Fast chemical reactions can cause the vertical mass flux densities of gases to change with height in the atmospheric surface layer. Theoretical and numerical studies have shown that either the reaction of NO with O3 to produce NO2 or the photodissociation of NO2 can cause large changes with height in the vertical fluxes of NO and NO2 (e.g., Lenschow, 1982; Fitzjarrald and Lenschow 1983; Kramm 1989; Gao et al. 1991, 1993; Vila-Guerau de Arllano et al. 1995, Padro et al. 1998). The flux is often affected most strongly, because its ambient concentration is typically smaller than those of the other primary reactants (NO2 and O3). The flux of O3 is usually the least affected.