A dynamic model for the production of H⁺ NO₃⁻, and SO₄²⁻ in urban fog

The chemical composition of nighttime urban fog has been investigated using a hybrid kinetic and equilibrium model. Extremely high acidity may be imparted to the droplets by condensation and growth on acidic condensation nuclei or by in situ S(IV) oxidation. Important oxidants of S(IV) were found to be O 2 as catalyzed by Fe(III) and Mn(II), H 2 O 2 , and O 3 . Formation of hydroxymethanesulfonate ion (HMSA) via the nucleophilic addition of HSO 3 − to CH 2 O(1) significantly increased the droplet capacity for S(IV) but did not slow down the net S(IV) oxidation rate leading to fog acidification. Gas phase nitric acid, ammonia, and hydrogen peroxide were scavenged efficiently, although aqueous phase hydrogen peroxide was depleted rapidly by reduction with S(IV). Nitrate production in the aqueous phase was found to be dominated by HNO 3 gas phase scavenging. Major aqueous phase species concentrations were controlled primarily by condensation, evaporation, and p H.