Diurnal variations in oxygen and nitrogen isotopes of atmospheric nitrogen dioxide and nitrate: implications for tracing NOx oxidation pathways and emission sources

Abstract

The oxygen (î"17O) and nitrogen (15N) 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 î"17O and 15N in NO2 (nCombining double low line16) and NO3- (nCombining double low line14). The measurements were carried out at sub-daily (3h) 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. î"17O 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 î"17O(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 î"17O(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 î"17O(NO3-) do not match, particularly daytime values; the possible effects on î"17O(NO3-) of a Saharan dust event that occurred during this sampling period and of winter boundary layer dynamics are discussed. 15N 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 15N(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. 15N(NO3-) is closely linked to 15N(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 î"17O and 15N in NO2 and NO3- in order to better constrain quantitative inferences about the sources and formation chemistry of NO3- in urban environments in winter.