Nitrogen isotopes (&#948;15N) and oxygen isotope anomalies (&#916;17O, &#948;18O) in atmospheric nitrogen dioxide : a new perspective for isotopic constraints on oxidation and aerosols formation processes
Authors: Albertin Sarah , Bekki Slimane , Savarino Joël 
Published: March 2021

&lt;p&gt;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.&lt;/p&gt;&lt;p&gt;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 (&lt;em&gt;&amp;#916;&lt;/em&gt;&lt;sup&gt;17&lt;/sup&gt;O) 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&lt;sub&gt;x&lt;/sub&gt; = NO + NO&lt;sub&gt;2&lt;/sub&gt;), the precursors of nitrate in the atmosphere, and the chemical state of the atmosphere.&lt;/p&gt;&lt;p&gt;In order to interpret more quantitatively the fate of reactive nitrogen using isotopic records, we have developed an effective active method to trap atmospheric NO&lt;sub&gt;2 &lt;/sub&gt;on denuder tubes and have measured, for the first time, its multi-isotopic composition (&lt;em&gt;&amp;#948;&lt;/em&gt;&lt;sup&gt;15&lt;/sup&gt;N, &lt;em&gt;&amp;#948;&lt;/em&gt;&lt;sup&gt;18&lt;/sup&gt;O, and &lt;em&gt;&amp;#916;&lt;/em&gt;&lt;sup&gt;17&lt;/sup&gt;O). The &lt;em&gt;&amp;#948;&lt;/em&gt;&lt;sup&gt;15&lt;/sup&gt;N values of NO&lt;sub&gt;2&lt;/sub&gt; trapped at our site in Grenoble, France, show little variability (-11.8 to -4.9 &amp;#8240;) with negligible N isotope&lt;sub&gt;&lt;/sub&gt;fractionations during the NO and NO&lt;sub&gt;2 &lt;/sub&gt;interconversion due to high NO&lt;sub&gt;2&lt;/sub&gt;/NO&lt;sub&gt;x&lt;/sub&gt; ratios. The main sources of NO&lt;sub&gt;x&lt;/sub&gt; emissions are estimated using a stable isotope model applied to our &lt;em&gt;&amp;#948;&lt;/em&gt;&lt;sup&gt;15&lt;/sup&gt;N measurements; the results indicate the predominance of traffic NO&lt;sub&gt;x&lt;/sub&gt; emissions in this area. The &lt;em&gt;&amp;#916;&lt;/em&gt;&lt;sup&gt;17&lt;/sup&gt;O values exhibit an important diurnal cycle with a late morning peak at (39.2 &amp;#177; 1.7) &amp;#8240; and a night-time decrease with a late-night minimum at (20.5 &amp;#177; 1.7) &amp;#8240;. On top of this general diurnal cycle,&amp;#160;&amp;#160;&lt;em&gt;&amp;#916;&lt;/em&gt;&lt;sup&gt;17&lt;/sup&gt;O also shows substantial variability during the day (from 29.7 to 39.2 &amp;#8240;), certainly driven by changes in the O&lt;sub&gt;3&lt;/sub&gt; to peroxyl radicals ratio. The night-time decay of &lt;em&gt;&amp;#916;&lt;/em&gt;&lt;sup&gt;17&lt;/sup&gt;O(NO&lt;sub&gt;2&lt;/sub&gt;) appears to be driven by slow removal of NO&lt;sub&gt;2&lt;/sub&gt;, mostly from its conversion into N&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;5&lt;/sub&gt;, and its formation from the reaction between O&lt;sub&gt;3&lt;/sub&gt; and emitted NO. As expected, our &lt;em&gt;&amp;#916;&lt;/em&gt;&lt;sup&gt;17&lt;/sup&gt;O(NO&lt;sub&gt;2&lt;/sub&gt;) values measured towards the end of the night are quantitatively consistent with typical values of &lt;em&gt;&amp;#916;&lt;/em&gt;&lt;sup&gt;17&lt;/sup&gt;O(O&lt;sub&gt;3&lt;/sub&gt;).&lt;/p&gt;&lt;p&gt;These preliminary results are very promising for the use of&amp;#160;&lt;em&gt;&amp;#916;&lt;/em&gt;&lt;sup&gt;17&lt;/sup&gt;O of NO&lt;sub&gt;2&lt;/sub&gt; as a probe of the atmospheric oxidative activity and for the interpretation of &amp;#160;NO&lt;sub&gt;3&lt;/sub&gt;&lt;sup&gt;-&lt;/sup&gt; isotopic composition records. In the future, samplings and multi-isotopic analysis of atmospheric nitrate performed in parallel with those of NO&lt;sub&gt;2&lt;/sub&gt; will be of great interest for the study of the full reactive nitrogen cycle.&lt;/p&gt;