The exchange of NO2, NO, and 0 3 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, J•idrafis (Scots pine) and Simlfingsdalen (Scots pine and Norway spruce). In Simlfingsdalen, the flux of NO2 was found to be quantitatively determined by the stomatal openings. Generally, the flux of NO 2 was towards the vegetation surface, and only in a few cases a small emission of NO was detected. The average deposition velocities per projected needle area were 1.5 _+ 1.3 mm s -• for Norway spruce and 1.4 m 1.1 mm s -1 for Scots pine. The results from J•idrafis 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 J•idrafis was 0.8 -+ 0.7 mm s -• . In only a few cases, both for J•idrafis and Simlf. ngsdalen, the uptake of NO 2 was limited by mesophyllic resistance. The difference between Simlfingsdalen and J•idrafis 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 2 1 detection limit of the method (0.1 ng N m-s-). The deposition velocity per projected needle area for 03 varied typically from 0 to 15.7 mm s -• in J•idra•ts and between 0 and 9.0 mm s -• in Simlfingsdalen, with average values of 3.1 and 2.5 mm s -•, 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 NO 2 and 03. The dry deposition of NO 2 to the forest floor is of the same magnitude as the deposition to the canopy. The average deposition velocity of NO 2 to the forest floor in Jgdrafis was equal to 4 mm s-1. The maximum contribution from the dry deposition of NO 2 to the total deposition of oxidized nitrogen compounds to the forest was estimated to be about 5% and 30% (as nitrogen) for J•idra•ts and Simlfingsdalen, respectively. The emission flux of NO from the forest soil is at least 4 times smaller than the deposition rate of NO 2 to the floor. 1. Several authors [Hill, 1971; Bennett and Hill, 1973; Eastman et al., 1981; Wesely, 1981; Ennis et al., 1990] have studied the possible pathways for the uptake of NO.,. and 0 3 by vegetation. The generally existing idea is that the uptake of NO,c and 03 by vegetation is mainl...

NO fluxes from soils with a wide range of soil moistures, soil inorganic‐N concentrations, and soil temperatures were measured during the wet and the dry season at a Venezuelan savannah site. Maximum NO emissions (∼12 ngN m−2 s−1) were observed at soil gravimetric moistures between 10% and 18%. Deviation from this optimum range results in decreased NO fluxes; very low emissions ( < 2 ngN m−2 s−1 ) were recorded at low ( < 2% ) and high ( > 25% ) soil moistures. Both NO production in soil and its transport within the soil play important roles in the emission of NO to the atmosphere. Under most conditions no temperature effect was observed. NO emission was strongly stimulated by the addition of NO3− and only very weakly by the addition of NH4+; at low and moderate soil moistures, soil nitrate and the NO flux were positively correlated. At low (natural) soil nitrate content and comparable soil moisture and temperature, NO emissions were greater during the dry season than during the rainy season, suggesting that other factors (i.e., soil physical structure) may also govern NO flux from savannah soil.

Measurements of the deposition of NO2 to seedlings of Scots pine and Norway spruce at ppbv and sub ppbv levels were made in the laboratory using a flow through chamber technique. The results show that NO2 deposition at air concentrations above 1 ppbv was governed by stomatal uptake. The possible influence of other factors controlling the NO2 deposition was also explored. Mean NO2 compensation points for both Scots pine and Norway spruce were lower than 0.2 ppbv, which can be considered negligible. Throughout the experiments, there was no evidence of internal resistance to NO2 uptake by the seedlings. At low NO2 concentrations (0.2–1 ppbv), the NO2 flux was generally below the detection limit of the technique.

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