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- Evaluation of SO2 dry deposition over short vegetation in PortugalPublication . Feliciano, Manuel; Pio, Casimiro; Vermeulen, AlexSO2 dry deposition was studied over short vegetation, in Portugal, by means of the concentration gradient method. The experimental study involved one first phase of long-term measurements carried out in a grassland and, subsequently, a second period of several 1997 intensive field campaigns performed in three places representing different climate and surface conditions. Temporal and spatial patterns of dry deposition parameters show that downward fluxes of SO2 are by some extent affected by surface processes. Median Rc varied from 140 s cm-1 to values around 200 s cm-1, in a wide range of environmental conditions. Stomatal uptake is an important sink when vegetation is biologically active, but its contribution is effectively low when compared with non-stomatal mechanisms, especially when the surface is wet. Under dry conditions Rc increases by a factor of two, but SO2 deposition rates then still are signifficant. The parameterisation of the surface resistance for SO2 proved to be difficult, but Vd derived with the Erisman parameterisation (Erisman et al., Atmos. Environ. 28 (16) (1994) 2595) compared best with measured values, at low time resolution scale and especially under moisture conditions.
- Dry deposition of ozone and sulphur dioxide over low vegetation in moderate southern European weather conditions. Measurements and modelingPublication . Pio, Casimiro; Feliciano, ManuelLong-term measurements of SO2 and O3 dry deposition were carried out during one year over a grass field, located in the west coast of Portugal. An automatic system based on micrometeorological techniques provided, on a routine basis, an important data set for both gases. The results show strong daily and seasonal cycles in dry deposition velocity and surface resistance, for ozone, being also verified a seasonal trend for SO2. For both gases maximum Rc values were observed in summertime with the minimum values occurring in winter and spring. These differences match with surface conditions. In Portugal the vegetation reaches its maximum of activity in spring and becomes partially dry in the end of summer, after several months without rainfall. The stomatal intake seems to be the most important controlling factor on dry deposition, specially for ozone. A parameterization of surface resistance for ozone based on conclusions from recent dry deposition studies yields Re values in good agreement with the observed values. For SO2, Rc values are influenced by other factors besides stoma opening such as surface wetness, being, therefore, more difficult to evaluate the applicability of existent parameterization schemes.
- Seasonal variability of ozone dry deposition under southern European climate conditions, in PortugalPublication . Pio, Casimiro; Feliciano, Manuel; Vermeulen, Alex; Sousa, ElsaOzone 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.