Deposition velocity of O3 and SO2 in the dry and wet season above a tropical forest in northern Thailand

In order to investigate ozone (O3) and sulfur dioxide (SO2) dry deposition above a forest in a tropical savanna climate in Southeast Asia, field experiments were performed in a teak deciduous forest in Mea Moh, Lampang Province, located in northern Thailand. O3 and SO2 fluxes were observed on the basis of the aerodynamic gradient method. The experimental period from January to August in 2004 covered the dry (Jan–Apr) and wet (May–Aug) seasons. Both gas concentrations increased in the dry season and decreased in the wet season. Interval estimation of average with a confidence interval of 95% on the deposition velocity data between 25 and 75 percentiles was performed. The intervals of average O3 deposition velocity were estimated to be 0.37–0.39 cm s−1 (daytime) and 0.12–0.13 cm s−1 (nighttime) in the dry season; and 0.62–0.65 cm s−1 (daytime) and 0.25–0.27 cm s−1 (nighttime) in the wet season. The intervals of average SO2 deposition velocity were estimated to be 0.10–0.31 cm s−1 (daytime) and 0.08–0.11 cm s−1 (nighttime) in the dry season; and 0.95–1.39 cm s−1 (daytime) and 0.26–0.42 cm s−1 (nighttime) in the wet season. SO2 deposition velocity in the rain period was significantly higher than that in the no-rain period. Higher deposition velocities in the wet season were mainly caused by non-stomatal uptake of wet canopy due to a lot of wet days, especially in the case of SO2. Much higher daytime deposition velocities in the wet season were additionally caused by stomatal uptake of leafy trees. The applicability of Wesely's parameterization of deposition velocity to a tropical savanna climate was examined. In both gases, the input resistance of transitional spring to the dry season and that of midsummer to the wet season could be applied as a first approximation, although strictly the parameterization of O3 non-stomatal resistance needs modification in the dry-season daytime. The applicability of some other parameterizations was also considered. Including the enhanced effect of wet canopy uptake in the non-stomatal resistance was essential in parameterizing SO2 deposition velocity in the wet season of the region.