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 (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 O-3/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 (delta N-15, delta O-18, and Delta O-17). The Delta O-17 values of NO2 trapped at our site in Grenoble, France, show a large diurnal cycle peaking in late morning at (39.2 +/- 0.3)parts per thousand and decreasing at night until (20.5 +/- 0.3)parts per thousand. On top of this diurnal cycle, Delta O-17 also exhibits substantial daytime variability (from 29.7 parts per thousand to 39.2 parts per thousand), certainly driven by changes in the O-3 to peroxyl radicals (RO2) ratio. The nighttime decay of Delta O-17(NO2) appears to be driven by NO2 slow removal, mostly from conversion into N2O5, and its formation from the reaction between O-3 and freshly emitted NO. As expected from a nighttime Delta O-17(NO2) expression, our Delta O-17(NO2) measured towards the end of the night is quantitatively consistent with typical values of Delta O-17(O-3). Daytime N isotope fractionation is estimated using a general expression linking it to Delta O-17(NO2). An expression is also derived for the nighttime N isotope fractionation. In contrast to Delta O-17(NO2), delta N-15(NO2) measurements exhibit little diurnal variability (-11.8 parts per thousand to -4.9 parts per thousand) 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 mixingmodel, indicating the predominance of traffic emissions in this area. These preliminary results are very promising for using the combination of Delta O-17 and delta N-15 of NO2 as a probe of the NOx sources and fate and for interpreting nitrate isotopic composition records.