Isotopic composition (δ15N, δ18O) of nitrate in high-frequency precipitation events differentiate atmospheric processes and anthropogenic NOx emissions

Abstract

Reactive nitrogen (Nr) species from the atmosphere contribute to adverse impacts like acid deposition and eutrophication; thus, information about local or distal emission N sources towards wet deposition of nitrogen oxides (NOx) is important information. Given N emissions source regions and atmospheric chemistry processes can change on an hourly basis, a thorough assessment of Nr removal from the atmosphere requires high resolution precipitation sampling. We measured nitrate isotopes (δ15Ν, δ18O of NO3−) and δ18Ο-H2O of all precipitation events at high-frequency (5–30 min) for Vienna, Austria, in 2019. Average monthly δ15N of NO3− ranged from −7.0‰ (AIR) to −0.1‰ with lowest and higher values in summer and winter, respectively. Similarly, the average δ18O of NO3− was highest in winter (+75.0‰, VSMOW) and lowest in summer (+61.2‰). Nitrate in nighttime precipitation and intense rainouts had higher δ15N-NO3− values (−4.4‰ and − 4.6‰, respectively) compared to daytime and washout samples (−5.7‰ and − 5.5‰). Atmospheric oxidation and cloud scavenging on a diel basis were correlated to air temperature, with an opposite non-linear effect between the δ18O of NO3− and H2O. The δ15N of NO3− showed a negative correlation with rain intensity, but a positive relationship to the number of days without rain prior to a precipitation event. Overall, nitrate source partitioning with and without photochemical equilibrium isotope fractionation suggested strong year-round influences from vehicle emissions and biomass burning by day and night. Biogenic, fertilized soil, and coal combustion N sources (power plants) showed a lower contribution to precipitation NO3− and was linked to both harvest periods and long-range transport of air masses from North-Eastern and southern Europe. N emissions from local municipal waste incineration were significant in the autumn. Our study reveals that nitrate isotopes in wet deposition combined with δ18O-H2O contribute new information to inform NOx source management strategies aimed at improving environmental air and water quality.