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 NO3- is an effective method to determine NO3- 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 NO3- and NH4+ concentrations and N isotopic compositions. Volume-weighted mean concentrations were 3.9 μmol L−1 for NO3- and 13.5 μmol L−1 for NH4+. The wet deposition samples showed pronounced seasonal variation in the N isotopic composition of NO3- with δ15N–NO3- ranging from −2.0‰ to +9.2‰ over the course of a year, with a mean value of +2.6 ± 2.6‰. Higher δ15N–NO3- 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 δ15N–NO3- values were clearly different (+0.9‰ and +6.7‰, respectively). Various factors are shown to lead to variation in the δ15N–NO3- values in wet deposition. One effect is changing ratios of N derived from agricultural/soil sources (low δ15N–NO3-, dominant in the warm season) and fossil fuel-derived NOX   (high δ15N–NO3-, relatively more important in the cool season). However, in the cool season δ15N–NO3- 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 δ15N–NO3-. The NOX   reacts in different ways depending on temperature, leading to a correlation of δ15N–NO3- 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 NO3- and NH4+ and air pollutant precursor data, the δ15N of NO3- 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.

► Evaluated the correlation between meteorological factors and δ15N–NO3-.
► Source changes are not the only factors to bring about δ15N–NO3- oscillation.
► Air pollutants and meteorological conditions lead to changes in the δ15N–NO3-.
► Reactions involving SO2 lead to negative correlation between δ15N–NO3- and SO2.