Seasonal variability of ozone dry deposition under southern European climate conditions, in Portugal

Ozone dry deposition measurements were carried out during approximately one year over a flat grass field in Portugal. The results show prominent diurnal and seasonal patterns in deposition flux, dry deposition velocity and surface resistance, especially for the daytime period. Dry deposition velocities vary diurnally from a minimum of 0.1 cm s−1, during the night to a maximum of 0.2–0.5 cm s−1 during the day. The observed canopy resistance (Rc) varies from values higher than 500 s m−1, at night, to a minimum of 200 s m−1, around noon. Seasonal variation is characterised by daytime Rc values much larger in summer than in winter and spring, while nighttime values do not show any evident seasonal pattern. This behaviour can be ascribed to the stomatal intake, which represents the most important controlling factor on ozone dry deposition. The Wesely parameterisation scheme of surface resistance predicts Rc diurnal cycles reasonably well. However, the observed canopy resistance seasonal cycle is completely different from Wesely predictions, since season parameters in Weselyʹs parameterisation were defined for a vegetation growing cycle different from that prevailing in our conditions.