# 1000 years of Nitrogen Oxide Sources in Western Europe: Evidence from Nitrogen Stable Isotopes (δ15N) of Nitrate in a Mont Blanc Ice Core

Alexis Lamothe, Pete Douglas Akers, Sarah Albertin, Nicolas Caillon, Sophie

Darfeuil, Elsa Gautier, Patrick Ginot, Shohei Hattori, Joel Savarino

► To cite this version:

Alexis Lamothe, Pete Douglas Akers, Sarah Albertin, Nicolas Caillon, Sophie Darfeuil, et al., 1000 years of Nitrogen Oxide Sources in Western Europe: Evidence from Nitrogen Stable Isotopes (δ15N) of Nitrate in a Mont Blanc Ice Core. 2025. hal-04803024

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# 1000 years of Nitrogen Oxide Sources in Western Europe: Evidence from Nitrogen Stable Isotopes (δ15N) of Nitrate in a Mont Blanc Ice Core

Alexis Lamothe¹, Pete Douglas Akers², Sarah Albertin³, Nicolas Caillon⁴, Sophie Darfeuil¹,
Elsa Gautier³, Patrick Ginot⁵, Shohei Hattori⁶, and Joel Savarino⁷

¹Univ. Grenoble Alpes, CNRS, IRD, INRAE, Grenoble INP, IGE

²Trinity College Dublin

³Institut des Géosciences de l'Environnement

⁴Univ. Grenoble Alpes, CNRS, IRD, Grenoble INP (Institute of Engineering), IGE

⁵IRD

⁶Nanjing University

⁷Laboratoire de Glaciologie et Géophysique de l'Environnement

June 08, 2024

## Abstract

Nitrogen stable isotopes (δ15N) of ice core nitrate of nitrate (NO3-) are often subject to various interpretations, associated with source, atmospheric, and/or post-depositional changes. Here, an analysis of a Mont-Blanc (French Alps) ice core was conducted to investigate δ15N(NO3-) records over the last 1000 years. We find that the glacially archived δ15N(NO3-) signal reliably preserves a record of NOx emission sources with limited post-emission isotopic changes. We use the δ15N record to reconstruct a history of NOx emissions and compare it with existing estimated NOx inventories for Western Europe. Notably, our ice-derived record suggests that inventories for the early 20th-century may have underestimated NOx emissions resulting from agriculture. During the 20th century, the δ15N value substantially decreased, which we attribute to the increasing emissions from oil combustion. Lastly, the ice core signal highlights the success of mitigation policies in reducing fossil fuel-induced NOx emissions, albeit 20 years later than anticipated.
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# 1000 years of Nitrogen Oxide Sources in Western Europe: Evidence from Nitrogen

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## Stable Isotopes (δ¹⁵N) of Nitrate in a Mont Blanc Ice Core

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Alexis Lamothe¹, Pete D. Akers², Sarah Albertin³,¹, Nicolas Caillon¹, Sophie Darfeuil¹,

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Elsa Gautier¹, Patrick Ginot¹, Shohei Hattori⁴,⁵, Joël Savarino¹

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¹Univ. Grenoble Alpes, CNRS, IRD, INRAE, Grenoble INP*, IGE, 38000 Grenoble, France

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*Institute of Engineering and Management Univ. Grenoble Alpes

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²Discipline of Geography, School of Natural Sciences, Trinity College Dublin, Dublin, Ireland

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³LATMOS/IPSL, Sorbonne Université, UVSQ, CNRS, 75005 Paris, France

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⁴International Center for Isotope Effects Research (ICIER), Nanjing University, Nanjing,

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210023, China

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⁵Frontiers Science Center for Critical Earth Material Cycling, State Key Laboratory for

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Mineral Deposits Research, School of Earth Sciences and Engineering, Nanjing University,

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Nanjing 210023, China

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Corresponding author: Alexis Lamothe (lamothe@cerege.fr) or Joël Savarino (joel.savarino@cnrs.fr)

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## Key Points:

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* A Mont Blanc ice core provides 1000 years of ¹⁵N isotopic composition of nitrate

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* The ice core ¹⁵N signal of nitrate reflects the relative contribution of NOₓ emission sources.

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* NOₓ emission inventory may have underestimated agricultural changes at the beginning of the 20th century in Western Europe

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**Abstract**

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Nitrogen stable isotopes ($\delta^{15}$N) of ice core nitrate of nitrate ($NO_3^-$) are often subject to various interpretations, associated with source, atmospheric, and/or post-depositional changes. Here, an analysis of a Mont-Blanc (French Alps) ice core was conducted to investigate $\delta^{15}$(NO$_3^-$) records over the last 1000 years. We find that the glacially archived $\delta^{15}$N(NO$_3^-$) signal reliably preserves a record of NO$_x$ emission sources with limited post-emission isotopic changes. We use the $\delta^{15}$N record to reconstruct a history of NO$_x$ emissions and compare it with existing estimated NO$_x$ inventories for Western Europe. Notably, our ice-derived record suggests that inventories for the early 20$^{th}$-century may have underestimated NO$_x$ emissions resulting from agriculture. During the 20$^{th}$ century, the $\delta^{15}$N value substantially decreased, which we attribute to the increasing emissions from oil combustion. Lastly, the ice core signal highlights the success of mitigation policies in reducing fossil fuel-induced NO$_x$ emissions, albeit 20 years later than anticipated.

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**Plain Language Summary**

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This study aims to understand changes in nitrogen (N) stable isotopes, specifically the enrichment of ¹⁵N to ¹⁴N ($\delta^{15}$N), in atmospheric nitrate. $\delta^{15}$N records of nitrate can provide information about its gaseous precursor (nitrogen oxides = NO$_x$) sources. However, debates exist regarding their interpretation in glacial archives as a source signal only. We analyze a glacial archive from Mont-Blanc (French Alps), which preserves the $\delta^{15}$N source signal. We discuss the evolution of NO$_x$ emissions over the past 1000 years in light of existing emission inventories in Western Europe. Notably, our NO$_x$ history traces both the rise in fossil fuel combustion over the 20$^{th}$ century and its more recent emission reductions resulting from mitigation programs.

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**1 Introduction**

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During the 20$^{th}$ century, anthropogenic emissions of nitrogen oxides (NO$_x$ = NO + NO$_2$) have soared mainly from combustion of fossil fuels and biomass (Hoesly et al., 2018). NO$_x$ accounts for a substantial proportion of total aerosol mass, given its oxidation into nitric acid (HNO$_3$) that is involved in secondary inorganic aerosol formation, and contributes to particulate air pollution in large proportions (e.g., 24 % in Grenoble, France (Favez et al., 2010)).
Additionally, NOₓ plays a role in the production of tropospheric ozone (O₃) (Fishman & Crutzen, 1978), a central atmospheric reactant, while the deposition of NOₓ' final oxidized form, nitrate (NO₃⁻ = HNO₃ + particulate nitrate (p-NO₃⁻)), alters the nutrient balance of terrestrial and aquatic ecosystems leading to eutrophication (Durand et al., 2011). Hence, NOₓ has become a key focus of pollution mitigation, and many countries and supranational organizations have implemented policies to control NOₓ emissions from the 1980s onwards.

Since more than 82 % of NOₓ is reported to result from the combustion of fossil fuel for Western Europe (European Environment Agency, 2021), mitigation policies have targeted these sources, mainly through the implementation of catalytic converters (Leip et al., 2011). Subsequently, NOₓ emissions have been reported to decrease by 58 % between 1990 and 2014 (Hoesly et al., 2018). However, based on nitrogen (N) stable isotopic composition (δ¹⁵N = (¹⁵N/¹⁴N)sample/(¹⁵N/¹⁴N)air - 1) expressed in per mill (‰)) measured in NO₃⁻, a recent study has suggested that fossil fuel combustion only accounted for (54 ± 13) % of anthropogenic NOₓ emissions, challenging the current mitigation policies (Song et al., 2021). The isotopic composition of NOₓ at given time and location is the mean of the isotopic signatures of each source (Extended Table 1), weighted according to their relative mass contributions. This isotopic fingerprint is then transferred to nitrate, although N isotopic fractionation may occur (e.g., Freyer, 1991; Vicars et al., 2013; Walters et al., 2016). However, Song et al. (2021) based their conclusion on the extrapolation of 23 measurement points to the entire continent and also lacked consideration for natural NOₓ sources. To circumvent these two limitations, we measured the isotopic signal of NO₃⁻ (δ¹⁵N(NO₃⁻)) in an ice core drilled in the Alps to reconstruct a NOₓ emission history that naturally aggregates emissions from across Western Europe.

Ice cores sample glacial archives that have preserved the aerosol atmospheric composition, including NO₃⁻, for millennia. Since many glaciers have archived preindustrial isotopic composition of aerosols, ice cores provide unparalleled clues to understanding the natural sources and character of NOₓ emissions at an average regional signal (Eichler et al., 2023). In more recent glacial layers, the glacial NO₃⁻ composition can directly be compared to NOₓ regional inventories to examine their accuracy (e.g., Preunkert et al., 2003).

However, the interpretation of δ¹⁵N(NO₃⁻) in ice cores remains controversial. The same 300-year δ¹⁵N(NO₃⁻) record from Summit, Greenland, has been interpreted to follow 1) the increase in fossil fuel burning (Hastings et al., 2009), 2) the development of agriculture during
the 20th century (Felix & Elliott, 2013), and 3) the evolution of isotopic fractionation between HNO₃ and *p*-NO₃⁻ resulting from changes in atmospheric acidity (Geng et al., 2014). This latter fractionation interpretation was prompted by the need to explain the high isotopic value measured in the pre-industrial period (> 11 ‰), as no natural NOₓ source with such an isotopic signature is known.

To date, all these studies fail to explain pre-industrial δ¹⁵N(NO₃⁻) values without considering post-depositional phenomena. Post-depositional phenomena are any physical or chemical processes that alter the chemical composition of a molecule, and therefore its isotopic composition, after its atmospheric deposition. HNO₃ volatilization and *p*-NO₃⁻ photolysis are such phenomena that can decrease the NO₃⁻ concentration in near-surface snow layers surface while enriching the snow in ¹⁵NO₃⁻ (Erbland et al., 2013). These phenomena generally have a greater impact at lower accumulation sites where NO₃⁻ is exposed to the atmosphere and sunlight for longer periods of time. Thus, across the very dry East Antarctic plateau where snow accumulation is typically < 0.2 m of water equivalent per year (w e a⁻¹), photolytic changes in NO₃⁻ isotopic composition have been well documented (Akers et al., 2022). At intermediate accumulation sites, including Summit, Greenland, debates remain and air-snow interface models need to be used to quantify the influence of photolysis on NO₃⁻ (Jiang et al., 2021).

The analysis of a 126 m long ice core drilled in 2016 as part of the Ice Memory program at the Col Du Dôme glacier (CDD, 4250 m asl, 45.8421°N, 6.8474°E) in the Mont Blanc massif can help address the limitations previously mentioned for developing a NOₓ emission record from ice core δ¹⁵N(NO₃⁻). Indeed, the CDD is a cold glacier whose high accumulation rate (≈ 2.5 m w e a⁻¹) (Preunkert et al., 2000) largely prevents impactful post-depositional phenomena. Additionally, the CDD is situated in the heart of Western Europe and faithfully records the regional atmospheric composition (Preunkert & Legrand, 2013). This close proximity to European NOₓ emission sources can help distinguish the influences of transport-related isotopic fractionation and source signatures when compared to distant Greenland drilling sites. Here, after assessing the influence of photolysis and atmospheric isotopic fractionation on the glacially archived NO₃⁻ signal, we can then compare the observed δ¹⁵N(NO₃⁻) values with NOₓ inventories to investigate potential inventory inaccuracies.
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## 2 2 Methods

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### 2.1 Ice core processing

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CDD16-C01 is a 126 meter-long, 10 cm diameter ice core drilled in 2016. Ice segments were always stored and processed in a cold room (≈ -20 °C), stored in clean PE bags, and handled with clean nitrile gloves. Each core segment was cut longitudinally into several sections using saws decontaminated with ultrapure ice. A (34 × 34) mm² section dedicated to measurement of chemical concentrations is further decontaminated during continuous flow analysis (CFA) since the ice melt head discards meltwater coming from the potentially contaminated external faces.

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The outermost section was used to measure tritium activity between 100.8 m and 106.0 m using liquid scintillation counting at the Paul Scherrer Institute, Villigen, Switzerland. The external faces of the section dedicate to isotopic measurements were meticulously decontaminated by removing the outermost 2 mm of ice with a clean ceramic knife.

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### 2.2 Concentration measurement

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Concentrations of selected ions and elements were measured on the CFA system from the PANDA platform at IGE (https://panda.osug.fr/, Grenoble, France). As part of this CFA, an online colorimetry module measured $NO_3^-$ and ammonium ($NH_4^+$) concentrations (Röthlisberger et al., 2000). $NO_3^-$ concentration is calibrated with a set of standards (74, 478, and 1742 ppb). Due to the $NO_3^-$ conversion to $NO_2^-$, any $NO_2^-$ naturally present in the ice will inflate the resulting $NO_3^-$ concentration values. To constrain this possible error, we verified the mean $NO_2^-/(NO_2^-+ NO_3^-)$ ratio with ion chromatography over the uppermost 23 m and determined it to be 1.6 % ($\sigma$ = 2.5 %).

