Two case studies on the interaction of large-scale transport, mesoscale photochemistry, and boundary-layer processes on the lower tropospheric ozone dynamics in early spring

The vertical distribution of ozone in the lower troposphere over the Swiss Plateau is investigated in detail for two episodes in early spring (Februar y 1998 and March 1999). Profile measurements of boundary-layer ozone perfor med during two field campaigns with a tethered balloon sounding system and a kite are investigated using regular aerological and ozone soundings from a nearby site, measurements from monitoring stations at various altitudes, backward trajectories, and synoptic analyses of meteorological fields. Addi tionally, the effect of in situ photochemistry was estimated for one of the episodes employing the Metphomod Eulerian photochemical model. Although th e meteorological situations were completely different, both cases had eleva ted layers with high ozone concentrations, which is not untypical for late winter and early spring. In the February episode, the highest ozone concent rations of 55 to 60 ppb, which were found at around 1100 m asl, were partly advected from Southern France, but a considerable contribution of in situ photochemistry is also predicted by the model. Below that elevation, the lo cal chemical sinks and surface deposition probably overcompensated chemical production, and the vertical ozone distribution was governed by boundary-l ayer dynamics. In the March episode, the results suggest that ozone-rich ai r parcels, probably of stratospheric or upper tropospheric origin, were adv ected aloft the boundary layer on the Swiss Plateau.