Innovative approach for new estimation of NOx snow-source on the Antarctic Plateau
Authors: Barbero Albane , Grilli Roberto , Blouzon Camille , Picard Ghislain , Frey Markus , Caillon Nicolas , Savarino Joel 
Published: March 2021

&lt;p&gt;Previous Antarctic summer campaigns have shown unexpectedly high levels of oxidants in the continental interior as well as at coastal regions, with atmospheric hydroxyl radical (OH) concentrations up to 4 x 10&lt;sup&gt;6&lt;/sup&gt; cm&lt;sup&gt;-3&lt;/sup&gt;. It is now well established that such high reactivity of the summer Antarctic boundary layer results in part from the emissions of nitrogen oxides (NO&lt;sub&gt;x&lt;/sub&gt; &amp;#8801; NO + NO&lt;sub&gt;2&lt;/sub&gt;) produced during the photo-denitrification of the snowpack. Despite the numerous observations collected at various sites during previous campaigns such as ISCAT 1998, 2000, ANTCI, NITE-DC and OPALE, a robust quantification of the NO&lt;sub&gt;x&lt;/sub&gt; emissions on a continental scale over Antarctica is still lacking. Only NO emissions were measured during ISCAT and the ratio NO&lt;sub&gt;2&lt;/sub&gt;:NO was measured during NITE-DC and OPALE using indirect NO&lt;sub&gt;2&lt;/sub&gt; measurements. This leaves significant uncertainties on the snow-air-radiation interaction. To overcome this crucial lack of information, direct NO&lt;sub&gt;2&lt;/sub&gt; measurements are needed to estimate the NO&lt;sub&gt;x&lt;/sub&gt; flux emissions with reduced uncertainties.&lt;/p&gt;&lt;p&gt;For the first time, new developed optical instruments based on the IBB-CEAS technique and allowing direct measurement of NO&lt;sub&gt;2&lt;/sub&gt; with detection limit of 10 x 10&lt;sup&gt;-12&lt;/sup&gt; mol mol&lt;sup&gt;-1&lt;/sup&gt;, (1&amp;#963;), (Barbero et al., 2020) were deployed on the field during the 2019&amp;#8211;2020 summer campaign at Dome C (75&amp;#176;06'S, 123&amp;#176;20'E, 3233m a.s.l). They were coupled with new designed dynamic flux chamber experiments. Snows of different ages ranging from newly formed drift snow to 16-20 year-old firn were sampled. Unexpectedly, the same daily average photolysis constant rate of (2.18 &amp;#177; 0.38) x 10&lt;sup&gt;-8&lt;/sup&gt; s&lt;sup&gt;-1&lt;/sup&gt; (1&amp;#963;) was estimated for the different type of snow samples, suggesting that the photolabile nitrate behaves as a single-family source with common photochemical properties. Daily summer NO&lt;sub&gt;x&lt;/sub&gt; fluxes were estimated to be (4.4 &amp;#177; 2.3) x 10&lt;sup&gt;7&lt;/sup&gt; molec cm&lt;sup&gt;-2&lt;/sup&gt; s&lt;sup&gt;-1&lt;/sup&gt;, 10 to 70 times less than what has been estimated in previous studies at Dome C and with uncertainties reduced by a factor up to 30. Using these results, we extrapolated an annual continental snow source NO&lt;sub&gt;x&lt;/sub&gt; budget of 0.025 &amp;#177; 0.013 Tg.N y&lt;sup&gt;-1&lt;/sup&gt;, more than three times the N-budget of the stratospheric denitrification estimated to be 0.008 &amp;#177; 0.003 Tg.N y&lt;sup&gt;-1&lt;/sup&gt; for Antarctica (Savarino et al., 2007), making the snowpack source a rather significant source in Antarctica. This innovative approach for the parameterization of nitrate photolysis using flux chamber experiments could &amp;#160;significantly improve future global atmospheric models.&lt;/p&gt;