Variations in the concentration and isotopic composition of nitrate nitrogen in wet deposition and their relation with meteorological conditions in Xi’an city, Northwest China

The characterization of N isotopes in NO 3 - is an effective method to determine NO 3 - sources and NOX transformation mechanisms. Xi’an city is located in a semi-arid to arid region and is representative of most cities in the north and NW of China. Few studies have been carried out on N isotopes in wet deposition of this region. In this study, 29 wet deposition samples were collected from Xi’an city in northwestern China between March 2008 and May 2009 and analyzed for their volume-weighted NO 3 - and NH 4 + concentrations and N isotopic compositions. Volume-weighted mean concentrations were 3.9 μmol L−1 for NO 3 - and 13.5 μmol L−1 for NH 4 + . The wet deposition samples showed pronounced seasonal variation in the N isotopic composition of NO 3 - with δ 15 N – NO 3 - ranging from −2.0‰ to +9.2‰ over the course of a year, with a mean value of +2.6 ± 2.6‰. Higher δ 15 N – NO 3 - values were observed in the cool season (November–May, +3.8‰) than in the warm season (June–October, +1.0‰). In autumn and winter in particular, the mean δ 15 N – NO 3 - values were clearly different (+0.9‰ and +6.7‰, respectively). Various factors are shown to lead to variation in the δ 15 N – NO 3 - values in wet deposition. One effect is changing ratios of N derived from agricultural/soil sources (low δ 15 N – NO 3 - , dominant in the warm season) and fossil fuel-derived NOX (high δ 15 N – NO 3 - , relatively more important in the cool season). However, in the cool season δ 15 N – NO 3 - unexpectedly shows a strong negative correlation with SO2 concentration, which should correlate with fossil fuel combustion inputs. This may be explained by interactions of N-species with other atmospheric pollutants. The results also suggest that temperature may be a significant factor affecting δ 15 N – NO 3 - . The NOX reacts in different ways depending on temperature, leading to a correlation of δ 15 N – NO 3 - with temperature. Moreover, the effects of dilution cannot be neglected here, as there is clear seasonal variation in precipitation. When combined with meteorological parameters, the volume-weighted concentrations of NO 3 - and NH 4 + and air pollutant precursor data, the δ15N of NO 3 - in wet deposition can be used as a reliable tool for the precise identification of NOX transformation mechanisms in the environment and the fate of NOX emissions.