Estimating the uptake of traffic-derived NO2 from 15N abundance in Norway spruce needles

The 15N ratio of nitrogen oxides (NOx) emitted from vehicles, measured in the air adjacent to a highway in the Swiss Middle Land, was very high [δ15N(NO2) = +5.7‰]. This high 15N abundance was used to estimate long-term NO2 dry deposition into a forest ecosystem by measuring δ15N in the needles and the soil of potted and autochthonous spruce trees [Picea abies (L.) Karst] exposed to NO2 in a transect orthogonal to the highway. δ15N in the current-year needles of potted trees was 2.0‰ higher than that of the control after 4 months of exposure close to the highway, suggesting a 25% contribution to the N-nutrition of these needles. Needle fall into the pots was prevented by grids placed above the soil, while the continuous decomposition of needle litter below the autochthonous trees over previous years has increased δ15N values in the soil, resulting in parallel gradients of δ15N in soil and needles with distance from the highway. Estimates of NO2 uptake into needles obtained from the δ15N data were significantly correlated with the inputs calculated with a shoot gas exchange model based on a parameterisation widely used in deposition modelling. Therefore, we provide an indication of estimated N inputs to forest ecosystems via dry deposition of NO2 at the receptor level under field conditions

Atmospheric near surface nitrate at coastal Antarctic sites

Records of atmospheric nitrate were obtained by year-round aerosol sampling at Neumayer and Dumont D'Urville stations, located in the Atlantic and Pacific sector of coastal Antarctica, respectively. Where possible, evaluation of the nitrate records is mainly based on concurrently measured radioisotopes (10Be, 7Be, 210Pb) as well as δ15N in nitrate nitrogen. Observations made at these (and two other coastal Antarctic sites [Savoie et al., 1993]) reveal a uniform nitrate background near 10 ng m−3 persisting throughout coastal Antarctica between approximately April and June. The dominant seasonal nitrate maximum, which occurred between spring and midsummer and ranged from 20 to 70 ng m−3, tended to increase with latitude. An estimate based on Neumayer mineral dust concentrations suggests that an average of less than 5% of the observed atmospheric nitrate load may be associated with continental tropospheric sources, while a separate estimate based on 210Pb records implies a much higher proportion of up to 60%. Stratospheric nitrate influx rates seen at coastal sites, deduced from Neumayer 10Be/7Be records for stratospheric air mass intrusions and from tritium for the sedimentation of polar stratospheric clouds (PSC), exceed the theoretical stratospheric odd nitrogen production rate from N2O oxidation by almost a factor of 5 and are found to be in close agreement with the observed surface nitrate flux, implying again that the continental source contribution is relatively unimportant. Consideration of nitrate reemission from near-surface snow layers reveals a minor effect of this flux on the global Antarctic troposphere but possibly a substantial influence on the nitrate load of a persistent surface inversion layer. Evaluation of the mean seasonal nitrate pattern, based on concurrent 10Be, 210Pb, and δ15N records at Neumayer and on tritium in precipitation at Halley, suggests that the period of significant enhancement above the background mainly reflects inputs of stratospheric nitrate with secondary peaks in winter and late summer most likely dominated by PSC sedimentation and stratospheric air mass intrusions, respectively