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The oxygen (Δ17 O) and nitrogen (δ15 N) isotopic compositions of atmospheric nitrate (NO 3-) are widely used as tracers of its formation pathways, precursor (nitrogen oxides (NO x) ≡ nitric oxide (NO) + nitrogen dioxide (NO 2)) emission sources, and physico-chemical processing. However, the lack of observations on the multi-isotopic composition of NO 2 perpetuates significant uncertainties regarding the quantitative links between the isotopic composition of NO x and NO 3- , which ultimately may bias inferences about NO 3- formation processes and the distribution of sources, particularly in winter urban atmospheres. We report here on the first simultaneous atmospheric observations of Δ17 O and δ15 N in NO 2 (n=16) and NO 3- (n=14). The measurements were carried out at sub-daily (∼3 h) resolution over 2 non-consecutive days in an Alpine city in February 2021. A strong diurnal signal is observed in both NO 2 and NO 3- multi-isotopic composition. Δ17 O of NO 2 and NO 3- ranges from 19.6 ‰ to 40.8 ‰ and from 18.3 ‰ to 28.1 ‰, respectively. During the day and night, the variability in Δ17 O(NO 2) is mainly driven by the oxidation of NO by ozone, with a substantial contribution from peroxy radicals in the morning. NO 3- mass balance equations, constrained by observed Δ17 O(NO 2), suggest that during the first day of sampling, most of the NO 3- was formed locally from the oxidation of NO 2 by hydroxyl radicals by day and via heterogeneous hydrolysis of dinitrogen pentoxide at night. For the second day, calculated and observed Δ17 O(NO 3-) do not match, particularly daytime values; the possible effects on Δ17 O(NO 3-) of a Saharan dust event that occurred during this sampling period and of winter boundary layer dynamics are discussed. δ15 N of NO 2 and NO 3- ranges from - 10.0 ‰ to 19.7 ‰ and from - 4.2 ‰ to 14.9 ‰, respectively. Consistent with theoretical predictions of N isotope fractionation, the observed variability in δ15 N(NO 2) 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. δ15 N(NO 3-) is closely linked to δ15 N(NO 2) variability, bringing further support to relatively fast and local NO x processing. Uncertainties in current N fractionation factors during NO 2 to NO 3- conversion are underlined. Overall, this detailed investigation highlights the potential and necessity of simultaneously using Δ17 O and δ15 N in NO 2 and NO 3- in order to better constrain quantitative inferences about the sources and formation chemistry of NO 3- in urban environments in winter. [ABSTRACT FROM AUTHOR]

Ammonia (NH3) participates in nucleation and growth of aerosols and thus plays a major role in atmospheric transparency, pollution, health, and climate related issues. Understanding its emission sources through nitrogen stable isotopes is therefore a major focus of current work to mitigate the adverse effects of aerosol formation. Since ice cores can preserve the past chemical composition of the atmosphere for centuries, they are a top tool of choice for understanding past NH3 emissions through ammonium (NH4+), the form of NH3 archived in ice. However, the remote or high-altitude sites where glaciers and ice sheets are typically localized have relatively low fluxes of atmospheric NH4+ deposition which makes ice core samples very sensitive to laboratory NH3 contamination. As a result, accurate techniques for identifying and tracking NH3 emissions through concentration and isotopic measurements are highly sought to constrain uncertainties in NH3 emission inventories and atmospheric reactivity unknowns. Here, we describe a solid phase extraction method for NH4+ samples of low concentration that limits external contamination and produces precise isotopic results. By limiting NH3atm exposure with a scavenging fume hood and concentrating the targeted NH4+ through ion exchange resin, we successfully achieve isotopic analysis of 50 nmol NH4+ samples with a 0.6 ‰ standard deviation. This extraction method is applied to an alpine glacier ice core from Col Du Dôme, Mont-Blanc, where we successfully demonstrate the analytical approach through the analysis of two replicate 8 m water equivalent ice cores representing 4 years of accumulation with a reproducibility of ± 2.1 ‰. Applying this methodology to other ice cores in alpine and polar environments will open new opportunities for understanding past changes in NH3 emissions and atmospheric chemistry. [ABSTRACT FROM AUTHOR]

The oxygen (훥17O) and nitrogen (훿15N) isotopic compositions of atmospheric nitrate (NO3-) are widely used as tracers of its formation pathways, precursor (nitrogen oxides NOx = 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 NOx 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 훥17O and 훿15N 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. 훥17O 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 훥17O(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 훥17O(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 훥17O(NO3-) do not match, particularly daytime values. The effects on 훥17O(NO3-) of a Saharan dust event that occurred during the second day and winter boundary layer dynamics are discussed. 훿15N 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 훿15N(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 NOx emissions at the sampling site. 훿15N(NO3-) is closely linked to 훿15N(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 훥17O and 훿15N in NO2 and NO3- to trace quantitatively the sources and formation chemistry of NO3-, particularly in urban environments in winter. [ABSTRACT FROM AUTHOR]

