Nitrogen isotopes (δ15N) and oxygen isotope anomalies (Δ17O, δ18O) in atmospheric nitrogen dioxide : a new perspective for isotopic constraints on oxidation and aerosols formation processes

Urban polar areas can be subject to severe pollution in winter, linked to sharp temperature inversions that trap pollutants close to the surface. However, the formation of secondary aerosols (sulphates, nitrates, organics) in these cold and dark conditions and the role of the Arctic boundary layer are still poorly understood. To address this issue, an intensive international measurement campaign, called ALPACA (ALaskan Pollution And Chemical Analysis), will be conducted in January/February 2021 in and around Fairbanks, Alaska. Among the various atmospheric chemical and physical measurements, gas and particles collections will be carried out for multiple isotopic analyses.

The use of stable isotopes over the past decades has demonstrated its ability to provide information relevant for tracing emission sources, individual chemical processes and budgets of atmospheric trace gases. Of particular interest is the propagation of the ozone distinctive oxygen-17 anomaly (Δ¹⁷O) into the reactive nitrogen cycle which has led to a better understanding of nitrate formation pathways in various environments. However, there remain some difficulties to interpret the isotopic composition of the nitrate, mainly due to the lack of clearly established understanding about the link between the oxygen and nitrogen isotopic composition of the nitrogen oxides (NO_x = NO + NO₂), the precursors of nitrate in the atmosphere, and the chemical state of the atmosphere.

In order to interpret more quantitatively the fate of reactive nitrogen using isotopic records, we have developed an effective active method to trap atmospheric NO₂ on denuder tubes and have measured, for the first time, its multi-isotopic composition (δ¹⁵N, δ¹⁸O, and Δ¹⁷O). The δ¹⁵N values of NO₂ trapped at our site in Grenoble, France, show little variability (-11.8 to -4.9 ‰) with negligible N isotopefractionations during the NO and NO₂ interconversion due to high NO₂/NO_x ratios. The main sources of NO_x emissions are estimated using a stable isotope model applied to our δ¹⁵N measurements; the results indicate the predominance of traffic NO_x emissions in this area. The Δ¹⁷O values exhibit an important diurnal cycle with a late morning peak at (39.2 ± 1.7) ‰ and a night-time decrease with a late-night minimum at (20.5 ± 1.7) ‰. On top of this general diurnal cycle,  Δ¹⁷O also shows substantial variability during the day (from 29.7 to 39.2 ‰), certainly driven by changes in the O₃ to peroxyl radicals ratio. The night-time decay of Δ¹⁷O(NO₂) appears to be driven by slow removal of NO₂, mostly from its conversion into N₂O₅, and its formation from the reaction between O₃ and emitted NO. As expected, our Δ¹⁷O(NO₂) values measured towards the end of the night are quantitatively consistent with typical values of Δ¹⁷O(O₃).

These preliminary results are very promising for the use of Δ¹⁷O of NO₂ as a probe of the atmospheric oxidative activity and for the interpretation of  NO₃^- isotopic composition records. In the future, samplings and multi-isotopic analysis of atmospheric nitrate performed in parallel with those of NO₂ will be of great interest for the study of the full reactive nitrogen cycle.