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The elemental [$^{23}$Na] (abundance 100 %), [$^{24}$Mg] (79 %), [$^{27}$Al] (100 %), [$^{40}$Ca] (97 %), and [$^{32}$S] (95 %) concentrations were measured with an online ICP-MS (iCAP TQ, ThermoFischer) and are assumed to be representative of the inorganic $[Na^+]$, $[Mg^{2+}]$, $[Al^{3+}]$, $[Ca^{2+}]$, and $[SO_4^{2-}]$ concentrations, respectively. The quantification of $[Cl^-]$ was performed by calculating $[Cl^-] = 1.61 \times [Na^+]$ (Legrand et al., 2002). The acidity profile was reconstructed following an ionic balance and agrees well with previous publications (Legrand et al., 2003; Preunkert & Legrand, 2013). The influence of anthropogenic Cl on acidity has previously been shown to be insignificant (Lamothe et al., in review).
144 **2.3 Dating**

145 The NH₄⁺ concentration profile is characterized by strong summer to winter variations which 146 were used to date the top 120 m of the ice core (Preunkert et al., 2000) covering from 2016 to 147 1895. This NH₄⁺-based chronology was verified and refined with time horizons using known 148 Saharan Dust Events (Clifford et al., 2019; Preunkert et al., 2000) and the 1963 tritium peak 149 activity. A large jump in time suggested by our initial chronology was also recently discussed 150 independently and hypothesized to reflect a crevasse (Preunkert et al., 2023). A 3D model based 151 on glacier flow and temperature characteristics (Gilbert et al., 2014) was used to validate our 152 measurements, estimate the uncertainty of the summer-winter variability technique, and 153 approximatively date the bottommost 5 meters. In our final resulting chronology, we estimated 154 the time scale uncertainty to be ±2 years for 1890–2016, ±5 years for 1890–1980, and ±30 years 155 before 1890. One can refer to Lamothe et al, *in review* for additional chronological details.

156 **2.4 δ¹⁵N(NO₃⁻) stable isotope measurement**

157 Samples for stable isotope analysis of NO₃⁻ were cut from the designated ice core section at 158 subannual resolution for the first 110 m (i.e., after 1928) and at 25 cm resolution for the 159 remaining 16 m (≈ 2-year intervals). Samples were then sealed in clean plastic bags (Whirl- 160 Pak) and left to melt at room temperature. The melted samples were initially passed through a 161 solid cationic extraction resin to extract NH₄⁺ for a coinciding study (Lamothe et al., 2023), 162 from which the remaining anionic solution was further separated with a Metrohm IC 850 in 163 suppressed mode (NaOH at 8 mM, suppressor H₂SO₄ at 50 mM, Dionex AG11 column) to 164 recover ≈ 100 nmol NO₃⁻ solutions. This anionic separation allows future research to measure 165 sulfate isotopes.

166 The denitrifying bacteria method (Casciotti et al., 2002; Kaiser et al., 2007) was used to 167 convert NO₃⁻ into N₂O gas that is then split into O₂ and N₂ on a 900 °C gold surface for 168 simultaneous measurement of δ¹⁵N, δ¹⁸O, and δ¹⁷O of NO₃⁻ with a Thermo Finnigan™ MAT- 169 253 Isotopic Ratio Mass Spectrometer equipped with a GasBench II™. Although oxygen 170 isotopic data were collected, they are not further discussed here other than examining potential 171 post-depositional effects. This method does not discriminate between NO₃⁻ and NO₂⁻, and thus 172 any NO₂⁻ present in the ice core will be converted to N₂O and potentially impact the NO₃⁻ 173 isotopic results; however, the concentration results indicate that this interference is negligible. 174 A calibration was performed against international isotopic standards (USGS 32, USGS 34, and
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USGS 35) to account for differing sample sizes (Frey et al., 2009; Morin et al., 2009). The measurement $\delta^{15}$N(NO$_3$⁻) mean square error of standards was ± 1.4 ‰. Finally, the $\delta^{15}$N(NO$_3$⁻) and [NO$_3$⁻] data spanning the last millennium were aggregated into 10-year and 25-year average periods for 1900–2010 and 1800–1900, respectively, for NO$_x$ inventory comparison.

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## 2.5 NO$_x$ inventory and its isotopic composition

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CDD is proven to record air masses originating from France, Germany, Switzerland, and Italy (Arienzo et al., 2021; Preunkert et al., 2003; Preunkert & Legrand, 2013). We compiled several datasets that cover the period 1750-2016 to construct a reference NO$_x$ inventory for these countries. NO$_x$ emissions from brown coal, biomass, coke, diesel oil, hard coal, heavy oil, light oil, gas, and industry were retrieved from the Community Emissions Data System (O'Rourke et al., 2021). Agricultural emissions of NO$_x$ were retrieved from Hoesly et al. (2018). Natural background emissions are reported for the preindustrial period (1860) and the present day (since 1990) at the global scale (Galloway et al., 2004), and we assumed these natural emissions varied linearly between these two dates. Based on the reported isotopic signatures of the NO$_x$ anthropogenic sources (Table S1) and of the preindustrial natural source, we calculated the estimated annual $\delta^{15}$N signal resulting from the inventory ($\delta^{15}$N$_{inv-mean}$). This isotopic signal is also calculated for the lowest and highest isotopic signatures of all the sources ($\delta^{15}$N$_{inv-low}$ and $\delta^{15}$N$_{inv-high}$, respectively) to account for the variance in isotopic value across the reported isotopic range of the source.

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# 3 Results and discussion

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## 3.1 Results

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We highlight five periods whose $\delta^{15}$N(NO$_3$⁻) values and NO$_3$⁻ concentrations ([NO$_3$⁻]) in CDD16-C01 are considered to reflect the history of NO$_3$⁻ emission variations in Western Europe (Figure 1):

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- Albeit coarsely resolved, the ‘pre-industrial’ period before 1700 presents very limited variations of $\delta^{15}$N(NO$_3$⁻) values and [NO$_3$⁻] with respective mean values ± standard deviation of (5.0 ± 1.0) ‰ and (119 ± 26) ppb, respectively.

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- During the 18th and 19th centuries, the minimum 68.5 ppb [NO$_3$⁻] value is reached in 1825 and is associated with a minimum $\delta^{15}$N(NO$_3$⁻) value of (2.4 ± 0.3) ‰. Both [NO$_3$⁻]
and $\delta^{15}$N(NO$_3$$^-) values gradually increase over the 19$^{th}$ century to respectively reach 81 ppb and $(4.8 \pm 0.5)$ ‰ in 1900.

- Between 1900 and 1950, [NO$_3$$^-$] is generally constant ((64.3 ± 9.5) ppb) while $\delta^{15}$N(NO$_3$$^-$) decreases from (4.8 ± 0.5) ‰ to (0.8 ± 0.5) ‰.

- For 1950–2000, [NO$_3$$^-$] increases substantially from 65.8 to 509 ppb, whereas $\delta^{15}$N(NO$_3$$^-$) first decreases from (0.8 ± 0.5) ‰ before varying around (-1.7 ± 1.1) ‰.

- From 2000 to 2016, [NO$_3$$^-$] decreases greatly from 509 to 287 ppb, and $\delta^{15}$N(NO$_3$$^-$) values decrease from (-1.5 ± 1.2) ‰ to (-4.1 ± 0.5) ‰.

<figure><img src="image_2.png" /><figcaption>**Figure 1:** NO$_3$$^-$ concentration in the CDD16 ice core (green) (/ppb) and decadal $\delta^{15}$N(NO$_3$$^-$) average (yellow) (/‰). The black dots represent the high resolution $\delta^{15}$N(NO$_3$$^-$) signal. The shaded yellow area is the 1$\sigma$ measurement uncertainty. The x axis is divided into two time frames with a low resolution before 1800, and a higher resolution afterwards.</figcaption></figure>

## 3.2 $\delta^{15}$N(NO$_3$$^-$) seasonality

Unlike previous measurements of $\delta^{15}$N(NO$_3$$^-$) in a CDD firn core (Freyer et al., 1996), no significant seasonal difference emerges in our subannually-resolved record spanning 1981 to 2016 (Figure S1). We assume that the relatively short range of time in the previous study (3 years) distorted the interpretation. With 35 years of $\delta^{15}$N(NO$_3$$^-$) variations at the seasonal scale, we confidently argue that $\delta^{15}$N(NO$_3$$^-$) values are not affected by seasonal phenomena (t-test, *p* = 0.006, n = 97).

In the subsequent discussion, all $\delta^{15}$N(NO$_3$$^-$) values will be mean annual (or decadal) values weighted according to NO$_3$$^-$ quantity.
226 **3.3 Post-depositional effects**

227 NO₃⁻ burial into the CDD firn is rapid due to high accumulation of ≈2.5 m w e a⁻¹ (Preunkert et al., 2000). As a result, post-depositional effects on NO₃⁻ concentration and isotopic composition are expected to be limited (Preunkert et al., 2003). Additionally, our NO₃⁻ data for the first meter of the core do not offer any evidence for post-depositional effects as no decreases of [NO₃⁻] are found in the top 80 cm roughly representing a month of accumulation (Figure S2a). More convincing, the δ¹⁵N(NO₃⁻) values in the top meter also suggest an insignificant influence of post-depositional phenomena. Any isotopic effect from volatilization would likely be minor (Shi et al., 2019), whereas we expect that photolysis would leave a definite enrichment in ¹⁵N (Erbland et al., 2013) that is not found in CDD (Figure S2a). This conclusion is also supported by oxygen NO₃⁻ stable isotopes. Indeed, photolysis of NO₃⁻ also results in a decrease in δ¹⁸O and Δ¹⁷O of NO₃⁻ with depth in the snowpack (Blunier et al., 2005; Erbland et al., 2013; Frey et al., 2009), but no such variations are measured in the first meter of snow at CDD (Figure S2b). Hence, it seems highly unlikely that these post-depositional phenomena have a notable impact on archived NO₃⁻ at CDD.

241 **3.4 Isotopic fractionation in the atmosphere**

242 Nitrogen isotopic fractionations can occur in the atmosphere during the NOₓ cycle (Freyer et al., 1993; Walters et al., 2016), NO₂ conversion into HNO₃ (Fang et al., 2021), and/or HNO₃(gas)-p-NO₃⁻ partitioning during transportation (Freyer, 1991; Geng et al., 2014). Here we show that these potential fractionations likely do not influence the δ¹⁵N(NO₃⁻) record at CDD.

246 Previous studies evaluated the importance of isotopic fractionations sensitivity tests. 247 This demonstrated that isotopic fractionation is negligible on ice core δ¹⁵N(NO₃⁻) during NOₓ cycling (Geng et al., 2014; Li et al., 2020) and NO₂ conversion into HNO₃ (Freyer, 1991; Li et al., 2020), but acidity-driven isotopic fractionation during HNO₃(gas)-NO₃⁻(particle) partitioning is substantial (Geng et al., 2014; Li et al., 2020). In that way, 20th century δ¹⁵N(NO₃⁻) fluctuations in a Summit ice core were proposed to be solely explained through this fractionation effect (Geng et al., 2014). Although the presence of atmospheric HNO₃ around CDD (Preunkert et al., 2007) could favor gas-particle exchanges until nitrate's deposition in the snowpack, no anticorrelation is noticed between acidity ([H⁺]) and the δ¹⁵N(NO₃⁻) values in CDD ice core (Figure S3), unlike the Summit ice core. Additionally, the application of the Rayleigh distillation model developed by Geng et al. (2014) cannot explain the δ¹⁵N(NO₃⁻) variations for
CDD (Figure S3). We therefore assume that either acidity-related isotopic fractionation does not actually influence the $\delta^{15}$N(NO$_3$⁻) measured in ice cores, or the close proximity of CDD to NO$_x$ sources, unlike Greenland, drastically reduces the isotopic fractionation related to the HNO$_3$(gas)-NO$_3$⁻(particle) partitioning, making the related isotopic fractionation negligible. Recent analysis of a second Greenland ice core suggests that this acidity-driven isotopic fractionation cannot solely explain the $\delta^{15}$N(NO$_3$⁻) signal after emission control started from the 1980s (Wei et al., *under review*).

Finally, other isotopic fractionations may still not be captured by current knowledge that could result from atmospheric transportation of particles (Li et al., 2020). In Alpine sites, transportation from emission sites to the higher elevation of the drilling sites is marked by upward convection with seasonal variations. In summer strong convection favors upward transport, while being deeply limited in winter. As a result, isotopic fractionations originating from transportation should follow this seasonal pattern. Since we showed that $\delta^{15}$N(NO$_3$⁻) signal does not display seasonal variations (Figure S1), such fractionation likely remains negligible.

While isotopic fractionations that occur during chemical conversions and/or during NO$_3$⁻ transportation cannot be absolutely excluded, previous research and our results at CDD suggest that no clear impacts from atmospheric fractionations are detected or expected. We caution that this conclusion is perhaps only the case for this site and this would need to be verified for other sites before this assumption is applied. As a result, the $\delta^{15}$N(NO$_3$⁻) values from CDD16-C01 likely recorded the changes of NO$_x$ sources in Western Europe throughout the last millennium.

## 3.5 Discussions on NO$_x$ sources

As post-emission isotopic changes appear limited for the CDD NO$_3$⁻ record, the $\delta^{15}$N(NO$_3$⁻) signal can be directly compared with the $\delta^{15}$N(NO$_3$⁻) predicted from the source inventory. Firstly, the preindustrial isotopic and concentration composition can be attributed to natural emissions of NO$_x$ from lightning, soil microorganisms, and oceans, as the anthropogenic emissions of NO$_x$ remain limited at that time (Galloway et al., 2004). Thus, we assign the preindustrial CDD $\delta^{15}$N(NO$_3$⁻) value, (5.0 ± 1.0) ‰, to represent the isotopic signature of the mean NO$_x$ natural background. Between 1000 and 1700, the $\delta^{15}$N(NO$_3$⁻) remains stable, while the concentration fluctuates (Figure 1). This supports consistent relative contributions from the various natural sources despite potential variations in the overall magnitude of natural NO$_x$
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emissions. However, the low resolution of ice core measurements during this period of time does not allow us to determine the causes of such concentration fluctuations.

