Abstract. The oxygen (????¹⁷O) and nitrogen (????¹⁵N) isotopic compositions of atmospheric nitrate (NO₃-) are widely used as tracers of its formation pathways, precursor (nitrogen oxides NO_x = nitric oxide NO + nitrogen NO₂) emission sources, and physico-chemical processing. However, the critical lack of observations on the multi-isotopic composition of NO₂ maintains significant uncertainties regarding the links between the isotopic composition of NO_x and NO₃-, which may bias estimates of the NO₃- formation processes and the distribution of sources. We report here on the first simultaneous atmospheric observations of ????¹⁷O and ????¹⁵N in NO₂ and NO₃-. The measurements were carried out at sub-daily (ca. 3 h) resolution over two non-consecutive days in an Alpine city in February 2021. Important diurnal variabilities are observed in both NO₂ and NO₃- multi-isotopic composition. ????¹⁷O of NO₂ and NO₃- range from 19.6 to 40.8 ‰ and 18.7 to 26 ‰, respectively. During both daytime and nighttime, the variability of ????¹⁷O(NO₂) is mainly driven by the oxidation of NO by ozone, with a substantial contribution from peroxy radicals in the morning. NO₃- local mass balance equations, constrained by observed ????¹⁷O(NO₂), suggest that during the first day of sampling NO₃- was formed locally from the oxidation of NO₂ by hydroxyl radicals during the day, and via heterogeneous hydrolysis of dinitrogen pentoxide during the night. For the second day, calculated and observed ????¹⁷O(NO₃-) do not match, particularly daytime values. The effects on ????¹⁷O(NO₃-) of a Saharan dust event that occurred during the second day and winter boundary layer dynamics are discussed. ????¹⁵N of NO₂ and NO₃- ranged from -10.0 to 19.7 ‰ and -4.2 to 14.8 ‰, respectively. Consistent with theoretical predictions of N isotope fractionation, the important variability of ????¹⁵N(NO₂) is explained by significant post-emission equilibrium N fractionation. After accounting for this effect, vehicle exhaust is found to be the primary source of NO_x emissions at the sampling site. ????¹⁵N(NO₃-) is closely linked to ????¹⁵N(NO₂) variability, which bring further evidence of fast and local processing, but uncertainties on current N fractionation factors during NO₂ to NO₃- conversion are underscored. Overall, this detailed investigation highlights the potential and the necessity to use ????¹⁷O and ????¹⁵N in NO₂ and NO₃- to trace quantitatively the sources and formation chemistry of NO₃-, particularly in urban environments in winter.

Les milieux polaires et subpolaires ne sont pas dépourvus de parasites. Chez les manchots royaux, les ectoparasites tels que les tiques (Ixodes uriae) sont suspectés d’avoir des effets marqués sur la santé et le succès reproducteur des adultes. En utilisant un traitement expérimental réduisant drastiquement l’abondance d’ectoparasites chez des adultes reproducteurs, nous avons pu démontrer que l’infestation par les ectoparasites induit une augmentation du stress physiologique caractérisé par des taux élevés de corticostérone (glucocorticoïde) et une situation de stress oxydant. Bien que l’éradication des ectoparasites n’ait pas d’influence notable sur le succès d’éclosion des œufs ou la survie des poussins durant l’élevage, le traitement améliore visiblement la croissance des poussins jusqu’à l’émancipation, avec des effets positifs probables sur leur survie future. Il semble donc que les ectoparasites exercent une pression de sélection naturelle non-négligeable chez le manchot royal.