On the nonlinearity of the tropospheric ozone production

The relationship of photochemical ozone production versus photochemical loss of an ozone precursor, that is, either NO_x 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 NO_x, 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 NO_x. 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 NO_x 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 NO_x. 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.