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During the 18th and 19th centuries, although changes in temperature (Ahmed et al., 2013) and soil usage (Kaplan et al., 2009) could have affected the natural NOₓ emissions, we attribute the increase in δ¹⁵N(NO₃⁻) value to anthropogenic sources. Inventories report a substantial increase in anthropogenic NOₓ emissions, particularly from coal combustion (O'Rourke et al., 2021), which tends to emit NOₓ with a high isotopic signature (Felix et al., 2012). However, while industrial and agricultural sources were also increasing during this time, natural emissions were decreasing (Galloway et al., 2004; Hoesly et al., 2018; O'Rourke et al., 2021). The observed δ¹⁵N(NO₃⁻) signal reflects the combined impact of these source changes. In particular, if only one source was considered as the driver of NOₓ emission evolution, i.e., coal combustion with the others considered constant, the δ¹⁵N(NO₃⁻) vs 1/[NO₃⁻] plot would result in a linear function. This is commonly referred as the Keeling plot method (Köhler et al., 2006). During this period, this δ¹⁵N(NO₃⁻) vs 1/[NO₃⁻] plot is not linear, indicating that multiple changes of NOₓ sources occurred.

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During the early 19th century, the observed δ¹⁵N(NO₃⁻) falls within the range of δ¹⁵N(NO₃⁻) values predicted in our reference inventory (Figure 2). This provides confidence in these estimations of anthropogenic sources but also, and primarily, to the estimate of the natural contribution, which is the main source of NOₓ. However, from 1850, δ¹⁵N(NO₃⁻) diverges from the mean predicted values, reaching the lowest δ¹⁵Nₗₐᵥ⁻ₐže at the end of the century (Figure 2). Investigating the next period may provide clues to explain this divergence.

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The period from 1900 to 1950 saw significant economic development but was also marked by major crises such as wars and the 1930s depression. NOₓ emissions increased during this period, except during the crises. The ice core δ¹⁵N(NO₃⁻) values during this period are substantially lower than the mean values predicted by inventories and are even lower than the lowest inventory-derived estimate range (Figure 2). This implies that sources with high δ¹⁵N values, such as coal, natural background, and biomass burning, contributed relatively less to NOₓ emissions than reported in the inventories. Natural background and biomass burning are unlikely to drive this divergence from expected δ¹⁵N(NO₃⁻) values, as even reducing their inventory contribution to zero does not match observed ice core values. Therefore, the observed δ¹⁵N(NO₃⁻) can only be explained by decreasing NOₓ emissions from coal and/or increasing emissions from oil and/or agriculture. The coal-induced NOₓ emissions are correlated to the
quantity of coal that is burnt. The measurement of thallium concentration in another CDD ice core tends to confirm support the inventory of coal-induced NOₓ (Legrand et al., 2022). Similarly, oil combustion is unlikely to be underestimated since no increase in non-crustal vanadium concentration, associated with oil combustion, in a CDD ice core (Arienzo et al., 2021). Hence, it is reasonable to argue that, during the first half of the 20th century, the difference between the ice core and the inventory results from underestimated agricultural emissions. Based on an isotopic mass balance, a crude estimate would suggest agricultural-induced NOₓ emissions to be 10 times higher than expected if the difference between inventory and ice core measurements was only explained by agriculture. Given the fact that agricultural emissions are difficult to quantify (Lagerwerf et al., 2019), this conclusion seems plausible. Thus, we suggest that from the late 19th century, Western-European agriculture had already increased substantially its productivity. This increasing importance of agriculture, before the discovery of the Haber Bosch process, is concomitant with increasing imports of Chile saltpeter, used as fertilizer (Soledad Ureta-Zañartu, 2022).

<figure><img src="image_3.png" /><figcaption>**Figure 2:** CDD16 ice core δ¹⁵N(NO₃⁻) measurements (red) and calculated based on the existing NOₓ emission inventories (blue). Shaded areas represent uncertainties in δ¹⁵N(NO₃⁻) measurements (red) and isotopic signatures (blue). All data can be found in supplementary.</figcaption></figure>

From 1900 onwards, the CDD16 δ¹⁵N(NO₃⁻) signal substantially decreases in accordance with observations in other archives (Holtgrieve et al., 2011; Mason et al., 2022), including ice cores (Hastings et al., 2009; Li et al., 2020; Vega et al., 2015). By applying the Keeling plot method between 1910 and 2000, we found that an additional source was added to a constant background with an isotopic signature of (-2.7 ± 0.4) ‰ (Figure 3, R² = 0.71, p =
3×10⁻⁴). This signature falls within the range of oil combustion (Miller et al., 2017). We note that this interpretation of the Keeling plot assumes that the other emission components of the background (e.g., natural) are constant and that simply having a linear correlation between δ¹⁵N(NO₃⁻) and 1/NO₃⁻ does not on its own demonstrate that these other components are nonvarying. However, the Keeling plot's identification of oil combustion is in line with historical expectations and provides potential insight into the interpretation of the evolution in natural NOₓ emissions. We should remind that this conclusion is only applicable to the CDD ice core and that debates exist regarding the interpretation of the δ¹⁵N decline in other archives.

<figure><img src="image_4.png" /><figcaption>**Figure 3:** Keeling plot for the period 1910-2000 with 5-year average δ¹⁵N(NO₃⁻) (/%) as a function of 1/[NO₃⁻] (/ppb⁻¹). Shaded area is the 95 % confidence interval of the linear regression. The 1940 value is discarded (green cross) likely being an outlier value (residue to linear regression statistically different (>2σ)).</figcaption></figure>

Previous comparisons of Alpine ice core [NO₃⁻] with Western European NOₓ inventories have reported a discrepancy between NOₓ inventories and ice core [NO₃⁻] since the 1990s (Eichler et al., 2023; Preunkert et al., 2003). In fact, inventories report a steady decrease of total NOₓ emissions since 1980s due to enacted mitigation strategies, but ice cores' [NO₃⁻] only decreased since 2000. A mis-quantification of mitigation strategies has been proposed to explain these discrepancies. However, these studies also suggested that changes in aerosol
reactivity and formation could also explain this difference between NOₓ emissions and aerosol deposition in the ice. Taking the advantage of our new ice core record that extends to 2016, we can answer this question using δ¹⁵N(NO₃⁻). For NOₓ emissions to differ from aerosol deposition, an atmospheric isotopic fractionation should occur, and the decrease in atmospheric acidity due to decreasing SO₂ emissions would increase δ¹⁵N(NO₃⁻) values. Instead, the opposite is observed, with a further decrease in δ¹⁵N(NO₃⁻) after 2000 (Figure S3). As a result, we can convincingly argue that the difference between ice core [NO₃⁻] and inventory NOₓ emissions results from miscalculated effects of the mitigation strategies. Our ice core records that mitigation had substantial impact on the deposited NO₃⁻ concentration with a decrease of -60 % between 2000 and 2016, but this is 20 years later than expected. The Keeling plot allows to decipher the changes of sources during this period. The intercept value of (0.2 ± 1.2) ‰ suggests that combustion sources drive the decrease in NOₓ emissions. We note that this value is higher than the 20th century value of -2.7 ‰, but implementation of recent NOₓ-limiting technologies are reported to increase the isotopic signature of oil combustion (Walters et al., 2015). Consequently, the decrease in combustion-induced NOₓ emissions increases the relative contribution of agricultural sources.

As previously mentioned, Song et al. suggested that recent emission inventories underestimate the NOₓ emissions from non-fossil fuel sources, which account for (54 ± 13) % of total NOₓ emissions (2021). However, our ice core results do not support that strong disagreement with emission inventories, and the sampling differences between their study and ours help explain this discrepancy. Our ice core signal records a homogenized spatial record of Western European emission, while Song et al. (2021) gathered data from NO₃⁻ in precipitation from 23 individual European point sites. This emphasizes that the extrapolation of point measurements, often from anthropized environments, will often differ from the true regional character, as well as highlighting the strength of Alpine ice cores for capturing more accurate regional records. Additionally, Song et al.'s conclusions lack of consideration of natural sources. Indeed, because to the best of our knowledge no value of isotopic signature for the natural background had been reported before our study, the authors could not account for the natural contribution, leading to an overestimate of the non-fossil fuel NOₓ emissions.

# 4 Synthesis and conclusion

Undeniably, anthropogenic activities have deeply affected the nitrogen cycle over the past century, through considerably increased NOₓ emissions and changes in NOₓ sources. Because
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chemical and physical processes involved in the nitrogen cycle are recorded in ice core $\delta^{15}$N(NO$_3$⁻) values, we have analyzed a Col du Dôme ice core (Mont Blanc, France) to further examine this anthropogenic impact. This ice core benefits from close proximity to European anthropogenic sources, which limits possible isotopic fractionation from transport, and has a high annual accumulation rate, which limits NO$_3$⁻ photolysis and other post-depositional effects. The source-tracer $\delta^{15}$N(NO$_3$⁻) indicates substantial changes since pre-industrial times that do not fully align with established NO$_x$ inventories.

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Using the pre-industrial $^{15}$N composition of NO$_3$⁻ as the isotopic signature of NO$_x$ emissions from natural sources, we show that the 18$^{th}$ and 19$^{th}$ century NO$_x$ inventories agree with the observed ice core $\delta^{15}$N(NO$_3$⁻) values. However, from 1850 onwards, inventories and ice core values diverge. Between 1900 and 1950, we confidently argue that this divergence can only result from the mis-quantification of the NO$_x$ source inventory. In particular, we suggest that agricultural sources may have been underestimated. The steep rise in NO$_x$ emissions after 1950 is primarily attributed to increasing emissions in oil combustion. Finally, after 2000, ice records a decrease in NO$_3$⁻ concentrations due to the implementation of mitigation strategies. It is worth noting that this decrease in ice core NO$_3$⁻ concentration occurs 20 years later than expected in inventories for unknown reasons.

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Our study shows that glacially-archived $\delta^{15}$N(NO$_3$⁻) from Alpine glaciers can be directly compared with NO$_x$ emission inventories. Hence, at a time when Alpine glaciers are increasingly under threat of melting, we must remember that they offer unparalleled clues to understanding aerosol sources across time and at regional levels.

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## Acknowledgement

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We acknowledge the LabEx OSUG@2020 (“Investissements d’avenir” – ANR10 LABX56), the ANR project ANR-15-IDEX-02 (IDEX-UGA), the University Grenoble Alpes Foundation, Ice Memory Foundation and its funders, and the technical support from the F2G (French National platform for Coring and Drilling, handled by INSU, the MESRI (Ministry of Higher Education, Research, and Innovation) for supporting this research. LEFE-IMAGO, a scientific program of the Institut National des Sciences de l’Univers (INSU/CNRS), has also provided partial funding for this study. We deeply thank Selin Bagci for lab handling, Luc Piard, Bruno Jourdain, Christian Vincent, Vladimir Mikhalenko, Victor Zagorodnov and other colleagues for the ice core drilling, and Meredith Hastings and Lei Geng for scientific discussion.
426 **Competing interests**

427 The authors do not declare any conflict of interests.

428

429 **Data availability statement**

430 Data are accessible in supplementary (Table S2).

431

432 **References**

433 Ahmed, M., Anchukaitis, K. J., Asrat, A., Borgaonkar, H. P., Braida, M., Buckley, B. M., Büntgen, U.,

434 Chase, B. M., Christie, D. A., Cook, E. R., Curran, M. A. J., Diaz, H. F., Esper, J., Fan, Z.-X., Gaire,

435 N. P., Ge, Q., Gergis, J., González-Rouco, J. F., Goosse, H., ... PAGES 2k Consortium. (2013).

436 Continental-scale temperature variability during the past two millennia. *Nature Geoscience*,

437 6(5), Article 5. https://doi.org/10.1038/ngeo1797

438 Akers, P. D., Savarino, J., Caillon, N., Servettaz, A. P., Le Meur, E., Magand, O., Martins, J., Agosta, C.,

439 Crockford, P., & Kobayashi, K. (2022). Sunlight-driven nitrate loss records Antarctic surface

440 mass balance. *Nature Communications*.

441 Arienzo, M., Legrand, M., Preunkert, S., Stohl, A., Chellman, N., Eckhardt, S., Gleason, K., &

442 McConnell, J. (2021). Alpine ice-core evidence of a large increase in vanadium and

443 molybdenum pollution in Western Europe during the 20th century. *JGR Atmosphere*.

