Innovative approach for new estimation of NOx snow-source on the Antarctic Plateau

<p>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 10⁶ cm^-3. 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_x &#8801; NO + NO₂) 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_x emissions on a continental scale over Antarctica is still lacking. Only NO emissions were measured during ISCAT and the ratio NO₂:NO was measured during NITE-DC and OPALE using indirect NO₂ measurements. This leaves significant uncertainties on the snow-air-radiation interaction. To overcome this crucial lack of information, direct NO₂ measurements are needed to estimate the NO_x flux emissions with reduced uncertainties.</p><p>For the first time, new developed optical instruments based on the IBB-CEAS technique and allowing direct measurement of NO₂ with detection limit of 10 x 10^-12 mol mol^-1, (1&#963;), (Barbero et al., 2020) were deployed on the field during the 2019&#8211;2020 summer campaign at Dome C (75&#176;06'S, 123&#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 &#177; 0.38) x 10^-8 s^-1 (1&#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_x fluxes were estimated to be (4.4 &#177; 2.3) x 10⁷ molec cm^-2 s^-1, 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_x budget of 0.025 &#177; 0.013 Tg.N y^-1, more than three times the N-budget of the stratospheric denitrification estimated to be 0.008 &#177; 0.003 Tg.N y^-1 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 &#160;significantly improve future global atmospheric models.</p>