A comparison of observed and parameterized SO2 dry deposition over a grassy clearing in Duke Forest

Deposition of trace gases, such as sulfur dioxide (SO2), can affect plant and soil chemistry in different ecosystems. Measurements over a complex ecosystem, like a forest clearing, are necessary to determine more accurate deposition rates that can be used to improve parameterizations and models. The flux-gradient technique was used to determine SO2 fluxes over grass in a clearing at Duke Forest, North Carolina, USA on 25 June – 2 July 2008. The mean flux was −0.037 ± 0.024 μg m−2 s−1. Dew on the canopy enhanced the uptake of SO2, which increased deposition rates. Deposition velocities (Vd) fluctuated greatly with a mean of 1.00 ± 0.48 cm s−1. The large variation in Vd was not fully captured by estimates determined from a multilayer model (MLM) and a big-leaf model (BLM). Mean deposition velocities derived from the MLM and BLM were 1.25 ± 0.21 cm s−1 and 0.63 ± 0.12 cm s−1, respectively. The model estimations of Vd in this study were probably affected by uncertainties associated with canopy resistance, particularly with stomatal and non-stomatal processes.

► Stomatal and non-stomatal resistances were significant contributors to canopy resistance for SO2 deposition.
► Deposition rates increased when the canopy was wet with rain or dew.
► Deposition velocities from a multilayer model generally agreed with observed deposition velocities.