Measurement report: Nitrogen isotopes (δ15N) and first quantification of oxygen isotope anomalies (Δ17O, δ18O) in atmospheric nitrogen dioxide.

The isotopic composition of nitrogen and oxygen in nitrogen dioxide (NO 2) potentially carries a wealth of information about the dynamics of the nitrogen oxides (NO x = nitric oxide (NO) + NO 2) chemistry in the atmosphere. While nitrogen isotopes of NO 2 are subtle indicators of NO x emissions and chemistry, oxygen isotopes are believed to reflect only the O 3 / NO x / 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 NO 2 on denuder tubes and measured, for the first time, its multi-isotopic composition (δ15 N, δ18 O, and Δ17 O). The Δ17 O values of NO 2 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, Δ17 O also exhibits substantial daytime variability (from 29.7 ‰ to 39.2 ‰), certainly driven by changes in the O 3 to peroxyl radicals (RO 2) ratio. The nighttime decay of Δ17 O(NO 2) appears to be driven by NO 2 slow removal, mostly from conversion into N 2 O 5 , and its formation from the reaction between O 3 and freshly emitted NO. As expected from a nighttime Δ17 O(NO 2) expression, our Δ17 O(NO 2) measured towards the end of the night is quantitatively consistent with typical values of Δ17 O(O 3). Daytime N isotope fractionation is estimated using a general expression linking it to Δ17 O(NO 2). An expression is also derived for the nighttime N isotope fractionation. In contrast to Δ17 O(NO 2), δ15 N(NO 2) measurements exhibit little diurnal variability (- 11.8 ‰ to - 4.9 ‰) with negligible isotope fractionations between NO and NO 2 , mainly due to high NO 2 / NO x ratios, excepted during the morning rush hours. The main NO x 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 Δ17 O and δ15 N of NO 2 as a probe of the NO x sources and fate and for interpreting nitrate isotopic composition records. [ABSTRACT FROM AUTHOR]