Dry deposition to pine of sulphur dioxide and ozone at low concentration

Deposition velocities of SO2 and O3 were measured with an enclosure technique onto a stand of pine in central Sweden, mainly in autumn at low SO2 concentrations (3–5 ppbv). The deposition velocities for both gases were substantially larger than corresponding stomatal conductances, estimated from simultaneous measurements of water transpiration. The additional deposition is explained as an external deposition which was found to vary over the day with a maximum at noon and almost no deposition at night. Estimated for a dry canopy of pine (leaf area INDEX = 3 m2 m−2), the 24 h average external deposition velocity of SO2, was 3.3 mm s−1 while the stomatal conductance for the same gas was only 1.3 mm s−1. A few additional measurements made during summer indicate that the external deposition can be even larger than what was obtained during October. If so, current estimates of SO2 deposition to dry canopies of conifers based on the stomatal resistance approach need to be revised upwards.