Factors affecting dry deposition of SO 2 on forests and grasslands

Abstract

Deposition velocities for SO 2 over forests and grasslands are derived through a mass conservation approach utilizing established empirical relations descriptive of the atmospheric transport of a gaseous contaminant above and within a vegetational canopy. Of particular interest are situations where surface resistance to uptake is relatively small and does not overwhelm atmospheric transport in the determination of deposition. To this end, emphasis is placed on cases assuming the vegetation surface is a perfect sink or has resistance approximating minimum stomatal resistance. This study indicates that deposition velocity, ν g , is relatively insensitive to canopy height, shape, roughness length, and extinction coefficients, but ν g is relatively sensitive to atmospheric stability, zero-plane displacement, surface resistance, leaf area index, and friction velocity. It follows that the latter parameters should be reported to allow for meaningful comparisons between ν g 's determined under various experimental conditions. Using reasonable parameter choices, predictions by this method agree well with previous observations, yielding ν g in the neighborhood of 1 cm s −1 over grass (or other low-set vegetation) and a value of 2 to 3 times as great over a forest. Only if a forest canopy could be considered a perfect sink would ν g 's approaching 10 cm s −1 be expected.