POST-FOG NUCLEATION OF H2SO4-H2O PARTICLES IN SMOG

Homogeneous nucleation of sulfuric acid and water associated with urba n fogs was examined. We approached the problem by analyzing the factor s determining the gas-phase sulfuric acid concentration, and thereby i dentifying the ambient conditions where new particle production is exp ected to occur. We showed that H2SO4(g) concentration is in steady sta te with respect to its production and depletion via condensation in ur ban environment. Based on this, a simple formula was derived for estim ating the nucleation probability from temperature, fine dry aerosol ma ss, and SO2(g) concentration and its oxidation rate during the post-fo g conditions. The major uncertainty is associated with the value of th e accommodation coefficient for sulfuric acid condensation on water. N ucleation was shown to be most favorable around 90% relative humidity after the dissipation of fog. We calculated a very high nucleation pro bability associated with fog observed in the San Joaquin Valley of Cal ifornia. In the South Coast Air Basin of Los Angeles, post-fog nucleat ion was predicted to occur frequently in Hawthorne and Long Beach, and occasionally at several other locations.