444 https://doi.org/10.1029/2020JD033211

445 Blunier, T., Floch, G. L., Jacobi, H.-W., & Quansah, E. (2005). Isotopic view on nitrate loss in Antarctic

446 surface snow. *Geophysical Research Letters*, 32(13). https://doi.org/10.1029/2005GL023011

447 Casciotti, K. L., Sigman, D. M., Hastings, M. G., Böhlke, J. K., & Hilkert, A. (2002). Measurement of the

448 Oxygen Isotopic Composition of Nitrate in Seawater and Freshwater Using the Denitrifier

449 Method. *Analytical Chemistry*, 74(19), 4905-4912. https://doi.org/10.1021/ac020113w
450 Clifford, H. M., Spaulding, N. E., Kurbatov, A. V., More, A., Korotkikh, E. V., Sneed, S. B., Handley, M.,
451 Maasch, K. A., Loveluck, C. P., Chaplin, J., McCormick, M., & Mayewski, P. A. (2019). A 2000
452 Year Saharan Dust Event Proxy Record from an Ice Core in the European Alps. *Journal of
453 Geophysical Research: Atmospheres, 124*(23). https://doi.org/10.1029/2019JD030725

454 Durand, P., Breuer, L., Johnes, P. J., Billen, G., Butturini, A., Pinay, G., van Grinsven, H., Garnier, J.,
455 Rivett, M., Reay, D. S., Curtis, C., Siemens, J., Maberly, S., Kaste, Ø., Humborg, C., Loeb, R.,
456 de Klein, J., Hejzlar, J., Skoulikidis, N., ... Wright, R. (2011). Nitrogen processes in aquatic
457 ecosystems. In M. A. Sutton, C. M. Howard, J. W. Erisman, G. Billen, A. Bleeker, P. Grennfelt,
458 H. van Grinsven, & B. Grizzetti (Éds.), *The European Nitrogen Assessment : Sources, Effects
459 and Policy Perspectives* (p. 126-146). Cambridge University Press.
460 https://doi.org/10.1017/CBO9780511976988.010

461 Eichler, A., Legrand, M., Jenk, T. M., Preunkert, S., Andersson, C., Eckhardt, S., Engardt, M., Plach, A.,
462 & Schwikowski, M. (2023). Consistent histories of anthropogenic western European air
463 pollution preserved in different Alpine ice cores. *The Cryosphere*, 17(5), 2119-2137.
464 https://doi.org/10.5194/tc-17-2119-2023

465 Erbland, J., Vicars, W. C., Savarino, J., Morin, S., Frey, M. M., Frosini, D., Vince, E., & Martins, J. M. F.
466 (2013). Air-snow transfer of nitrate on the East Antarctic Plateau &ndash; Part 1 : Isotopic
467 evidence for a photolytically driven dynamic equilibrium in summer. *Atmospheric Chemistry
468 and Physics*, 13(13), 6403-6419. https://doi.org/10.5194/acp-13-6403-2013

469 European Environment Agency. (2021). *European Union emission inventory report 1990-2019 under
the UNECE Convention on Long-range Transboundary Air Pollution (Air Convention)*
470 [Publication]. https://www.eea.europa.eu//publications/lrtap-1990-2019
471

472 Fang, H., Walters, W. W., Mase, D., & Michalski, G. (2021). iNRACM: incorporating 15N into the
473 Regional Atmospheric Chemistry Mechanism (RACM) for assessing the role photochemistry
474 plays in controlling the isotopic composition of NOx, NOy, and atmospheric nitrate.
475 *Geoscientific Model Development*, 14(8). https://doi.org/10.5194/gmd-14-5001-2021
476

Favez, O., El Haddad, I., Piot, C., Boréave, A., Abidi, E., Marchand, N., Jaffrezo, J.-L., Besombes, J.-L.,

477

Personnaz, M.-B., Sciare, J., Wortham, H., George, C., & D'Anna, B. (2010). Inter-comparison

478

of source apportionment models for the estimation of wood burning aerosols during

479

wintertime in an Alpine city (Grenoble, France). *Atmospheric Chemistry and Physics*, 10(12),

480

5295-5314. https://doi.org/10.5194/acp-10-5295-2010

481

Felix, J. D., & Elliott, E. M. (2013). The agricultural history of human-nitrogen interactions as recorded

482

in ice core $\delta^{15}$N-$NO_3^-$*. Geophysical Research Letters, 40*(8), 1642-1646.

483

https://doi.org/10.1002/grl.50209

484

Felix, J. D., Elliott, E. M., & Shaw, S. L. (2012). Nitrogen Isotopic Composition of Coal-Fired Power

485

Plant NOx: Influence of Emission Controls and Implications for Global Emission Inventories.

486

*Environmental Science & Technology*, 46(6), 3528-3535. https://doi.org/10.1021/es203355v

487

Fishman, J., & Crutzen, P. J. (1978). The origin of ozone in the troposphere. *Nature*, 274(5674),

488

855-858. https://doi.org/10.1038/274855a0

489

Frey, M. M., Savarino, J., Morin, S., Erbland, J., & Martins, J. M. F. (2009). Photolysis imprint in the

490

nitrate stable isotope signal in snow and atmosphere of East Antarctica and implications for

491

reactive nitrogen cycling. *Atmospheric Chemistry and Physics*, 9(22), 8681-8696.

492

https://doi.org/10.5194/acp-9-8681-2009

493

Freyer, H. D. (1991). Seasonal variation of 15N/14N ratios in atmospheric nitrate species. *Tellus B*,

494

43(1), 30-44. https://doi.org/10.1034/j.1600-0889.1991.00003.x

495

Freyer, H. D., Kley, D., Volz-Thomas, A., & Kobel, K. (1993). On the interaction of isotopic exchange

496

processes with photochemical reactions in atmospheric oxides of nitrogen. *Journal of*

497

*Geophysical Research, 98*(D8). https://doi.org/10.1029/93JD00874

498

Freyer, H. D., Kobel, K., Delmas, R. J., Kley, D., & Legrand, M. R. (1996). First results of 15N/14N ratios

499

in nitrate from alpine and polar ice cores. *Tellus B: Chemical and Physical Meteorology*, 48(1),

500

93-105. https://doi.org/10.3402/tellusb.v48i1.15671
Galloway, J. N., Dentener, F. J., Capone, D. G., Boyer, E. W., Howarth, R. W., Seitzinger, S. P., Asner, G.

P., Cleveland, C. C., Green, P. A., Holland, E. A., Karl, D. M., Michaels, A. F., Porter, J. H.,

Townsend, A. R., & Vöosmarty, C. J. (2004). Nitrogen Cycles: Past, Present, and Future.

*Biogeochemistry*, 70(2), 153-226. https://doi.org/10.1007/s10533-004-0370-0

Geng, L., Alexander, B., Cole-Dai, J., Steig, E. J., Savarino, J., Sofen, E. D., & Schauer, A. J. (2014).

Nitrogen isotopes in ice core nitrate linked to anthropogenic atmospheric acidity change.

*Proceedings of the National Academy of Sciences*, 111(16).

https://doi.org/10.1073/pnas.1319441111

Gilbert, A., Gagliardini, O., Vincent, C., & Wagnon, P. (2014). A 3-D thermal regime model suitable for

cold accumulation zones of polythermal mountain glaciers. *Journal of Geophysical Research: Earth Surface*, 119(9), 1876-1893. https://doi.org/10.1002/2014JF003199

Hastings, M. G., Jarvis, J. C., & Steig, E. J. (2009). Anthropogenic Impacts on Nitrogen Isotopes of Ice-Core Nitrate. *Science*, 324(5932), 1288. https://doi.org/10.1126/science.1170510

Hoesly, R. M., Smith, S. J., Feng, L., Klimont, Z., Janssens-Maenhout, G., Pitkanen, T., Seibert, J. J., Vu, L., Andres, R. J., Bolt, R. M., Bond, T. C., Dawidowski, L., Kholod, N., Kurokawa, J., Li, M., Liu, L., Lu, Z., Moura, M. C. P., O'Rourke, P. R., & Zhang, Q. (2018). Historical (1750-2014) anthropogenic emissions of reactive gases and aerosols from the Community Emissions Data System (CEDS). *Geoscientific Model Development*, 11(1). https://doi.org/10.5194/gmd-11-369-2018

Holtgrieve, G. W., Schindler, D. E., Hobbs, W. O., Leavitt, P. R., Ward, E. J., Bunting, L., Chen, G., Finney, B. P., Gregory-Eaves, I., Holmgren, S., Lisac, M. J., Lisi, P. J., Nydick, K., Rogers, L. A., Saros, J. E., Selbie, D. T., Shapley, M. D., Walsh, P. B., & Wolfe, A. P. (2011). A Coherent Signature of Anthropogenic Nitrogen Deposition to Remote Watersheds of the Northern Hemisphere. *Science*, 334(6062), 1545-1548. https://doi.org/10.1126/science.1212267
Jiang, Z., Alexander, B., Savarino, J., Erbland, J., & Geng, L. (2021). Impacts of the photo-driven post-depositional processing on snow nitrate and its isotopes at Summit, Greenland: A model-based study. *The Cryosphere*. https://doi.org/10.5194/tc-15-4207-2021

Kaiser, J., Hastings, M. G., Houlton, B. Z., Röckmann, T., & Sigman, D. M. (2007). Triple Oxygen Isotope Analysis of Nitrate Using the Denitrifier Method and Thermal Decomposition of N₂O. *Analytical Chemistry*, 79(2). https://doi.org/10.1021/ac061022s

Kaplan, J. O., Krumhardt, K. M., & Zimmermann, N. (2009). The prehistoric and preindustrial deforestation of Europe. *Quaternary Science Reviews*, 28(27), 3016-3034. https://doi.org/10.1016/j.quascirev.2009.09.028

Köhler, P., Fischer, H., Schmitt, J., & Munhoven, G. (2006). On the application and interpretation of Keeling plots in paleo climate research &ndash; deciphering &delta;^13C of atmospheric CO₂ measured in ice cores. *Biogeosciences*, 3(4), 539-556. https://doi.org/10.5194/bg-3-539-2006

Lagerwerf, L. A., Bannink, A., Bruggen, C. van, Groenestein, C. M., Huijsmans, J. F. M., Kolk, J. W. H. van der, Luesink, H. H., Sluis, S. M. van der, Velthof, G. L., & Vonk, J. (2019). *Methodology for estimating emissions from agriculture in the Netherlands: Calculations of CH₄, NH₃, N₂O, NOx, NMVOC, PM10, PM2.5 and CO₂ with the National Emission Model for Agriculture* (NEMA), *Update 2019* (148; p.). WOT Natuur & Milieu. https://library.wur.nl/WebQuery/wurpubs/549203

Lamothe, A., Savarino, J., Ginot, P., Soussaintjean, L., Gautier, E., Akers, P. D., Caillon, N., & Erbland, J. (2023). An extraction method for nitrogen isotope measurement of ammonium in a low-concentration environment. *Atmospheric Measurement Techniques*, 16(17), 4015-4030. https://doi.org/10.5194/amt-16-4015-2023

Legrand, M., McConnell, J. R., Preunkert, S., Bergametti, G., Chellman, N. J., Desboeufs, K., Plach, A., Stohl, A., & Eckhardt, S. (2022). Thallium Pollution in Europe Over the Twentieth Century. Recorded in Alpine Ice: Contributions From Coal Burning and Cement Production.
551 *Geophysical Research Letters*, 49(13), e2022GL098688.

552 https://doi.org/10.1029/2022GL098688

553 Legrand, M., Preunkert, S., Wagenbach, D., Cachier, H., & Puxbaum, H. (2003). A historical record of formate and acetate from a high-elevation Alpine glacier : Implications for their natural versus anthropogenic budgets at the European scale. *Journal of Geophysical Research: Atmospheres*, 108(D24). https://doi.org/10.1029/2003JD003594

557 Legrand, M., Preunkert, S., Wagenbach, D., & Fischer, H. (2002). Seasonally resolved Alpine and Greenland ice core records of anthropogenic HCl emissions over the 20th century. *Journal of Geophysical Research*, 107(D12), 4139-4153. https://doi.org/10.1029/2001JD001165

560 Leip, A., Achermann, B., Billen, G., Bleeker, A., Bouwman, A. F., de Vries, W., Dragosits, U., Döring, U., Fernall, D., Geupel, M., Herolstab, J., Johnes, P., Le Gall, A. C., Monni, S., Nevečeřal, R., Orlandini, L., Prud'homme, M., Reuter, H. I., Simpson, D., ... Winiwarter, W. (2011). Integrating nitrogen fluxes at the European scale. In M. A. Sutton, C. M. Howard, J. W. Erisman, G. Billen, A. Bleeker, P. Grennfelt, H. van Grinsven, & B. Grizzetti (Éds.), *The European Nitrogen Assessment : Sources, Effects and Policy Perspectives* (p. 345-376). Cambridge University Press. https://doi.org/10.1017/CBO9780511976988.019

567 Li, Z., Hastings, M. G., Walters, W. W., Tian, L., Clemens, S. C., Song, L., Shao, L., & Fang, Y. (2020).

568 Isotopic evidence that recent agriculture overprints climate variability in nitrogen deposition to the Tibetan Plateau. *Environment International*, 138, 105614.

570 https://doi.org/10.1016/j.envint.2020.105614

571 Mason, R. E., Craine, J. M., Lany, N. K., Jonard, M., Ollinger, S. V., Groffman, P. M., Fulweiler, R. W., Angerer, J., Read, Q. D., Reich, P. B., Templer, P. H., & Elmore, A. J. (2022). Evidence, causes, and consequences of declining nitrogen availability in terrestrial ecosystems. *Science*.

574 https://doi.org/10.1126/science.abh3767
575 Miller, D. J., Wojtal, P. K., Clark, S. C., & Hastings, M. G. (2017). Vehicle NOx emission plume isotopic

576 signatures: Spatial variability across the eastern United States. *Journal of Geophysical*

577 *Research: Atmospheres*, 122(8), 4698-4717. https://doi.org/10.1002/2016JD025877

578 Morin, S., Savarino, J., Frey, M. M., Domine, F., Jacobi, H.-W., Kaleschke, L., & Martins, J. M. F. (2009).

579 Comprehensive isotopic composition of atmospheric nitrate in the Atlantic Ocean boundary

580 layer from 65°S to 79°N. *Journal of Geophysical Research: Atmospheres*, 114(D5).

