Trace gas exchange above the floor of a deciduous forest: 2. SO₂ and O₃ deposition

The pollutant uptake capacity of the surface beneath forest canopies is poorly understood. In this study we show that the eddy correlation method can be applied within a canopy. Direct measurements of SO 2 and O 3 deposition to the forest floor of two deciduous forests were made with the eddy correlation method. For dry periods the surface uptake resistance for SO 2 was of the order of 700 s m −1 . When the soil and litter were moist, the SO 2 surface uptake resistance was small, resulting in deposition velocities at the forest floor that were about 2 times greater than during dry periods. SO 2 deposition to the forest floor constitutes between 20 and 30% of total uptake to the forest ecosystem when the surface is moist. Soil surface uptake resistances for O 3 were about 2000 s m −1 for both wet and dry periods.