2-D studies of the kinetic photochemistry of the urban troposphere. I. Air stagnation conditions

Abstract A two-dimensional model of the photochemistry of the urban troposphere has been developed to study gradients in air quality in both the vertical and horizontal directions. The calculational approach combines representative chemical kinetics (51 reactions in 44 species) with turbulent diffusion and gas-aerosol interaction processes. The study reported here simulates a 3-d air stagnation episode over an urban region in which advection is assumed to be negligible. Vertical profiles on the third day for O 3 , CH 4 , CO and SO 2 are consistent with species profiles measured in urban areas during air stagnation episodes. Concentrations of primary contaminants CO, NO x x HC and SO 2 in the lowest level increase throughout the episode. Concentrations of most photochemical products decrease throughout the episode because of decreasing HO · and HO 2 · radical concentrations. The results suggest that the largest accumulation of photochemical products may occur under conditions of moderate, but not total, air stagnation.

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