Intense field campaigns over the past 3 decades have studied the Arctic and Antarctic boundary layer composition and photochemistry (, and references therein). The relatively unpolluted nature of these regions, free of local anthropogenic emissions, has led to the growing interest in their atmospheric chemistry and the collection and interpretation of polar ice cores . Antarctica, in particular, is the most isolated continent on Earth; therefore, providing the most suitable continent-scale laboratory for studying past and present natural atmospheric cycles . Because of this presumed pristine nature, the scientific community was puzzled by initial observations of high oxidative capacity in the polar boundary layer, i.e. an oxidation process initiated by oxidants (including ozone, hydroxyl and peroxyl radicals, hydrogen peroxide, halogen radicals), which resembled those seen in urban environments . It is today well established that such high reactivity in the summer Antarctic boundary layer results from the presence of highly reactive species, such as nitrogen oxides (NOy ≡ NO, NO2, HONO, HO2NO2, HNO3 etc.), hydroxyl and peroxyl radicals (ROx ≡ OH, HO2, RO2), and halogen oxides (XO ≡ BrO, IO, ClO), due to precursor emissions from the snowpack (, and references therein). Despite their short lifetime and low abundance in the atmosphere, they control the oxidative capacity and the atmospheric chemistry of these regions due to their high reactivity. Today, this phenomenon is still not yet fully understood due to the difficulty of performing accurate and free of interference NOx measurements, combined with a complex oxidation–reduction reaction involving NOx, O3, and radicals in the snow and atmosphere at wavelengths, λ, below 420 nm (Fig. ).

The photolysis of nitrate ions (NO3-) in the snow produces NOx (≡ NO2 + NO) . Gas-phase nitrogen dioxide (NO2) photolyses to produce nitrogen oxide (NO), which also results in production of ozone (O3), which then reacts together to reform NO2 (bold cycle in Fig. -1). In this recycling reaction, no net production or loss of any species is involved.

The following list of campaigns and publications is not exhaustive but gives an idea of the intensive studies of the Antarctic atmospheric chemistry for evaluating the air–snow transfer function. ISCAT 2000 (Investigation of Sulfur Chemistry of the Antarctic Troposphere) at South Pole station in 2000 revealed a strong oxidizing environment at South Pole (SP). The measurements established the recycling of reactive nitrogen as a critical component of this unique environment. ANTCI (The Antarctic Tropospheric Chemistry Investigation) deployed two field studies between 2003 and 2005 with large ground-based sampling during winter components at SP station and aircraft chemistry and photochemistry measurements . The CHABLIS (CHemistry of the Antarctic Boundary Layer and the Interface with Snow) measurement campaign was conducted at Halley station in coastal Antarctica from January 2004 through February 2005 . SUNITE DC (Sulfate and NItraTe surface snow Evolution at Dome C) aimed to document and use isotopic anomalies of oxyanions (sulfate and nitrate) to constrain the sources, transformations, and transports of these compounds to the polar regions where they are archived over thousands of years . The OPALE (Oxidative Production over Antarctic Land and its Export) campaign took place at both coastal Antarctica (Terre Adélie, Dumont D'Urville station) and the plateau (Dome C, Concordia station), with the aim to quantify, understand, and model the level of oxidants present in the lower atmosphere of East Antarctica . Despite the numerous observations of nitrogen-bearing species, i.e. HONO, NOx, and NO, collected during those cited previous campaigns, no direct NO2 measurements, free of interferences from other NOx species , have so far been performed on the Antarctic plateau. Indeed, previous studies used chemiluminescent detectors (CLDs), which directly measure NO and estimate NOx and NO2 from photolytic conversion to NO, which is conversion that is subject to interferences. This absence hinders our efforts to correctly evaluate the NOx cycle over the snowpack, leaving significant uncertainties in modelled values, which affect our full understanding of snow–air–radiation interactions. To overcome this lack of information, direct and accurate NO2 measurements with simultaneous detection of NO are needed. Here, we deployed newly developed optical instruments that allow direct measurement of NO2 and indirect measurement of NO with detection limits of 30 and 63 pptv (3σ), respectively . Although indirect, the NO measurement is well constrained. Potential artefacts have been identified and discussed in . Even though they may occur during night-time in an urban polluted environment, caused by the presence of N2O5, they are negligible during the summer period in Antarctica. We present new results on summer NO2:NO variability over the Antarctic Plateau and explore the mechanisms involved in the atmospheric boundary layer of the Antarctic Plateau during the photolytic season in light of these new data. While the overall NO2:NO ratio can be explained by the extended Leighton's relationship (including peroxy radicals, ROx), a high NO2:NO ratio was estimated in the morning during the early photolysis season, deviating from steady-state equilibrium and not explained by the extended Leighton's relationship taken from . Additionally, calculations show that the previous assumption of an additional conversion of NO to NO2 through XO or ROx is insufficient to fully explain the observations. In this work, using results of dynamic flux chamber experiments, , which allowed the NOx snow source to be better characterized, we were able to highlight that the NO2:NO ratio is very sensitive to the position of the sun. Indeed, results show that a 5 % difference in the solar zenith angle (SZA) between December and January may explain the equilibrium deviation observed in the NO2:NO ratio during the summer campaign.

