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 diurn al and seasonal patterns in deposition flux, dry deposition velocity and su rface resistance, especially for the daytime period. Dry deposition velocit ies vary diurnally from a minimum of 0.1 cm s(-1) during the night to a max imum of 0.7-0.5 cm s(-1) during the day. The observed canopy resistance (R- c) varies from values higher than 500 s m(-1), at night, to a minimum of 20 0 s m(-1), around noon. Seasonal variation is characterised by daytime R-c values much larger in summer than in winter and spring, while nighttime val ues do not show any evident seasonal pattern. This behaviour can be ascribe d to the stomatal intake, which represents the most important controlling f actor on ozone dry deposition. The Wesely parameterisation scheme of surfac e resistance predicts R-c diurnal cycles reasonably well. However, the obse rved canopy resistance seasonal cycle is completely different from Wesely p redictions, since season parameters in Wesely's parameterisation were defin ed for a vegetation growing cycle different from that prevailing in our con ditions. (C) 1999 Elsevier Science Ltd. All rights:reserved.