Photo-oxidant chemistry in the polluted boundary layer under changing UV-Bradiation

UV-B radiation is a driving factor for the chemistry of the polluted bounda ry layer. It is involved in the formation of radicals and consequently infl uences the formation and concentration of photo-oxidants. The 3-D mesoscale photochemical Metphomod model was employed to study the effect of changes in UV-B radiation on the concentration of photo-oxidants in the boundary la yer over the Swiss Plateau. The model chemistry is based on the RACM mechan ism and a two-stream approximation of radiative transfer. A summer (July) a nd a late winter (February) episode were simulated. All simulations were re plicated with relatively large changes in the prescribed total ozone. The r esults for an increase in UV-B radiation show increases in PAN, HNO3, and o zone at noon in NOx-rich areas and a decrease in NOx. In NOx-poor areas in summer the effect on ozone is weak and has a negative sign, the main effect being an increase in H2O2. The spatial variability of NOx concentrations i n the Swiss Plateau in the summer case is such that the effect of increased UV-B radiation on ozone is spatially variable. The effect on the ozone pro duction rate in summer is strongest positive at the surface in the NOx-rich regions in the morning and strongest negative at some altitude above groun d in NOx-poor regions in the early afternoon. In the winter episode, NOx-ri ch conditions are found almost everywhere on the Swiss Plateau, the effect of increased UV-B radiation on the ozone production rate is positive all da y long and is largest at 300 m above ground at noon. In this case, in contr ast to the summer case, the increase in ozone is carried over to the next d ay. The model results for ozone are in good agreement with results from a c ase study and a time series analysis of surface ozone measurements. We esti mate the effect of day-to-day changes in total ozone on surface ozone peaks to range from 4 to 6 ppb at most. (C) 2001 Elsevier Science Ltd. All right s reserved.