On the nonlinearity of the tropospheric ozone production

The relationship of photochemical ozone production versus photochemical loss of an ozone precursor, that is, either NOx or nonmethane hydrocarbons (NMHCs), is studied by using a box model with particular emphasis on the nonlinearity problem of the relationship with respect to the concentration of the precursor. Model calculations indicate that the composition of NMHCs, the ratio of NMHCs to NOx, and the background concentrations of natural hydrocarbons, CO, and CH4 all play important roles in determining the nonlinearity of O3 production with respect to the loss of NOx. In addition, influences on the nonlinearity due to radical loss via reactions of HO2 with RO2, exchanges between PAN and NO2, and inclusion of nighttime NOx loss processes are also investigated. Mechanisms that contribute to the nonlinearity are discussed. The nonlinear property of O3 production versus loss of hydrocarbons and CO is different from that of NOx. When the sum of CO and all hydrocarbons, including CH4, natural NMHCs, and anthropogenic NMHCs, is used as the reference O3 precursor, the nonlinearity is much less pronounced for ambient conditions usually found in rural air.