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

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 ≡ 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.

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, (1σ), (Barbero et al., 2020) were deployed on the field during the 2019–2020 summer campaign at Dome C (75°06'S, 123°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 ± 0.38) x 10^-8 s^-1 (1σ) 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 ± 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 ± 0.013 Tg.N y^-1, more than three times the N-budget of the stratospheric denitrification estimated to be 0.008 ± 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  significantly improve future global atmospheric models.