The deposition of sulphur dioxide to pine forest assessed by 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 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. (Auth.)