Abstract. Polar stratospheric clouds (PSCs) play a critical role in the stratospheric ozone depletion processes. The last 30 years have seen significant improvements in our understanding of the PSC processes but PSC parametrization in global models still remains a challenge due to the necessary trade-off between the complexity of PSC microphysics and model parametrization constraints. The French Antarctic station Dumont d'Urville (DDU, 66.6^∘ S, 140.0^∘ E) has one of the few high latitude ground-based lidars in the Southern Hemisphere that has been monitoring PSCs for decades. This study focuses on the PSC data record during the 2007–2020 period. First, the DDU lidar record is analysed through three established classification schemes that prove to be mutually consistent: the PSC population observed above DDU is estimated to be of 30 % supercooled ternary solutions, more than 60 % nitric acid trihydrate mixtures and less than 10 % of water–ice dominated PSC. The Cloud–Aerosol Lidar with Orthogonal Polarization PSC detection around the station are compared to DDU PSC datasets and show a good agreement despite more water–ice PSC detection. Detailed 2015 lidar measurements are presented to highlight interesting features of PSC fields above DDU. Then, combining a temperature proxy to lidar measurements, we build a trend of PSC days per year at DDU from ERA5 (the fifth generation of European ReAnalysis) and NCEP (National Centers for Environment Protection reanalysis) reanalyses fitted on lidar measurements operated at the station. This significant 14-year trend of −4.6 PSC days per decade is consistent with recent temperature satellite measurements at high latitudes. Specific DDU lidar measurements are presented to highlight fine PSC features that are often sub-scale to global models and spaceborne measurements.

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.