Nitrate in Antarctic snow has seasonal cycles in its nitrogen and oxygen isotopic ratios that reflect its sources and atmospheric formation processes, and as a result, nitrate archived in Antarctic ice should have great potential to record atmospheric chemistry changes over thousands of years. However, sunlight that strikes the snow surface results in photolytic nitrate loss and isotopic fractionation that can completely obscure the nitrate's original isotopic values. To gain insight into how photolysis overwrites the seasonal atmospheric cycles, we collected 244 snow samples along a 850 km transect of East Antarctica during the 2013–2014 CHICTABA traverse. The CHICTABA route's limited elevation change, consistent distance between the coast and the high interior plateau, and intermediate accumulation rates offered a gentle environmental gradient ideal for studying the competing pre- and post-depositional influences on archived nitrate isotopes. We find that nitrate isotopes in snow along the transect are indeed notably modified by photolysis after deposition, and drier sites have more intense photolytic impacts. Still, an imprint of the original seasonal cycles of atmospheric nitrate isotopes is still present in the top 1–2 m of the snowpack and likely preserved through archiving in glacial ice at these sites. Despite this preservation, reconstructing past atmospheric values from archived nitrate along CHICTABA and in similar transitional regions remains a difficult challenge without having an independent proxy for photolytic loss to correct for post-depositional isotopic changes. Nevertheless, nitrate isotopes should function as a proxy for snow accumulation rate in such regions if multiple years of deposition are aggregated to remove the seasonal cycles, and this application can prove highly valuable in its own right. [ABSTRACT FROM AUTHOR]

The analysis of air bubbles from ice cores has yielded a precise record of atmospheric greenhouse gas concentrations, but the timing of changes in these gases with respect to temperature is not accurately known because of uncertainty in the gas age-ice age difference. We have measured the isotopic composition of argon in air bubbles in the Vostok core during Termination III (~240,000 years before the present). This record most likely reflects the temperature and accumulation change, although the mechanism remains unclear. The sequence of events during Termination III suggests that the C[O.sub.2] increase lagged Antarctic deglacial warming by 800 ± 200 years and preceded the Northern Hemisphere deglaciation., Ice cores are unique archives of past climatic and environmental conditions that provide detailed records of local temperature and atmospheric concentrations of greenhouse gases. Analyses of the Vostok ice core [...]

Previous Antarctic summer campaigns have shown unexpectedly high levels of oxidants in the lower atmosphere of the continental plateau as well as at coastal regions, with atmospheric hydroxyl radical (OH) concentrations up to 4 x 106 cm-3. Such high reactivity of the summer Antarctic boundary layer results in part from the emissions of nitrogen oxides (NOx = NO + NO2) produced during photo-denitrification of the snowpack, but its underlying mechanisms are not yet fully understood as some of the chemical species involved (NO2, in particular) have not yet been measured directly and accurately. To overcome this crucial lack of information, newly developed optical instruments based on absorption spectroscopy (incoherent broadband cavity enhanced absorption spectroscopy or IBBCEAS) were deployed for the first time at Dome C (-75.10 lat., 123.33 long., 3,233 m a.s.l) during the 2019-2020 summer campaign to refine uncertainties in snow-air-radiation interaction. These instruments directly measure NO2 with a detection limit of 30 pptv (parts per trillion by volume or 10-12 mol mol-1) (3a). We performed two sets of measurements in December 2019 (4th to 9th) and January 2020 (16th to 25th) to capture the early and late photolytic season, respectively. Late in the season, the daily averaged NO2 :NO ratio (0.4 ± 0.4) matches that expected for photochemical equilibrium through Leighton's extended relationship involving ROx (0.6 ± 0.3). In December, however, we observe a daily averaged NO2:NO ratio of 1.3 ± 1.1, which is approximately twice the daily ratio of 0.7 ± 0.4 calculated for Leighton equilibrium. This suggests that more NO2 is produced from the snowpack early in the photolytic season (December 4th to 9th) possibly due to stronger UV irradiance caused by a smaller solar zenith angle near the solstice. Such a high sensitivity of the NO2:NO ratio to the sun's position is of importance for consideration in atmospheric chemistry models. [ABSTRACT FROM AUTHOR]

The isotopic composition of nitrogen and oxygen in nitrogen dioxide (NO2) potentially carries a wealth of information about the dynamics of the nitrogen oxides (NOx = nitric oxide(NO) + NO2) chemistry in the atmosphere. While nitrogen isotopes of NO2 are subtle indicators of emissions, NOx chemistry and isotopic nitrogen exchange between NO and NO2, oxygen isotopes are believed to reflect only the O3/NOx/VOC chemical regime in different atmospheric environments. In order to access this potential tracer of the tropospheric chemistry, we have developed an efficient active method to trap atmospheric NO2 on denuder tubes and measured, for the first time, its multi-isotopic composition (δ15N, δ18O, and Δ17O). The δ15N values of NO2 trapped at our site in Grenoble, France, show little variability (-11.8 to -4.9 ‰) with negligible N isotope fractionations between NO and NO2 due to high NO2/NOx ratios. NOx emissions main sources are estimated using a stable isotope model indicating the predominance of traffic NOx emissions in this area. The Δ17O values, however, reveal an important diurnal cycle peaking in late morning at (39.2 ± 1.7) ‰ and decreasing at night until (20.5 ± 1.7) ‰. On top of this diurnal cycle, Δ17O also has substantial variability during the day (from 29.7 to 39.2 ‰), certainly driven by changes in the O3 to peroxyl radicals ratio. The night-time decay of Δ17O(NO2) appears to be driven by NO2 slow removal, mostly from conversion into N2O5, and its formation from the reaction between O3 and emitted NO. Our Δ17O(NO2) measured towards the end of the night is quantitatively consistent with typical values of Δ17O(O3). These preliminary results are very promising for using Δ17O of NO2 as a probe of the atmospheric oxidative activity and for interpreting NO3- isotopic composition records. [ABSTRACT FROM AUTHOR]