The exchange of N02, NO, and 03 between the atmosphere and coniferous forests has been studied by using a dynamic flow-through chamber technique. The measurements were performed during summer at two coniferous forest sites in Sweden, Jadraas (Scots pine) and Simlangsdalen (Scots pine and Norway spruce). In Simlangsdalen, the flux of N02 was found to be quantitatively determined by the stomatal openings. Generally, the flux of NO2 was towards the vegetation surface, and only in a few cases a small emission of N02 was detected. The average deposition velocities per projected needle area were 1.5 +/- 1.3 mm s-1 for Norway spruce and 1.4 +/- 1.1 mm s-1 for Scots pine. The results from Jadraas are slightly different; the deposition velocity of N02, at concentrations lower than 1 ppbv, was always lower than the stomatal conductance and net emissions of N02 were observed at concentrations below 0.5-0.7 ppbv. The average deposition velocity per projected needle area observed in Jadraas was 0.8 +/- 0.7 mm s - 1. In only a few cases, both for Jadraas and Simlangsdalen, the uptake of N02 was limited by mesophyllic resistance. The difference between Simlangsdalen and Jadraas may be attributed to physiological variations or nutrient supply. The exchange rate of NO between the air and vegetation per projected needle area was generally found to be less than the detection limit of the method (0. 1 ng N M-2 S- 1). The deposition velocity per projected needle area for 03 varied typically from 0 to 15.7 mm s-1 in Jadraas and between 0 and 9.0 mm s-1 in Simlangsdalen, with average values of 3.1 and 2.5 mm s-1, respectively. The ratio between the deposition velocity for 03 and stomatal conductance was in general larger than one, indicating deposition Of 03 to the external surfaces of the vegetation. The residual deposition Of 03 showed a marked diurnal variation with maximum values around noon. It seems to follow a complex mechanism in which several factors such as light intensity, temperature, humidity and chemical properties of the surface of the cuticle might be important. No significant differences between Norway spruce and Scots pine species were observed in terms of the dry deposition of N02 and 03. The dry deposition of N02 to the forest floor is of the same magnitude as the deposition to the canopy. The average deposition velocity of N02 to the forest floor in Jadraas was equal to 4 mm s - 1. The maximum contribution from the dry deposition of N02 to the total deposition of oxidized nitrogen compounds to the forest was estimated to be about 5% and 30% (as nitrogen) for JadraAs and Simlangsdalen, respectively. The emission flux of NO from the forest soil is at least 4 times smaller than the deposition rate of N02 to the floor.
