Measurement report: Nitrogen isotopes (<i>δ</i>¹⁵N) and first quantification of oxygen isotope anomalies (<i>Δ</i>¹⁷O, <i>δ</i>¹⁸O) in atmospheric nitrogen dioxide

Abstract

The isotopic composition of nitrogen and oxygen in nitrogen dioxide (NO2) potentially carries a wealth of information about the dynamics of the nitrogen oxides (NOx = nitric oxide (NO) + NO2) chemistry in the atmosphere. While nitrogen isotopes of NO2 are subtle indicators of NOx emissions and chemistry, oxygen isotopes are believed to reflect only the O3 / NOx / VOC chemical regime in different atmospheric environments. In order to access this potential tracer of the tropospheric chemistry, we have developed an efficient active method to trap atmospheric NO2 on denuder tubes and measured, for the first time, its multi-isotopic composition (δ15N, δ18O, and Δ17O). The Δ17O values of NO2 trapped at our site in Grenoble, France, show a large diurnal cycle peaking in late morning at (39.2 ± 0.3) ‰ and decreasing at night until (20.5 ± 0.3) ‰. On top of this diurnal cycle, Δ17O also exhibits substantial daytime variability (from 29.7 ‰ to 39.2 ‰), certainly driven by changes in the O3 to peroxyl radicals (RO2) ratio. The nighttime decay of Δ17O(NO2) appears to be driven by NO2 slow removal, mostly from conversion into N2O5, and its formation from the reaction between O3 and freshly emitted NO. As expected from a nighttime Δ17O(NO2) expression, our Δ17O(NO2) measured towards the end of the night is quantitatively consistent with typical values of Δ17O(O3). Daytime N isotope fractionation is estimated using a general expression linking it to Δ17O(NO2). An expression is also derived for the nighttime N isotope fractionation. In contrast to Δ17O(NO2), δ15N(NO2) measurements exhibit little diurnal variability (−11.8 ‰ to −4.9 ‰) with negligible isotope fractionations between NO and NO2, mainly due to high NO2 / NOx ratios, excepted during the morning rush hours. The main NOx emission sources are estimated using a Bayesian isotope mixing model, indicating the predominance of traffic emissions in this area. These preliminary results are very promising for using the combination of Δ17O and δ15N of NO2 as a probe of the NOx sources and fate and for interpreting nitrate isotopic composition records.