Dry deposition of ozone and sulphur dioxide over low vegetation in moderate southern European weather conditions. Measurements and modelling

Abstract

Long-term measurements of SO 2 and O 3 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 SO 2. 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 Rc values in good agreement with the observed values. For SO 2, 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.