Observation of SO2 dry deposition velocity at a high elevation flux tower over a evergreen broadleaf forest

A 60-m flux tower was built on a 2100 m mountain for the measurement of the air pollutant concentration and the evaluation of dry deposition velocity in Central Taiwan. The tower was constructed in an evergreen broadleaf forest, which is the dominant species of forest in the world. Multiple-level SO2 concentrations and meteorological variables at the site were measured from February to April 2008. The results showed that the mean dry deposition velocities of SO2 were 0.61 cm s-1 during daytime and 0.27 cm s-1 during nighttime. A tendency was observed that the dry deposition velocity increases with LAI and solar radiation from the comparison of the monthly data. Furthermore, it was observed that the deposition velocity was larger over wet canopy than over dry canopy, that higher deposition velocities in the wet season were mainly caused by non-stomatal uptake of wet canopy. Over wet canopy, the mean dry deposition velocities of SO2 were estimated to be 0.83 cm s-1 during daytime and 0.47 cm s-1 during nighttime; and 0.44 cm s-1 during daytime and 0.19 cm s-1 during nighttime over dry canopy. Comparisons of the results in this study and the predictions of Zhang et al. (2003a) are in a good agreement. The median (geometric mean) of derived rc during daytime are 233 (266) m s-1 over dry canopy and 147 (146) m s-1 over wet canopy. It was found that the solar radiation is the critical important meteorological variable determining the stomatal resistance during daytime. For non-stomatal resistance, clear dependencies were observed on the friction velocity and relative humidity.