Seasonal variation of ¹⁵N/¹⁴N ratios in atmospheric nitrate species

Nitrate in rain sampled in moderately polluted areas at Jülich/Germany and Ahrensburg/Germany, as well as in a rural area at Deuselbach/Germany and in a coastal area at Le Conquet/France, shows a pronounced seasonal variation in its ¹⁵N/¹⁴N ratio with lower data in summer (δ¹⁵N =— (5 ± 1)‰; mean between the stations) than in winter (δ¹⁵N = (0 ± 2)‰)). Particulate nitrate trapped in low-and high-volume sampling at Jülich shows a similar seasonal 15N/14N variation (δ¹⁵N of about + 3‰ and + 7‰ in summer and winter, respectively); the absolute data however differ from those in the rain samples. Nitric acid vapour, on the other hand, exhibits nearly constant 15N/14N data over the seasons (δ¹⁵N = - (2-3)‰). The data are discussed considering isotope exchange reactions and kinetic fractionations involved in transformations of atmospheric oxidized nitrogen species. Full explanations must await further measurements.