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

Abstract

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.