Dry Deposition Of SO2 Research Articles

Dry and wet removal of sulphur from the atmosphere

Dry deposition and removal in precipitation of SO 2 and of paniculate sulphate are considered in turn. Many assessments of the dry deposition of SO 2 to various surfaces give deposition velocities of about 0.8cm s −1, although variations with season and weather conditions are important. The deposition velocity of the sulphate aerosol is probably about 0.l cm s −1. Removal of SO 2 in rain is also a rather inefficient process and theoretical and experimental results suggest that the sulphur in precipitation results chiefly from the rainout of cloud condensation nuclei. The removal time constants for SO 2 and sulphate by moderate rain are probably of order 10 −5 and 10 −4s −1 respectively. A much simplified model suggests that about a half of the SO 2 emitted to the atmosphere is removed by dry deposition, the remainder is oxidised to sulphate and removed in precipitation and the atmospheric residence time is about 5 days for sulphur. The method and climatological statistics for a more realistic treatment do not yet appear to be available.

Factors affecting dry deposition of SO 2 on forests and grasslands

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.

The deposition of sulphur dioxide to pine forest assessed by a radioactive tracer method

Pine shoots, twigs and bark were exposed to ³⁵SO₂ in the field using a small exposure chamber. The rate of uptake by pine needles was found to be proportional to their conductivity for water vapour, indicating stomatal control of SO₂ exchange. Other live surfaces absorbed negligible amounts but considerable uptake occurred on dead twigs bearing lichen and algae. The results were used to predict that the deposition velocity for SO₂ to a pine forest canopy varies from about 0.1 cm s^-1 at night to a daytime maximum value of 0.6 cm s^-1. The rate of uptake may be an order of magnitude faster when the canopy is wet. Dry deposition of SO₂ is probably the major mechanism for sulphur input to forests in Europe.