Isotopic composition (δ15N, δ18O) of nitrate in high-frequency precipitation events differentiate atmospheric processes and anthropogenic NOx emissions
2022
Matiatos, Ioannis | Wassenaar, Leonard I. | Monteiro, Lucilena R. | Terzer-Wassmuth, Stefan | Douence, Cedric
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.
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