Dry deposition of nitrogen dioxide and ozone to coniferous forests

Abstract

The exchange of NO2, NO, and O3 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, Jädraås (Scots pine) and Simlångsdalen (Scots pine and Norway spruce). In Simlångsdalen, the flux of NO2 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 NO2 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 Jädraås are slightly different; the deposition velocity of NO2, at concentrations lower than 1 ppbv, was always lower than the stomatal conductance and net emissions of NO2 were observed at concentrations below 0.5–0.7 ppbv. The average deposition velocity per projected needle area observed in Jädraås was 0.8 ± 0.7 mm s−1. In only a few cases, both for Jädraås and Simlångsdalen, the uptake of NO2 was limited by mesophyllic resistance. The difference between Simlångsdalen and Jädraås 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 O3 varied typically from 0 to 15.7 mm s−1 in Jädraås and between 0 and 9.0 mm s−1 in Simlångsdalen, with average values of 3.1 and 2.5 mm s−1, respectively. The ratio between the deposition velocity for O3 and stomatal conductance was in general larger than one, indicating deposition of O3 to the external surfaces of the vegetation. The residual deposition of O3 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 NO2 and O3. The dry deposition of NO2 to the forest floor is of the same magnitude as the deposition to the canopy. The average deposition velocity of NO2 to the forest floor in Jädraås was equal to 4 mm s−1. The maximum contribution from the dry deposition of NO2 to the total deposition of oxidized nitrogen compounds to the forest was estimated to be about 5% and 30% (as nitrogen) for Jädraås and Simlångsdalen, respectively. The emission flux of NO from the forest soil is at least 4 times smaller than the deposition rate of NO2 to the floor.