Diurnal variations in oxygen and nitrogen isotopes of atmospheric nitrogen dioxide and nitrate: implications for tracing NO_<i>x</i> oxidation pathways and emission sources

Abstract

The oxygen (Δ17O) and nitrogen (δ15N) isotopic compositions of atmospheric nitrate (NO3-) are widely used as tracers of its formation pathways, precursor (nitrogen oxides (NOx) ≡ nitric oxide (NO) + nitrogen dioxide (NO2)) emission sources, and physico-chemical processing. However, the lack of observations on the multi-isotopic composition of NO2 perpetuates significant uncertainties regarding the quantitative links between the isotopic composition of NOx and NO3-, which ultimately may bias inferences about NO3- formation processes and the distribution of sources, particularly in winter urban atmospheres. We report here on the first simultaneous atmospheric observations of Δ17O and δ15N in NO2 (n=16) and NO3- (n=14). The measurements were carried out at sub-daily (∼3 h) resolution over 2 non-consecutive days in an Alpine city in February 2021. A strong diurnal signal is observed in both NO2 and NO3- multi-isotopic composition. Δ17O of NO2 and NO3- ranges from 19.6 ‰ to 40.8 ‰ and from 18.3 ‰ to 28.1 ‰, respectively. During the day and night, the variability in Δ17O(NO2) is mainly driven by the oxidation of NO by ozone, with a substantial contribution from peroxy radicals in the morning. NO3- mass balance equations, constrained by observed Δ17O(NO2), suggest that during the first day of sampling, most of the NO3- was formed locally from the oxidation of NO2 by hydroxyl radicals by day and via heterogeneous hydrolysis of dinitrogen pentoxide at night. For the second day, calculated and observed Δ17O(NO3-) do not match, particularly daytime values; the possible effects on Δ17O(NO3-) of a Saharan dust event that occurred during this sampling period and of winter boundary layer dynamics are discussed. δ15N of NO2 and NO3- ranges from −10.0 ‰ to 19.7 ‰ and from −4.2 ‰ to 14.9 ‰, respectively. Consistent with theoretical predictions of N isotope fractionation, the observed variability in δ15N(NO2) is explained by significant post-emission equilibrium N fractionation. After accounting for this effect, vehicle exhaust is found to be the primary source of NOx emissions at the sampling site. δ15N(NO3-) is closely linked to δ15N(NO2) variability, bringing further support to relatively fast and local NOx processing. Uncertainties in current N fractionation factors during NO2 to NO3- conversion are underlined. Overall, this detailed investigation highlights the potential and necessity of simultaneously using Δ17O and δ15N in NO2 and NO3- in order to better constrain quantitative inferences about the sources and formation chemistry of NO3- in urban environments in winter.