581 https://doi.org/10.1029/2008JD010696

582 O'Rourke, P. R., Smith, S. J., Mott, A., Ahsan, H., McDuffie, E. E., Crippa, M., Klimont, Z., McDonald, B.,

583 Wang, S., Nicholson, M. B., Feng, L., & Hoesly, R. M. (2021). *CEDS v_2021_02_05 Release*

584 *Emission Data* (v_2021_02_05) [jeu de données]. Zenodo.

585 https://doi.org/10.5281/zenodo.4509372

586 Preunkert, S., Bohleber, P., Legrand, M., Fischer, H., Gilbert, A., Erhardt, T., Purtschert, R., Zipf, L.,

587 Waldner, A., & McConnell, J. R. (2023). Impact of subsurface crevassing on the depth-age

588 relationship of high-alpine ice cores extracted at Col du D&ocirc;me between 1994 and 2012.

589 *The Cryosphere Discussions*, 1-21. https://doi.org/10.5194/tc-2022-259

590 Preunkert, S., & Legrand, M. (2013). Towards a quasi-complete reconstruction of past atmospheric

591 aerosol load and composition (organic and inorganic) over Europe since 1920 inferred from

592 Alpine ice cores. *Climate of the Past*, 9(4), 1403-1416. https://doi.org/10.5194/cp-9-1403-

593 2013

594 Preunkert, S., Legrand, M., Jourdain, B., & Dombrowski-Etchevers, I. (2007). Acidic gases (HCOOH,

595 CH3COOH, HNO3, HCl, and SO2) and related aerosol species at a high mountain Alpine site

596 (4360 m elevation) in Europe. *Journal of Geophysical Research: Atmospheres*, 112(D23).

597 https://doi.org/10.1029/2006JD008225

598 Preunkert, S., Wagenbach, D., & Legrand, M. (2003). A seasonally resolved alpine ice core record of

599 nitrate: Comparison with anthropogenic inventories and estimation of preindustrial
600 emissions of NO in Europe. *Journal of Geophysical Research: Atmospheres*, 108(D21).

601 https://doi.org/10.1029/2003JD003475

602 Preunkert, S., Wagenbach, D., Legrand, M., & Vincent, C. (2000). Col du Dôme (Mt Blanc Massif,

603 French Alps) suitability for ice-core studies in relation with past atmospheric chemistry over

604 Europe. *Tellus B: Chemical and Physical Meteorology*, 52(3), 993-1012.

605 https://doi.org/10.3402/tellusb.v52i3.17081

606 Röthlisberger, R., Bigler, M., Hutterli, M., Sommer, S., Stauffer, B., Junghans, H. G., & Wagenbach, D.

607 (2000). Technique for Continuous High-Resolution Analysis of Trace Substances in Firn and

608 Ice Cores. *Environmental Science & Technology*, 34(2), 338-342.

609 https://doi.org/10.1021/es9907055

610 Shi, G., Chai, J., Zhu, Z., Hu, Z., Chen, Z., Yu, J., Ma, T., Ma, H., An, C., Jiang, S., Tang, X., & Hastings, M.

611 G. (2019). Isotope Fractionation of Nitrate During Volatilization in Snow : A Field Investigation

612 in Antarctica. *Geophysical Research Letters*, 46(6), 3287-3297.

613 https://doi.org/10.1029/2019GL081968

614 Soledad Ureta-Zañartu, M. (2022). How chemistry shaped societies : The case of the Atacama Desert

615 in Chile. *ChemTexts*, 9(1), 1. https://doi.org/10.1007/s40828-022-00172-3

616 Song, W., Liu, X.-Y., Hu, C.-C., Chen, G.-Y., Liu, X.-J., Walters, W. W., Michalski, G., & Liu, C.-Q. (2021).

617 Important contributions of non-fossil fuel nitrogen oxides emissions. *Nature*

618 *Communications*, 12(1), 243. https://doi.org/10.1038/s41467-020-20356-0

619 Vega, C. P., Pohjola, V. A., Samyn, D., Pettersson, R., Isaksson, E., Björkman, M. P., Martma, T., Marca,

620 A., & Kaiser, J. (2015). First ice core records of NO3– stable isotopes from Lomonosovfonna,

621 Svalbard. *Journal of Geophysical Research: Atmospheres*, 120(1), 313-330.

622 https://doi.org/10.1002/2013JD020930

623 Vicars, W. C., Morin, S., Savarino, J., Wagner, N. L., Erbland, J., Vince, E., Martins, J. M. F., Lerner, B.

624 M., Quinn, P. K., Coffman, D. J., Williams, E. J., & Brown, S. S. (2013). Spatial and diurnal

625 variability in reactive nitrogen oxide chemistry as reflected in the isotopic composition of
626 atmospheric nitrate : Results from the CalNex 2010 field study. *Journal of Geophysical Research: Atmospheres*, 118(18), 10,567-10,588. https://doi.org/10.1002/jgrd.50680
627
628 Walters, W. W., Goodwin, S. R., & Michalski, G. (2015). Nitrogen Stable Isotope Composition (δ15N) of Vehicle-Emitted NOx. *Environmental Science & Technology*, 49(4), 2278-2285.
629
630 https://doi.org/10.1021/es505580v
631 Walters, W. W., Simonini, D. S., & Michalski, G. (2016). Nitrogen isotope exchange between NO and NO₂ and its implications for δ¹⁵N variations in tropospheric NOₓ and atmospheric nitrate. *Geophysical Research Letters*, 43(1), 440-448. https://doi.org/10.1002/2015GL066438
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# 1000 years of Nitrogen Oxide Sources in Western Europe: Evidence from Nitrogen

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## Stable Isotopes (δ¹⁵N) of Nitrate in a Mont Blanc Ice Core

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Alexis Lamothe¹, Pete D. Akers², Sarah Albertin³,¹, Nicolas Caillon¹, Sophie Darfeuil¹,

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Elsa Gautier¹, Patrick Ginot¹, Shohei Hattori⁴,⁵, Joël Savarino¹

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¹Univ. Grenoble Alpes, CNRS, IRD, INRAE, Grenoble INP*, IGE, 38000 Grenoble, France

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*Institute of Engineering and Management Univ. Grenoble Alpes

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²Discipline of Geography, School of Natural Sciences, Trinity College Dublin, Dublin, Ireland

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³LATMOS/IPSL, Sorbonne Université, UVSQ, CNRS, 75005 Paris, France

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⁴International Center for Isotope Effects Research (ICIER), Nanjing University, Nanjing,

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210023, China

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⁵Frontiers Science Center for Critical Earth Material Cycling, State Key Laboratory for

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Mineral Deposits Research, School of Earth Sciences and Engineering, Nanjing University,

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Nanjing 210023, China

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Corresponding author: Alexis Lamothe (lamothe@cerege.fr) or Joël Savarino (joel.savarino@cnrs.fr)

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## Key Points:

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* A Mont Blanc ice core provides 1000 years of ¹⁵N isotopic composition of nitrate

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* The ice core ¹⁵N signal of nitrate reflects the relative contribution of NOₓ emission sources.

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* NOₓ emission inventory may have underestimated agricultural changes at the beginning of the 20th century in Western Europe

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**Abstract**

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Nitrogen stable isotopes (δ¹⁵N) of ice core nitrate of nitrate (NO₃⁻) are often subject to various interpretations, associated with source, atmospheric, and/or post-depositional changes. Here, an analysis of a Mont-Blanc (French Alps) ice core was conducted to investigate δ¹⁵(NO₃⁻) records over the last 1000 years. We find that the glacially archived δ¹⁵N(NO₃⁻) signal reliably preserves a record of NOₓ emission sources with limited post-emission isotopic changes. We use the δ¹⁵N record to reconstruct a history of NOₓ emissions and compare it with existing estimated NOₓ inventories for Western Europe. Notably, our ice-derived record suggests that inventories for the early 20th-century may have underestimated NOₓ emissions resulting from agriculture. During the 20th century, the δ¹⁵N value substantially decreased, which we attribute to the increasing emissions from oil combustion. Lastly, the ice core signal highlights the success of mitigation policies in reducing fossil fuel-induced NOₓ emissions, albeit 20 years later than anticipated.

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**Plain Language Summary**

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This study aims to understand changes in nitrogen (N) stable isotopes, specifically the enrichment of ¹⁵N to ¹⁴N (δ¹⁵N), in atmospheric nitrate. δ¹⁵N records of nitrate can provide information about its gaseous precursor (nitrogen oxides = NOₓ) sources. However, debates exist regarding their interpretation in glacial archives as a source signal only. We analyze a glacial archive from Mont-Blanc (French Alps), which preserves the δ¹⁵N source signal. We discuss the evolution of NOₓ emissions over the past 1000 years in light of existing emission inventories in Western Europe. Notably, our NOₓ history traces both the rise in fossil fuel combustion over the 20th century and its more recent emission reductions resulting from mitigation programs.

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**1 Introduction**

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During the 20th century, anthropogenic emissions of nitrogen oxides (NOₓ = NO + NO₂) have soared mainly from combustion of fossil fuels and biomass (Hoesly et al., 2018). NOₓ accounts for a substantial proportion of total aerosol mass, given its oxidation into nitric acid (HNO₃) that is involved in secondary inorganic aerosol formation, and contributes to particulate air pollution in large proportions (e.g., 24 % in Grenoble, France (Favez et al., 2010)).
Additionally, NOₓ plays a role in the production of tropospheric ozone (O₃) (Fishman & Crutzen, 1978), a central atmospheric reactant, while the deposition of NOₓ' final oxidized form, nitrate (NO₃⁻ = HNO₃ + particulate nitrate (p-NO₃⁻)), alters the nutrient balance of terrestrial and aquatic ecosystems leading to eutrophication (Durand et al., 2011). Hence, NOₓ has become a key focus of pollution mitigation, and many countries and supranational organizations have implemented policies to control NOₓ emissions from the 1980s onwards.

Since more than 82 % of NOₓ is reported to result from the combustion of fossil fuel for Western Europe (European Environment Agency, 2021), mitigation policies have targeted these sources, mainly through the implementation of catalytic converters (Leip et al., 2011). Subsequently, NOₓ emissions have been reported to decrease by 58 % between 1990 and 2014 (Hoesly et al., 2018). However, based on nitrogen (N) stable isotopic composition (δ¹⁵N = (¹⁵N/¹⁴N)sample/(¹⁵N/¹⁴N)air - 1) expressed in per mill (‰)) measured in NO₃⁻, a recent study has suggested that fossil fuel combustion only accounted for (54 ± 13) % of anthropogenic NOₓ emissions, challenging the current mitigation policies (Song et al., 2021). The isotopic composition of NOₓ at given time and location is the mean of the isotopic signatures of each source (Extended Table 1), weighted according to their relative mass contributions. This isotopic fingerprint is then transferred to nitrate, although N isotopic fractionation may occur (e.g., Freyer, 1991; Vicars et al., 2013; Walters et al., 2016). However, Song et al. (2021) based their conclusion on the extrapolation of 23 measurement points to the entire continent and also lacked consideration for natural NOₓ sources. To circumvent these two limitations, we measured the isotopic signal of NO₃⁻ (δ¹⁵N(NO₃⁻)) in an ice core drilled in the Alps to reconstruct a NOₓ emission history that naturally aggregates emissions from across Western Europe.

Ice cores sample glacial archives that have preserved the aerosol atmospheric composition, including NO₃⁻, for millennia. Since many glaciers have archived preindustrial isotopic composition of aerosols, ice cores provide unparalleled clues to understanding the natural sources and character of NOₓ emissions at an average regional signal (Eichler et al., 2023). In more recent glacial layers, the glacial NO₃⁻ composition can directly be compared to NOₓ regional inventories to examine their accuracy (e.g., Preunkert et al., 2003).

However, the interpretation of δ¹⁵N(NO₃⁻) in ice cores remains controversial. The same 300-year δ¹⁵N(NO₃⁻) record from Summit, Greenland, has been interpreted to follow 1) the increase in fossil fuel burning (Hastings et al., 2009), 2) the development of agriculture during
the 20th century (Felix & Elliott, 2013), and 3) the evolution of isotopic fractionation between HNO₃ and *p*-NO₃⁻ resulting from changes in atmospheric acidity (Geng et al., 2014). This latter fractionation interpretation was prompted by the need to explain the high isotopic value measured in the pre-industrial period (> 11 ‰), as no natural NOₓ source with such an isotopic signature is known.

To date, all these studies fail to explain pre-industrial δ¹⁵N(NO₃⁻) values without considering post-depositional phenomena. Post-depositional phenomena are any physical or chemical processes that alter the chemical composition of a molecule, and therefore its isotopic composition, after its atmospheric deposition. HNO₃ volatilization and *p*-NO₃⁻ photolysis are such phenomena that can decrease the NO₃⁻ concentration in near-surface snow layers surface while enriching the snow in ¹⁵NO₃⁻ (Erbland et al., 2013). These phenomena generally have a greater impact at lower accumulation sites where NO₃⁻ is exposed to the atmosphere and sunlight for longer periods of time. Thus, across the very dry East Antarctic plateau where snow accumulation is typically < 0.2 m of water equivalent per year (w e a⁻¹), photolytic changes in NO₃⁻ isotopic composition have been well documented (Akers et al., 2022). At intermediate accumulation sites, including Summit, Greenland, debates remain and air-snow interface models need to be used to quantify the influence of photolysis on NO₃⁻ (Jiang et al., 2021).