Both NO2 and NO exhibited diurnal variations, with the highest concentrations in the afternoon and evening and lowest concentrations in the mid-morning to noon (Fig. ), in association with the collapse and rise of the polar boundary layer (PBL) . Generally, the rise and fall in NO2 lagged behind the NO peak by 4–6 h. As a result, the NO2:NO ratio had less diurnal amplitude than either NO2 or NO. Overall, the mean NO2:NO ratio was 1.3 ± 1.1 (1σ) for December and 0.4 ± 0.4 (1σ) for January. A possible reason for time lag are the different residence times in the porous snow after production, and transport through snow can indeed be slowed due to interaction with the snow interface, . However, both NO2 and NO are not adsorbed by snow . As explained in Sect. 2.3, RO2 plays an important role in the oxidation. Therefore, RO2 might be expected to be adsorbed to snow more than NO2, similar to HNO4 , and be released later.

The NO2:NO ratios calculated from the extended and simple Leighton relationships (Eqs. , ) share similar diurnal patterns to the observed NO2:NO ratios but do not reflect the difference in mean value between December and January (Fig. ). In December, the observed NO2:NO is systematically higher than the one estimated using simple and extended Leighton's equilibrium (Fig. a), while in January it more closely matches the extended equilibrium estimations (Fig. b).

We observed a dramatic loss of O3 at the end of the measurement period, with the mean mixing ratio dropping from 29.8 ± 3.6 ppbv during 16–20 January to 14.3 ± 0.8 ppbv during 23–25 January. HYSPLIT 5 d backward trajectories between 14:00 and 20:00 LT on 23 January (Appendix 2) reveal that the air mass over Dome C originated from the east coast of Antarctica, and this could explain the 10 ppbv drop of O3 observed between the two January periods shown in Fig. . It is therefore possible that this air mass was partially affected by marine influences, as concluded that O3 mixing ratios below 20 ppbv were observed when the air masses spent at least 1 d above the ocean during the previous 5 d. As a result, we excluded the 23–25 January data from the following discussions. Figure a and b show the daily averaged total number of molecules (Eq. ) of NNOx (cyan), NNO (black), and NNO2 (blue) for both observation periods, December 2019 (Fig. a) and January 2020 (Fig. b). The total number of molecules is used to cancel the PBL dynamic, as explained in Sect. . The NO2:NO ratios observed (red) in comparison with the theoretical NO2:NO calculated from Eq. () (NO2:NO)sim in grey and Eq. () (NO2:NO)ext in black are reported in the lower panels of Fig. a and b. With the objective of testing the consistency of the observed NO2:NO ratios with ozone production and destruction, in Fig. c and d we also reported the total number of O3 molecules in a column of 1 cm × 1 cm × 1 HPBL cm (green, left-hand scale), with its variation over time, i.e. dO3dt (in molec. h-1, blue-grey, right-hand scale). Production of ozone, PO3 (expressed in molec. h-1), calculated as the NO2 production rate from the reactions RO2 + NO → NO2 + RO and HO2 + NO → NO2 + OH, as reported by , is calculated following Eq. () and reported in violet in Fig. c and d. 5PO3=kRO2+NO[RO2]+kHO2+NO[HO2]×[NO]×V In the above equation, kRO2+NO and kHO2+NO are the kinetic rate coefficients of the reactions RO2 + NO → NO2 + RO and HO2 + NO → NO2 + OH, respectively (expressed in cm3 molec.-1 s-1); [RO2] and [HO2] are the species concentrations (expressed in molec. cm-3) and are derived from the correlation between JNO2 and [RO2] and the [RO2][HO2] ratio ; [NO] is the concentration of NO (expressed in molec. cm-3); and V the arbitrary volume (expressed as 1 cm × 1cm × HPBL cm, with HPBL given by the MAR regional model; see Appendix  for details). The overall daily mean PO3V calculated for this study (≈ 0.22 ppbv h-1) is close to the 0.3 ppbv h-1 calculated by and the 0.2 ppbv h-1 derived from the study of the ozone diurnal concentration by . In Fig. c and d, the deviations of the observed ratio (NO2:NO)obs from the simple and extended Leighton's equilibria are reported, i.e. Δsim(NO2:NO) = (NO2:NO)obs - (NO2:NO)sim and Δext(NO2:NO) = (NO2:NO)obs - (NO2:NO)ext for each period, respectively. Looking at Fig. a and b, the NOx, NO, and NO2 peaks are now more in phase with the variability of the UV radiation. The total number of NO molecules and their diurnal variability appear constant from December to January, with a maximum from 11:00 to 17:00 LT. NO2 shows a similar trend in December with a maximum from 10:00 to 18:00 LT, while in January the total number of NO2 molecules in the atmospheric boundary layer is somewhat constant during the day, with a slight increase from 09:00 to 12:00 LT.

