A numerical study of the effects of air chemistry on fluxes of NO, NO₂, and O₃ near the surface

Abstract

A numerical model was developed to investigate modifications of the vertical fluxes of NO, NO 2 , and O 3 by rapid atmospheric chemical reactions in a horizontally homogeneous, neutrally stratified atmospheric surface layer. The one‐dimensional model coupled second‐order budget equations for fluxes with conservation equations for concentrations and incorporated simplified homogeneous gas kinetics describing the photolysis of NO 2 and the recombination of NO and O 3 . The effects of hydrocarbons were ignored. Simulations were carried out for representative chemical concentrations at the top boundary, moderate atmospheric turbulent mixing, surface removal rates typical for vegetation in the daytime, and a range of NO emission rates from the surface. Results indicate that the effective eddy diffusivity for NO and NO 2 can be significantly altered by chemical reactions and that the fluxes of NO and NO 2 can vary strongly with height but that the impact of rapid atmospheric chemical reactions on O 3 is small relative to its total flux. When the NO 2 ‐to‐NO concentration ratio aloft is greater than the ratio for the photostationary state, photodecomposition of NO 2 dominates the reaction cycle. In this case, NO flux increases with height even when the flux at the surface is zero, while the downward NO 2 flux increases in magnitude with height and the magnitude of the downward O 3 flux decreases. When the value of [NO 2 ]/[NO] aloft is smaller than the photostationary value, these trends with height are reversed. Emission of NO at the surface strongly influences NO flux near the surface, while chemical reactions tend to determine the flux at heights above a few meters. A significant effect on NO 2 occurs at the intermediate heights, but the relative change in O 3 flux is minor.