The analysis of a 126 m long ice core drilled in 2016 as part of the Ice Memory program at the Col Du Dôme glacier (CDD, 4250 m asl, 45.8421°N, 6.8474°E) in the Mont Blanc massif can help address the limitations previously mentioned for developing a NOₓ emission record from ice core δ¹⁵N(NO₃⁻). Indeed, the CDD is a cold glacier whose high accumulation rate (≈ 2.5 m w e a⁻¹) (Preunkert et al., 2000) largely prevents impactful post-depositional phenomena. Additionally, the CDD is situated in the heart of Western Europe and faithfully records the regional atmospheric composition (Preunkert & Legrand, 2013). This close proximity to European NOₓ emission sources can help distinguish the influences of transport-related isotopic fractionation and source signatures when compared to distant Greenland drilling sites. Here, after assessing the influence of photolysis and atmospheric isotopic fractionation on the glacially archived NO₃⁻ signal, we can then compare the observed δ¹⁵N(NO₃⁻) values with NOₓ inventories to investigate potential inventory inaccuracies.
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## 2 2 Methods

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### 2.1 Ice core processing

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CDD16-C01 is a 126 meter-long, 10 cm diameter ice core drilled in 2016. Ice segments were always stored and processed in a cold room (≈ -20 °C), stored in clean PE bags, and handled with clean nitrile gloves. Each core segment was cut longitudinally into several sections using saws decontaminated with ultrapure ice. A (34 × 34) mm² section dedicated to measurement of chemical concentrations is further decontaminated during continuous flow analysis (CFA) since the ice melt head discards meltwater coming from the potentially contaminated external faces.

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The outermost section was used to measure tritium activity between 100.8 m and 106.0 m using liquid scintillation counting at the Paul Scherrer Institute, Villigen, Switzerland. The external faces of the section dedicate to isotopic measurements were meticulously decontaminated by removing the outermost 2 mm of ice with a clean ceramic knife.

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### 2.2 Concentration measurement

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Concentrations of selected ions and elements were measured on the CFA system from the PANDA platform at IGE (https://panda.osug.fr/, Grenoble, France). As part of this CFA, an online colorimetry module measured $NO_3^-$ and ammonium ($NH_4^+$) concentrations (Röthlisberger et al., 2000). $NO_3^-$ concentration is calibrated with a set of standards (74, 478, and 1742 ppb). Due to the $NO_3^-$ conversion to $NO_2^-$, any $NO_2^-$ naturally present in the ice will inflate the resulting $NO_3^-$ concentration values. To constrain this possible error, we verified the mean $NO_2^-/(NO_2^-+ NO_3^-)$ ratio with ion chromatography over the uppermost 23 m and determined it to be 1.6 % ($\sigma$ = 2.5 %).

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The elemental [$^{23}$Na] (abundance 100 %), [$^{24}$Mg] (79 %), [$^{27}$Al] (100 %), [$^{40}$Ca] (97 %), and [$^{32}$S] (95 %) concentrations were measured with an online ICP-MS (iCAP TQ, ThermoFischer) and are assumed to be representative of the inorganic [Na$^+$], [Mg$^{2+}$], [Al$^{3+}$], [Ca$^{2+}$], and [SO$_4$$^{2-}$] concentrations, respectively. The quantification of [Cl$^-$] was performed by calculating [Cl$^-$] = 1.61 × [Na$^+$] (Legrand et al., 2002). The acidity profile was reconstructed following an ionic balance and agrees well with previous publications (Legrand et al., 2003; Preunkert & Legrand, 2013). The influence of anthropogenic Cl on acidity has previously been shown to be insignificant (Lamothe et al., in review).
144 **2.3 Dating**

145 The NH₄⁺ concentration profile is characterized by strong summer to winter variations which 146 were used to date the top 120 m of the ice core (Preunkert et al., 2000) covering from 2016 to 147 1895. This NH₄⁺-based chronology was verified and refined with time horizons using known 148 Saharan Dust Events (Clifford et al., 2019; Preunkert et al., 2000) and the 1963 tritium peak 149 activity. A large jump in time suggested by our initial chronology was also recently discussed 150 independently and hypothesized to reflect a crevasse (Preunkert et al., 2023). A 3D model based 151 on glacier flow and temperature characteristics (Gilbert et al., 2014) was used to validate our 152 measurements, estimate the uncertainty of the summer-winter variability technique, and 153 approximatively date the bottommost 5 meters. In our final resulting chronology, we estimated 154 the time scale uncertainty to be ±2 years for 1890–2016, ±5 years for 1890–1980, and ±30 years 155 before 1890. One can refer to Lamothe et al, *in review* for additional chronological details.

156 **2.4 δ¹⁵N(NO₃⁻) stable isotope measurement**

157 Samples for stable isotope analysis of NO₃⁻ were cut from the designated ice core section at 158 subannual resolution for the first 110 m (i.e., after 1928) and at 25 cm resolution for the 159 remaining 16 m (≈ 2-year intervals). Samples were then sealed in clean plastic bags (Whirl- 160 Pak) and left to melt at room temperature. The melted samples were initially passed through a 161 solid cationic extraction resin to extract NH₄⁺ for a coinciding study (Lamothe et al., 2023), 162 from which the remaining anionic solution was further separated with a Metrohm IC 850 in 163 suppressed mode (NaOH at 8 mM, suppressor H₂SO₄ at 50 mM, Dionex AG11 column) to 164 recover ≈ 100 nmol NO₃⁻ solutions. This anionic separation allows future research to measure 165 sulfate isotopes.

166 The denitrifying bacteria method (Casciotti et al., 2002; Kaiser et al., 2007) was used to 167 convert NO₃⁻ into N₂O gas that is then split into O₂ and N₂ on a 900 °C gold surface for 168 simultaneous measurement of δ¹⁵N, δ¹⁸O, and δ¹⁷O of NO₃⁻ with a Thermo Finnigan™ MAT- 169 253 Isotopic Ratio Mass Spectrometer equipped with a GasBench II™. Although oxygen 170 isotopic data were collected, they are not further discussed here other than examining potential 171 post-depositional effects. This method does not discriminate between NO₃⁻ and NO₂⁻, and thus 172 any NO₂⁻ present in the ice core will be converted to N₂O and potentially impact the NO₃⁻ 173 isotopic results; however, the concentration results indicate that this interference is negligible. 174 A calibration was performed against international isotopic standards (USGS 32, USGS 34, and
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USGS 35) to account for differing sample sizes (Frey et al., 2009; Morin et al., 2009). The
measurement $\delta^{15}$N(NO$_3$⁻) mean square error of standards was ± 1.4 ‰. Finally, the $\delta^{15}$N(NO$_3$⁻)
and [NO$_3$⁻] data spanning the last millennium were aggregated into 10-year and 25-year average
periods for 1900–2010 and 1800–1900, respectively, for NO$_x$ inventory comparison.

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CDD is proven to record air masses originating from France, Germany, Switzerland, and Italy (Arienzo et al., 2021; Preunkert et al., 2003; Preunkert & Legrand, 2013). We compiled several datasets that cover the period 1750-2016 to construct a reference NOₓ inventory for these countries. NOₓ emissions from brown coal, biomass, coke, diesel oil, hard coal, heavy oil, light oil, gas, and industry were retrieved from the Community Emissions Data System (O'Rourke et al., 2021). Agricultural emissions of NOₓ were retrieved from Hoesly et al. (2018). Natural background emissions are reported for the preindustrial period (1860) and the present day (since 1990) at the global scale (Galloway et al., 2004), and we assumed these natural emissions varied linearly between these two dates. Based on the reported isotopic signatures of the NOₓ anthropogenic sources (Table S1) and of the preindustrial natural source, we calculated the estimated annual $\delta^{15}$N signal resulting from the inventory ($\delta^{15}$Ninv-mean). This isotopic signal is also calculated for the lowest and highest isotopic signatures of all the sources ($\delta^{15}$Ninv-low and $\delta^{15}$Ninv-high, respectively) to account for the variance in isotopic value across the reported isotopic range of the source.

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# 3 Results and discussion

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## 3.1 Results

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We highlight five periods whose $\delta^{15}$N(NO$_3$⁻) values and NO$_3$⁻ concentrations ([NO$_3$⁻]) in CDD16-C01 are considered to reflect the history of NO$_3$⁻ emission variations in Western Europe (Figure 1):

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- Albeit coarsely resolved, the ‘pre-industrial’ period before 1700 presents very limited variations of $\delta^{15}$N(NO$_3$⁻) values and [NO$_3$⁻] with respective mean values ± standard deviation of (5.0 ± 1.0) ‰ and (119 ± 26) ppb, respectively.

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- During the 18th and 19th centuries, the minimum 68.5 ppb [NO$_3$⁻] value is reached in 1825 and is associated with a minimum $\delta^{15}$N(NO$_3$⁻) value of (2.4 ± 0.3) ‰. Both [NO$_3$⁻]
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and $\delta^{15}$N(NO$_3$$^-)$ values gradually increase over the 19$^{th}$ century to respectively reach 81 ppb and $(4.8 \pm 0.5)$ ‰ in 1900.

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- Between 1900 and 1950, [NO$_3$$^-$] is generally constant ((64.3 ± 9.5) ppb) while $\delta^{15}$N(NO$_3$$^-$) decreases from (4.8 ± 0.5) ‰ to (0.8 ± 0.5) ‰.

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- For 1950–2000, [NO$_3$$^-$] increases substantially from 65.8 to 509 ppb, whereas $\delta^{15}$N(NO$_3$$^-$) first decreases from (0.8 ± 0.5) ‰ before varying around (-1.7 ± 1.1) ‰.

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- From 2000 to 2016, [NO$_3$$^-$] decreases greatly from 509 to 287 ppb, and $\delta^{15}$N(NO$_3$$^-$) values decrease from (-1.5 ± 1.2) ‰ to (-4.1 ± 0.5) ‰.

<figure><img src="image_5.png" /></figure>

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**Figure 1:** NO$_3$$^-$ concentration in the CDD16 ice core (green) (/ppb) and decadal $\delta^{15}$N(NO$_3$$^-$) average (yellow) (/‰). The black dots represent the high resolution $\delta^{15}$N(NO$_3$$^-$) signal. The shaded yellow area is the 1$\sigma$ measurement uncertainty. The x axis is divided into two time frames with a low resolution before 1800, and a higher resolution afterwards.

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## 3.2 $\delta^{15}$N(NO$_3$$^-$) seasonality

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Unlike previous measurements of $\delta^{15}$N(NO$_3$$^-$) in a CDD firn core (Freyer et al., 1996), no significant seasonal difference emerges in our subannually-resolved record spanning 1981 to 2016 (Figure S1). We assume that the relatively short range of time in the previous study (3 years) distorted the interpretation. With 35 years of $\delta^{15}$N(NO$_3$$^-$) variations at the seasonal scale, we confidently argue that $\delta^{15}$N(NO$_3$$^-$) values are not affected by seasonal phenomena (t-test, *p* = 0.006, *n* = 97).

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In the subsequent discussion, all $\delta^{15}$N(NO$_3$$^-$) values will be mean annual (or decadal) values weighted according to NO$_3$$^-$ quantity.
226 **3.3 Post-depositional effects**

227 NO₃⁻ burial into the CDD firn is rapid due to high accumulation of ≈2.5 m w e a⁻¹ (Preunkert et al., 2000). As a result, post-depositional effects on NO₃⁻ concentration and isotopic composition are expected to be limited (Preunkert et al., 2003). Additionally, our NO₃⁻ data for the first meter of the core do not offer any evidence for post-depositional effects as no decreases of [NO₃⁻] are found in the top 80 cm roughly representing a month of accumulation (Figure S2a). More convincing, the δ¹⁵N(NO₃⁻) values in the top meter also suggest an insignificant influence of post-depositional phenomena. Any isotopic effect from volatilization would likely be minor (Shi et al., 2019), whereas we expect that photolysis would leave a definite enrichment in ¹⁵N (Erbland et al., 2013) that is not found in CDD (Figure S2a). This conclusion is also supported by oxygen NO₃⁻ stable isotopes. Indeed, photolysis of NO₃⁻ also results in a decrease in δ¹⁸O and Δ¹⁷O of NO₃⁻ with depth in the snowpack (Blunier et al., 2005; Erbland et al., 2013; Frey et al., 2009), but no such variations are measured in the first meter of snow at CDD (Figure S2b). Hence, it seems highly unlikely that these post-depositional phenomena have a notable impact on archived NO₃⁻ at CDD.

241 **3.4 Isotopic fractionation in the atmosphere**

242 Nitrogen isotopic fractionations can occur in the atmosphere during the NOₓ cycle (Freyer et al., 1993; Walters et al., 2016), NO₂ conversion into HNO₃ (Fang et al., 2021), and/or HNO₃(gas)-p-NO₃⁻ partitioning during transportation (Freyer, 1991; Geng et al., 2014). Here we show that these potential fractionations likely do not influence the δ¹⁵N(NO₃⁻) record at CDD.

246 Previous studies evaluated the importance of isotopic fractionations sensitivity tests. 247 This demonstrated that isotopic fractionation is negligible on ice core δ¹⁵N(NO₃⁻) during NOₓ cycling (Geng et al., 2014; Li et al., 2020) and NO₂ conversion into HNO₃ (Freyer, 1991; Li et al., 2020), but acidity-driven isotopic fractionation during HNO₃(gas)-NO₃⁻(particle) partitioning is substantial (Geng et al., 2014; Li et al., 2020). In that way, 20th century δ¹⁵N(NO₃⁻) fluctuations in a Summit ice core were proposed to be solely explained through this fractionation effect (Geng et al., 2014). Although the presence of atmospheric HNO₃ around CDD (Preunkert et al., 2007) could favor gas-particle exchanges until nitrate's deposition in the snowpack, no anticorrelation is noticed between acidity ([H⁺]) and the δ¹⁵N(NO₃⁻) values in CDD ice core (Figure S3), unlike the Summit ice core. Additionally, the application of the Rayleigh distillation model developed by Geng et al. (2014) cannot explain the δ¹⁵N(NO₃⁻) variations for
CDD (Figure S3). We therefore assume that either acidity-related isotopic fractionation does not actually influence the δ¹⁵N(NO₃⁻) measured in ice cores, or the close proximity of CDD to NOₓ sources, unlike Greenland, drastically reduces the isotopic fractionation related to the HNO₃(gas)-NO₃⁻(particle) partitioning, making the related isotopic fractionation negligible. Recent analysis of a second Greenland ice core suggests that this acidity-driven isotopic fractionation cannot solely explain the δ¹⁵N(NO₃⁻) signal after emission control started from the 1980s (Wei et al., *under review*).