A possible reason for this productivity of the snowpack in the morning during the earlier phase of the photolysis season is the number of photons available, leading to higher rate of photolysis in December. Nitrate photolysis in snow occurs for wavelengths λ>300 nm (305 nm being the optimal wavelength). Measurements of solar UV spectral radiation have been continuously recorded at Concordia since 2007 as part of the Network for the Detection of Atmospheric Composition Change (NDACC). The instrument, SAOZ (Système d'Analyse par Observation Zenithal), is an UV–visible (310–650 nm) diode array spectrometer (1 nm resolution), looking at the scattered sunlight .

Figure  shows the relative radiation fluxes at different wavelengths in arbitrary units. One can see that the relative radiation flux is systematically higher in December than in January, which could be expected as 18 d separates the first observation period from the summer solstice versus 27 d for the second period. While this explains higher levels of NOx due to more photolysis activity in the snowpack in December than in January, it does not explain the early morning excess. Indeed, one can see in Fig.  that the diurnal cycle of the radiation is somewhat symmetrical in January (in blue), while an asymmetry appears in December (in red). This is also visible while comparing the shape of the SZA for the two periods, with the difference between the two SZA cycles (black line in Fig. ) showing a bump in the morning between 06:00 and 12:00 LT with a similar shape as the NO2:NO deviation from equilibrium reported in Fig. a. No different cloud cover during the two observation windows was experienced; therefore, the discontinuity in the sinusoidal shape of the radiation signal in December could be due to a smaller solar zenith angle at this latitude. Indeed, as shown in Fig. , the difference between the SZA in January and in December, Δ(SZAjan-SZAdec), normalized to the December value to get percentages (black line in Fig. ), indeed appears higher (close to 5 %) in the morning than during the rest of the day, coinciding with the bump in UV and the NO2:NO equilibrium deviation observed before.

A rapid parameterization of the dependence of the NOx production, PNOx (pptv h-1), to the position of the sun, SZA (∘), has been established. For the calculation, a normalization over the SZA in December has been made. Additionally, in Fig.  it appears that the NOx production is happening during daytime, with a SZA >70∘; therefore, only the daily values (06:00 to 18:00 LT) were taken into account for this parameterization. Figure  shows the results of linear regressions for both periods, suggesting that it is possible to decipher the parameters linking the NOx production to the SZA. Indeed, satisfactory R2 values (0.984 for December and 0.727 for January) were found. Of course, this result is only an approach, and a thorough study of the NOx production dependence on the sun's position would allow for the improvement of existing models and would reduce the uncertainties concerning the Antarctic nitrogen budget.

For the first time, direct and in situ atmospheric measurements of NO2 were carried out in the Antarctic Plateau at Dome C. The summer variability of the NO2:NO ratio was studied in light of this new set of observations. While the overall NO2:NO ratio can be explained by the extended Leighton's relationship in certain periods, a high NO2:NO ratio was estimated in the morning during the early photolysis season, which deviates from steady-state equilibrium and is not explained by the extended Leighton's relationship. The results of this study disagree with previous studies that found a systematic deviation from equilibrium, requiring around 50 pptv of halogenated radicals to explain such a NO2:NO ratio. However, the required levels of halogenated species were never observed at Dome C. While differences in atmospheric dynamics and snow cover during the different campaigns could explain the discrepancies with previous studies, it may also be due to different detection techniques being used. In our study, NO2 was measured directly for the first time at Dome C. The NOx snow source was studied in the light of new results presented in , where a nitrate daily average photolysis rate constant, JNO3- = (2.37 ± 0.35) × 10-8 s-1, for all different snow samples (at the depth and location of the Antarctic Plateau) taken from 10 December to 7 January is estimated. NOx fluxes (expressed in molec. cm−2 s−1) from the snow surface source were estimated and converted into a production rate (in molec. cm−3 h−1) using the polar boundary layer height (cm). A mean ratio of 1.92 ± 0.33 in the NOx production between December and January was estimated in the early morning, with a factor of up to 2.6 difference in the surface snow NOx production between December and January, corresponding to the deviation from photochemical equilibrium observed. Therefore, evaluation of the meteorological conditions and estimation of the NOx snow source tend to identify the NOx snow source and the position of the sun, through SZA measurement, as the main actors of the atmospheric oxidative capacity in the austral summer at Dome C. From our calculations, it appears that a 5 % difference in the SZA in the morning could lead to an excess of up to 5 times the NO2:NO ratio when at steady state. Such a high sensitivity of the NO2:NO ratio to the sun position is of importance to the atmospheric chemistry models, where such a parameter can be better adjusted in the future. However, even though the NOx snow source might explain the differences between December and January in the NOx cycle, this does not match the observed O3 variations. In addition, even though the depletion of the NOx reservoir throughout the light season might explain the lower levels of O3 toward the end of the photolytic season, additional investigations into possible unidentified mechanisms or unidentified sinks of O3 are needed to understand the O3 consumption. The link between the stratospheric ozone hole inducing an enhancement in the incident UV radiation, and in turn causing an increase in the ice-to-atmosphere iodine emissions, has been stated by . Therefore, in the future, additional field campaigns targeting halogenated radical measurements in snow and the troposphere in Antarctica are needed to allow the scientific community to fully understand the mechanisms driving the oxidative capacity of the polar atmosphere.