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 Wesely's parameterisation were defined for a vegetation growing cycle different from that
prevailing in our conditions.
