PHOTOCHEMICAL OXIDANT FORMATION OVER SOUTHERN SWITZERLAND .2. MODEL RESULTS

Measurements of photooxidants made during summer 1994 in the southern part of Switzerland are compared with results of a one-dimensional Lag rangian model. The agreement between modeled and measured ozone concen trations is good. Concentrations of highly soluble species such as H2O 2 and HNO3 are overestimated by the model. The model underestimates co ncentrations of isoprene, HCHO, and HONO. The impact of this discrepan cy on the question of NOx versus VOC limitation of the ozone productio n is discussed. According to the model, reduction of the NOx emissions is more effective in reducing O-3 than reduction of the VOC emissions for a large fraction of the area under investigation. This finding is in accordance with what we concluded from the data. The southern part of Switzerland is often impacted by a plume originating from Milan, w hich leads to additional ozone at the measuring site. We used the mode l to simulate the influence of this plume on the chemical regime. The plume emissions are picked up by the model at two different times. Bot h cases deliver the same concentrations for the end point of the calcu lations. If the emissions are picked up early in the morning, the ozon e production becomes VOC sensitive for a few hours before switching ba ck to being NO, sensitive. If the emissions are picked up later, the O -3 production stays NOx sensitive all the time. Forcing the model to r eproduce measured concentrations of HONO and isoprene shifts the O-3 p roduction toward being more NOx sensitive.