Finally, other isotopic fractionations may still not be captured by current knowledge that could result from atmospheric transportation of particles (Li et al., 2020). In Alpine sites, transportation from emission sites to the higher elevation of the drilling sites is marked by upward convection with seasonal variations. In summer strong convection favors upward transport, while being deeply limited in winter. As a result, isotopic fractionations originating from transportation should follow this seasonal pattern. Since we showed that δ¹⁵N(NO₃⁻) signal does not display seasonal variations (Figure S1), such fractionation likely remains negligible.

While isotopic fractionations that occur during chemical conversions and/or during NO₃⁻ transportation cannot be absolutely excluded, previous research and our results at CDD suggest that no clear impacts from atmospheric fractionations are detected or expected. We caution that this conclusion is perhaps only the case for this site and this would need to be verified for other sites before this assumption is applied. As a result, the δ¹⁵N(NO₃⁻) values from CDD16-C01 likely recorded the changes of NOₓ sources in Western Europe throughout the last millennium.

## 3.5 Discussions on NOₓ sources

As post-emission isotopic changes appear limited for the CDD NO₃⁻ record, the δ¹⁵N(NO₃⁻) signal can be directly compared with the δ¹⁵N(NO₃⁻) predicted from the source inventory. Firstly, the preindustrial isotopic and concentration composition can be attributed to natural emissions of NOₓ from lightning, soil microorganisms, and oceans, as the anthropogenic emissions of NOₓ remain limited at that time (Galloway et al., 2004). Thus, we assign the preindustrial CDD δ¹⁵N(NO₃⁻) value, (5.0 ± 1.0) ‰, to represent the isotopic signature of the mean NOₓ natural background. Between 1000 and 1700, the δ¹⁵N(NO₃⁻) remains stable, while the concentration fluctuates (Figure 1). This supports consistent relative contributions from the various natural sources despite potential variations in the overall magnitude of natural NOₓ
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emissions. However, the low resolution of ice core measurements during this period of time does not allow us to determine the causes of such concentration fluctuations.

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During the 18th and 19th centuries, although changes in temperature (Ahmed et al., 2013) and soil usage (Kaplan et al., 2009) could have affected the natural NOₓ emissions, we attribute the increase in δ¹⁵N(NO₃⁻) value to anthropogenic sources. Inventories report a substantial increase in anthropogenic NOₓ emissions, particularly from coal combustion (O'Rourke et al., 2021), which tends to emit NOₓ with a high isotopic signature (Felix et al., 2012). However, while industrial and agricultural sources were also increasing during this time, natural emissions were decreasing (Galloway et al., 2004; Hoesly et al., 2018; O'Rourke et al., 2021). The observed δ¹⁵N(NO₃⁻) signal reflects the combined impact of these source changes. In particular, if only one source was considered as the driver of NOₓ emission evolution, i.e., coal combustion with the others considered constant, the δ¹⁵N(NO₃⁻) vs 1/[NO₃⁻] plot would result in a linear function. This is commonly referred as the Keeling plot method (Köhler et al., 2006). During this period, this δ¹⁵N(NO₃⁻) vs 1/[NO₃⁻] plot is not linear, indicating that multiple changes of NOₓ sources occurred.

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During the early 19th century, the observed δ¹⁵N(NO₃⁻) falls within the range of δ¹⁵N(NO₃⁻) values predicted in our reference inventory (Figure 2). This provides confidence in these estimations of anthropogenic sources but also, and primarily, to the estimate of the natural contribution, which is the main source of NOₓ. However, from 1850, δ¹⁵N(NO₃⁻) diverges from the mean predicted values, reaching the lowest δ¹⁵Nₗₐᵥ⁻ₐᵥ at the end of the century (Figure 2). Investigating the next period may provide clues to explain this divergence.

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The period from 1900 to 1950 saw significant economic development but was also marked by major crises such as wars and the 1930s depression. NOₓ emissions increased during this period, except during the crises. The ice core δ¹⁵N(NO₃⁻) values during this period are substantially lower than the mean values predicted by inventories and are even lower than the lowest inventory-derived estimate range (Figure 2). This implies that sources with high δ¹⁵N values, such as coal, natural background, and biomass burning, contributed relatively less to NOₓ emissions than reported in the inventories. Natural background and biomass burning are unlikely to drive this divergence from expected δ¹⁵N(NO₃⁻) values, as even reducing their inventory contribution to zero does not match observed ice core values. Therefore, the observed δ¹⁵N(NO₃⁻) can only be explained by decreasing NOₓ emissions from coal and/or increasing emissions from oil and/or agriculture. The coal-induced NOₓ emissions are correlated to the
quantity of coal that is burnt. The measurement of thallium concentration in another CDD ice core tends to confirm support the inventory of coal-induced NOₓ (Legrand et al., 2022). Similarly, oil combustion is unlikely to be underestimated since no increase in non-crustal vanadium concentration, associated with oil combustion, in a CDD ice core (Arienzo et al., 2021). Hence, it is reasonable to argue that, during the first half of the 20th century, the difference between the ice core and the inventory results from underestimated agricultural emissions. Based on an isotopic mass balance, a crude estimate would suggest agricultural-induced NOₓ emissions to be 10 times higher than expected if the difference between inventory and ice core measurements was only explained by agriculture. Given the fact that agricultural emissions are difficult to quantify (Lagerwerf et al., 2019), this conclusion seems plausible. Thus, we suggest that from the late 19th century, Western-European agriculture had already increased substantially its productivity. This increasing importance of agriculture, before the discovery of the Haber Bosch process, is concomitant with increasing imports of Chile saltpeter, used as fertilizer (Soledad Ureta-Zañartu, 2022).

<figure><img src="image_6.png" /><figcaption>**Figure 2:** CDD16 ice core δ¹⁵N(NO₃⁻) measurements (red) and calculated based on the existing NOₓ emission inventories (blue). Shaded areas represent uncertainties in δ¹⁵N(NO₃⁻) measurements (red) and isotopic signatures (blue). All data can be found in supplementary.</figcaption></figure>

From 1900 onwards, the CDD16 δ¹⁵N(NO₃⁻) signal substantially decreases in accordance with observations in other archives (Holtgrieve et al., 2011; Mason et al., 2022), including ice cores (Hastings et al., 2009; Li et al., 2020; Vega et al., 2015). By applying the Keeling plot method between 1910 and 2000, we found that an additional source was added to a constant background with an isotopic signature of (-2.7 ± 0.4) ‰ (Figure 3, R² = 0.71, p =
3×10⁻⁴). This signature falls within the range of oil combustion (Miller et al., 2017). We note that this interpretation of the Keeling plot assumes that the other emission components of the background (e.g., natural) are constant and that simply having a linear correlation between δ¹⁵N(NO₃⁻) and 1/NO₃⁻ does not on its own demonstrate that these other components are nonvarying. However, the Keeling plot's identification of oil combustion is in line with historical expectations and provides potential insight into the interpretation of the evolution in natural NOₓ emissions. We should remind that this conclusion is only applicable to the CDD ice core and that debates exist regarding the interpretation of the δ¹⁵N decline in other archives.

<figure><img src="image_7.png" /><figcaption>**Figure 3:** Keeling plot for the period 1910-2000 with 5-year average δ¹⁵N(NO₃⁻) (/%) as a function of 1/[NO₃⁻] (/ppb⁻¹). Shaded area is the 95 % confidence interval of the linear regression. The 1940 value is discarded (green cross) likely being an outlier value (residue to linear regression statistically different (>2σ)).</figcaption></figure>

Previous comparisons of Alpine ice core [NO₃⁻] with Western European NOₓ inventories have reported a discrepancy between NOₓ inventories and ice core [NO₃⁻] since the 1990s (Eichler et al., 2023; Preunkert et al., 2003). In fact, inventories report a steady decrease of total NOₓ emissions since 1980s due to enacted mitigation strategies, but ice cores' [NO₃⁻] only decreased since 2000. A mis-quantification of mitigation strategies has been proposed to explain these discrepancies. However, these studies also suggested that changes in aerosol
reactivity and formation could also explain this difference between NOₓ emissions and aerosol deposition in the ice. Taking the advantage of our new ice core record that extends to 2016, we can answer this question using δ¹⁵N(NO₃⁻). For NOₓ emissions to differ from aerosol deposition, an atmospheric isotopic fractionation should occur, and the decrease in atmospheric acidity due to decreasing SO₂ emissions would increase δ¹⁵N(NO₃⁻) values. Instead, the opposite is observed, with a further decrease in δ¹⁵N(NO₃⁻) after 2000 (Figure S3). As a result, we can convincingly argue that the difference between ice core [NO₃⁻] and inventory NOₓ emissions results from miscalculated effects of the mitigation strategies. Our ice core records that mitigation had substantial impact on the deposited NO₃⁻ concentration with a decrease of -60 % between 2000 and 2016, but this is 20 years later than expected. The Keeling plot allows to decipher the changes of sources during this period. The intercept value of (0.2 ± 1.2) ‰ suggests that combustion sources drive the decrease in NOₓ emissions. We note that this value is higher than the 20th century value of -2.7 ‰, but implementation of recent NOₓ-limiting technologies are reported to increase the isotopic signature of oil combustion (Walters et al., 2015). Consequently, the decrease in combustion-induced NOₓ emissions increases the relative contribution of agricultural sources.

As previously mentioned, Song et al. suggested that recent emission inventories underestimate the NOₓ emissions from non-fossil fuel sources, which account for (54 ± 13) % of total NOₓ emissions (2021). However, our ice core results do not support that strong disagreement with emission inventories, and the sampling differences between their study and ours help explain this discrepancy. Our ice core signal records a homogenized spatial record of Western European emission, while Song et al. (2021) gathered data from NO₃⁻ in precipitation from 23 individual European point sites. This emphasizes that the extrapolation of point measurements, often from anthropized environments, will often differ from the true regional character, as well as highlighting the strength of Alpine ice cores for capturing more accurate regional records. Additionally, Song et al.'s conclusions lack of consideration of natural sources. Indeed, because to the best of our knowledge no value of isotopic signature for the natural background had been reported before our study, the authors could not account for the natural contribution, leading to an overestimate of the non-fossil fuel NOₓ emissions.

# 4 Synthesis and conclusion

Undeniably, anthropogenic activities have deeply affected the nitrogen cycle over the past century, through considerably increased NOₓ emissions and changes in NOₓ sources. Because
394

chemical and physical processes involved in the nitrogen cycle are recorded in ice core $\delta^{15}$N(NO$_3$⁻) values, we have analyzed a Col du Dôme ice core (Mont Blanc, France) to further examine this anthropogenic impact. This ice core benefits from close proximity to European anthropogenic sources, which limits possible isotopic fractionation from transport, and has a high annual accumulation rate, which limits NO$_3$⁻ photolysis and other post-depositional effects. The source-tracer $\delta^{15}$N(NO$_3$⁻) indicates substantial changes since pre-industrial times that do not fully align with established NO$_x$ inventories.

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Using the pre-industrial $^{15}$N composition of NO$_3$⁻ as the isotopic signature of NO$_x$ emissions from natural sources, we show that the 18$^{th}$ and 19$^{th}$ century NO$_x$ inventories agree with the observed ice core $\delta^{15}$N(NO$_3$⁻) values. However, from 1850 onwards, inventories and ice core values diverge. Between 1900 and 1950, we confidently argue that this divergence can only result from the mis-quantification of the NO$_x$ source inventory. In particular, we suggest that agricultural sources may have been underestimated. The steep rise in NO$_x$ emissions after 1950 is primarily attributed to increasing emissions in oil combustion. Finally, after 2000, ice records a decrease in NO$_3$⁻ concentrations due to the implementation of mitigation strategies. It is worth noting that this decrease in ice core NO$_3$⁻ concentration occurs 20 years later than expected in inventories for unknown reasons.

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Our study shows that glacially-archived $\delta^{15}$N(NO$_3$⁻) from Alpine glaciers can be directly compared with NO$_x$ emission inventories. Hence, at a time when Alpine glaciers are increasingly under threat of melting, we must remember that they offer unparalleled clues to understanding aerosol sources across time and at regional levels.

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## Acknowledgement

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We acknowledge the LabEx OSUG@2020 (“Investissements d'avenir” - ANR10 LABX56), the ANR project ANR-15-IDEX-02 (IDEX-UGA), the University Grenoble Alpes Foundation, Ice Memory Foundation and its funders, and the technical support from the F2G (French National platform for Coring and Drilling, handled by INSU, the MESRI (Ministry of Higher Education, Research, and Innovation) for supporting this research. LEFE-IMAGO, a scientific program of the Institut National des Sciences de l'Univers (INSU/CNRS), has also provided partial funding for this study. We deeply thank Selin Bagci for lab handling, Luc Piard, Bruno Jourdain, Christian Vincent, Vladimir Mikhalenko, Victor Zagorodnov and other colleagues for the ice core drilling, and Meredith Hastings and Lei Geng for scientific discussion.

