occasionally at several other locations.