Long-term (13 years) measurements of SO2 fluxes over a forest and their control by surface chemistry

Long-term fluxes of sulphur dioxide (SO2) have been measured over a mixed suburban forest subjected to elevated SO2 concentrations. The net exchange was shown to be highly dynamic with substantial periods of both upward and downward fluxes observed in excellent conditions for flux measurement. Upward fluxes constituted 30% of selected fluxes and appeared more frequently when the canopy was acidic. Upward fluxes were shown to be due to desorption from a drying surface or when ambient levels declined after periods of increased SO2 exposure. The long term average SO2 flux (F) was -59 ng SO2 m-2 s-1 for the period 1997-2009 corresponding to an average SO2 concentration of 12.3 μg SO2 m-3 and a deposition velocity υd of 5 mm s-1. The smallest deposition fluxes and υd were measured in dry conditions (-42 ng m-2 s-1 and 3.5 mm s-1, resp.), which represented 57% of all cases. Wet canopies were more efficient sinks for SO2 and a dew-wetted canopy had a smaller υd (6 mm s-1) than a rain-wetted canopy (ca 10 mm s-1). Seasonal variability reflected differences in chemical climate or canopy buffering properties. During the summer half-year when surface acidity was low due to higher NH3/SO2 ratios, a higher deposition efficiency (υd/υdmax) and lower non-stomatal resistance (Rw) were observed compared to winter conditions. Comparisons of Rc for different combinations of canopy wetness and surface acidity categories emphasized the importance of both factors in regulating the non-stomatal sinks of SO2. Increased surface water acidity gradually led to a lower υd/υdmax and an increased Rc for all considered canopy wetness categories. The smallest υd/υdmax ratio and highest Rc were obtained for a dry canopy with high surface acidity. Conversely, a rain-wetted canopy was the most efficient sink for SO2. The canopy sink strength was further enhanced by high friction velocities (u*), optimizing the mechanical mixing into the canopy. Long-term trends were strongly coupled to changes in the NH3/SO2 ratio, which has clearly enhanced the deposition efficiency of SO2 in recent years.