POST-FOG NUCLEATION OF H2SO4-H2O PARTICLES IN SMOG

Homogeneous 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 accommodation 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.