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425
426 **Competing interests**

427 The authors do not declare any conflict of interests.

428

429 **Data availability statement**

430 Data are accessible in supplementary (Table S2).

431

432 **References**

433 Ahmed, M., Anchukaitis, K. J., Asrat, A., Borgaonkar, H. P., Braida, M., Buckley, B. M., Büntgen, U.,

434 Chase, B. M., Christie, D. A., Cook, E. R., Curran, M. A. J., Diaz, H. F., Esper, J., Fan, Z.-X., Gaire,

435 N. P., Ge, Q., Gergis, J., González-Rouco, J. F., Goosse, H., ... PAGES 2k Consortium. (2013).

436 Continental-scale temperature variability during the past two millennia. *Nature Geoscience*,

437 6(5), Article 5. https://doi.org/10.1038/ngeo1797

438 Akers, P. D., Savarino, J., Caillon, N., Servettaz, A. P., Le Meur, E., Magand, O., Martins, J., Agosta, C.,

439 Crockford, P., & Kobayashi, K. (2022). Sunlight-driven nitrate loss records Antarctic surface

440 mass balance. *Nature Communications*.

441 Arienzo, M., Legrand, M., Preunkert, S., Stohl, A., Chellman, N., Eckhardt, S., Gleason, K., &

442 McConnell, J. (2021). Alpine ice-core evidence of a large increase in vanadium and

443 molybdenum pollution in Western Europe during the 20th century. *JGR Atmosphere*.

444 https://doi.org/10.1029/2020JD033211

445 Blunier, T., Floch, G. L., Jacobi, H.-W., & Quansah, E. (2005). Isotopic view on nitrate loss in Antarctic

446 surface snow. *Geophysical Research Letters*, 32(13). https://doi.org/10.1029/2005GL023011

447 Casciotti, K. L., Sigman, D. M., Hastings, M. G., Böhlke, J. K., & Hilkert, A. (2002). Measurement of the

448 Oxygen Isotopic Composition of Nitrate in Seawater and Freshwater Using the Denitrifier

449 Method. *Analytical Chemistry*, 74(19), 4905-4912. https://doi.org/10.1021/ac020113w
450 Clifford, H. M., Spaulding, N. E., Kurbatov, A. V., More, A., Korotkikh, E. V., Sneed, S. B., Handley, M.,
451 Maasch, K. A., Loveluck, C. P., Chaplin, J., McCormick, M., & Mayewski, P. A. (2019). A 2000
452 Year Saharan Dust Event Proxy Record from an Ice Core in the European Alps. *Journal of
453 Geophysical Research: Atmospheres, 124*(23). https://doi.org/10.1029/2019JD030725

454 Durand, P., Breuer, L., Johnes, P. J., Billen, G., Butturini, A., Pinay, G., van Grinsven, H., Garnier, J.,
455 Rivett, M., Reay, D. S., Curtis, C., Siemens, J., Maberly, S., Kaste, Ø., Humborg, C., Loeb, R.,
456 de Klein, J., Hejzlar, J., Skoulikidis, N., ... Wright, R. (2011). Nitrogen processes in aquatic
457 ecosystems. In M. A. Sutton, C. M. Howard, J. W. Erisman, G. Billen, A. Bleeker, P. Grennfelt,
458 H. van Grinsven, & B. Grizzetti (Éds.), *The European Nitrogen Assessment : Sources, Effects
459 and Policy Perspectives* (p. 126-146). Cambridge University Press.
460 https://doi.org/10.1017/CBO9780511976988.010

461 Eichler, A., Legrand, M., Jenk, T. M., Preunkert, S., Andersson, C., Eckhardt, S., Engardt, M., Plach, A.,
462 & Schwikowski, M. (2023). Consistent histories of anthropogenic western European air
463 pollution preserved in different Alpine ice cores. *The Cryosphere*, 17(5), 2119-2137.
464 https://doi.org/10.5194/tc-17-2119-2023

465 Erbland, J., Vicars, W. C., Savarino, J., Morin, S., Frey, M. M., Frosini, D., Vince, E., & Martins, J. M. F.
466 (2013). Air-snow transfer of nitrate on the East Antarctic Plateau &ndash; Part 1 : Isotopic
467 evidence for a photolytically driven dynamic equilibrium in summer. *Atmospheric Chemistry
468 and Physics*, 13(13), 6403-6419. https://doi.org/10.5194/acp-13-6403-2013

469 European Environment Agency. (2021). *European Union emission inventory report 1990-2019 under
the UNECE Convention on Long-range Transboundary Air Pollution (Air Convention)*
470 [Publication]. https://www.eea.europa.eu//publications/lrtap-1990-2019
471

472 Fang, H., Walters, W. W., Mase, D., & Michalski, G. (2021). iNRACM: incorporating 15N into the
473 Regional Atmospheric Chemistry Mechanism (RACM) for assessing the role photochemistry
474 plays in controlling the isotopic composition of NOx, NOy, and atmospheric nitrate.
475 *Geoscientific Model Development*, 14(8). https://doi.org/10.5194/gmd-14-5001-2021
476

Favez, O., El Haddad, I., Piot, C., Boréave, A., Abidi, E., Marchand, N., Jaffrezo, J.-L., Besombes, J.-L.,

477

Personnaz, M.-B., Sciare, J., Wortham, H., George, C., & D'Anna, B. (2010). Inter-comparison

478

of source apportionment models for the estimation of wood burning aerosols during

479

wintertime in an Alpine city (Grenoble, France). *Atmospheric Chemistry and Physics*, 10(12),

480

5295-5314. https://doi.org/10.5194/acp-10-5295-2010

481

Felix, J. D., & Elliott, E. M. (2013). The agricultural history of human-nitrogen interactions as recorded

482

in ice core $\delta^{15}$N-$NO_3^-$*. Geophysical Research Letters, 40*(8), 1642-1646.

483

https://doi.org/10.1002/grl.50209

484

Felix, J. D., Elliott, E. M., & Shaw, S. L. (2012). Nitrogen Isotopic Composition of Coal-Fired Power

485

Plant NOx: Influence of Emission Controls and Implications for Global Emission Inventories.

486

*Environmental Science & Technology*, 46(6), 3528-3535. https://doi.org/10.1021/es203355v

487

Fishman, J., & Crutzen, P. J. (1978). The origin of ozone in the troposphere. *Nature*, 274(5674),

488

855-858. https://doi.org/10.1038/274855a0

489

Frey, M. M., Savarino, J., Morin, S., Erbland, J., & Martins, J. M. F. (2009). Photolysis imprint in the

490

nitrate stable isotope signal in snow and atmosphere of East Antarctica and implications for

491

reactive nitrogen cycling. *Atmospheric Chemistry and Physics*, 9(22), 8681-8696.

492

https://doi.org/10.5194/acp-9-8681-2009

493

Freyer, H. D. (1991). Seasonal variation of 15N/14N ratios in atmospheric nitrate species. *Tellus B*,

494

43(1), 30-44. https://doi.org/10.1034/j.1600-0889.1991.00003.x

495

Freyer, H. D., Kley, D., Volz-Thomas, A., & Kobel, K. (1993). On the interaction of isotopic exchange

496

processes with photochemical reactions in atmospheric oxides of nitrogen. *Journal of*

497

*Geophysical Research, 98*(D8). https://doi.org/10.1029/93JD00874

498

Freyer, H. D., Kobel, K., Delmas, R. J., Kley, D., & Legrand, M. R. (1996). First results of 15N/14N ratios

499

in nitrate from alpine and polar ice cores. *Tellus B: Chemical and Physical Meteorology*, 48(1),

500

93-105. https://doi.org/10.3402/tellusb.v48i1.15671
Galloway, J. N., Dentener, F. J., Capone, D. G., Boyer, E. W., Howarth, R. W., Seitzinger, S. P., Asner, G.

P., Cleveland, C. C., Green, P. A., Holland, E. A., Karl, D. M., Michaels, A. F., Porter, J. H.,

Townsend, A. R., & Vöosmarty, C. J. (2004). Nitrogen Cycles: Past, Present, and Future.

*Biogeochemistry*, 70(2), 153-226. https://doi.org/10.1007/s10533-004-0370-0

Geng, L., Alexander, B., Cole-Dai, J., Steig, E. J., Savarino, J., Sofen, E. D., & Schauer, A. J. (2014).

Nitrogen isotopes in ice core nitrate linked to anthropogenic atmospheric acidity change.

*Proceedings of the National Academy of Sciences*, 111(16).

https://doi.org/10.1073/pnas.1319441111

Gilbert, A., Gagliardini, O., Vincent, C., & Wagnon, P. (2014). A 3-D thermal regime model suitable for

cold accumulation zones of polythermal mountain glaciers. *Journal of Geophysical Research: Earth Surface*, 119(9), 1876-1893. https://doi.org/10.1002/2014JF003199

Hastings, M. G., Jarvis, J. C., & Steig, E. J. (2009). Anthropogenic Impacts on Nitrogen Isotopes of Ice-Core Nitrate. *Science*, 324(5932), 1288. https://doi.org/10.1126/science.1170510

Hoesly, R. M., Smith, S. J., Feng, L., Klimont, Z., Janssens-Maenhout, G., Pitkanen, T., Seibert, J. J., Vu, L., Andres, R. J., Bolt, R. M., Bond, T. C., Dawidowski, L., Kholod, N., Kurokawa, J., Li, M., Liu, L., Lu, Z., Moura, M. C. P., O'Rourke, P. R., & Zhang, Q. (2018). Historical (1750–2014) anthropogenic emissions of reactive gases and aerosols from the Community Emissions Data System (CEDS). *Geoscientific Model Development*, 11(1). https://doi.org/10.5194/gmd-11-369-2018

Holtgrieve, G. W., Schindler, D. E., Hobbs, W. O., Leavitt, P. R., Ward, E. J., Bunting, L., Chen, G., Finney, B. P., Gregory-Eaves, I., Holmgren, S., Lisac, M. J., Lisi, P. J., Nydick, K., Rogers, L. A., Saros, J. E., Selbie, D. T., Shapley, M. D., Walsh, P. B., & Wolfe, A. P. (2011). A Coherent Signature of Anthropogenic Nitrogen Deposition to Remote Watersheds of the Northern Hemisphere. *Science*, 334(6062), 1545-1548. https://doi.org/10.1126/science.1212267
Jiang, Z., Alexander, B., Savarino, J., Erbland, J., & Geng, L. (2021). Impacts of the photo-driven post-depositional processing on snow nitrate and its isotopes at Summit, Greenland: A model-based study. *The Cryosphere*. https://doi.org/10.5194/tc-15-4207-2021

Kaiser, J., Hastings, M. G., Houlton, B. Z., Röckmann, T., & Sigman, D. M. (2007). Triple Oxygen Isotope Analysis of Nitrate Using the Denitrifier Method and Thermal Decomposition of N₂O. *Analytical Chemistry*, 79(2). https://doi.org/10.1021/ac061022s

Kaplan, J. O., Krumhardt, K. M., & Zimmermann, N. (2009). The prehistoric and preindustrial deforestation of Europe. *Quaternary Science Reviews*, 28(27), 3016-3034.

https://doi.org/10.1016/j.quascirev.2009.09.028

Köhler, P., Fischer, H., Schmitt, J., & Munhoven, G. (2006). On the application and interpretation of Keeling plots in paleo climate research &ndash; deciphering &delta;^13C of atmospheric CO₂ measured in ice cores. *Biogeosciences*, 3(4), 539-556. https://doi.org/10.5194/bg-3-539-2006

Lagerwerf, L. A., Bannink, A., Bruggen, C. van, Groenestein, C. M., Huijsmans, J. F. M., Kolk, J. W. H. van der, Luesink, H. H., Sluis, S. M. van der, Velthof, G. L., & Vonk, J. (2019). *Methodology for estimating emissions from agriculture in the Netherlands: Calculations of CH₄, NH₃, N₂O, NOx, NMVOC, PM10, PM2.5 and CO₂ with the National Emission Model for Agriculture* (NEMA), *Update 2019* (148; p.). WOT Natuur & Milieu.

https://library.wur.nl/WebQuery/wurpubs/549203

Lamothe, A., Savarino, J., Ginot, P., Soussaintjean, L., Gautier, E., Akers, P. D., Caillon, N., & Erbland, J. (2023). An extraction method for nitrogen isotope measurement of ammonium in a low-concentration environment. *Atmospheric Measurement Techniques*, 16(17), 4015-4030.

https://doi.org/10.5194/amt-16-4015-2023

Legrand, M., McConnell, J. R., Preunkert, S., Bergametti, G., Chellman, N. J., Desboeufs, K., Plach, A., Stohl, A., & Eckhardt, S. (2022). Thallium Pollution in Europe Over the Twentieth Century

Recorded in Alpine Ice: Contributions From Coal Burning and Cement Production.
551 *Geophysical Research Letters*, 49(13), e2022GL098688.

552 https://doi.org/10.1029/2022GL098688

553 Legrand, M., Preunkert, S., Wagenbach, D., Cachier, H., & Puxbaum, H. (2003). A historical record of formate and acetate from a high-elevation Alpine glacier : Implications for their natural versus anthropogenic budgets at the European scale. *Journal of Geophysical Research: Atmospheres*, 108(D24). https://doi.org/10.1029/2003JD003594

557 Legrand, M., Preunkert, S., Wagenbach, D., & Fischer, H. (2002). Seasonally resolved Alpine and Greenland ice core records of anthropogenic HCl emissions over the 20th century. *Journal of Geophysical Research*, 107(D12), 4139-4153. https://doi.org/10.1029/2001JD001165

560 Leip, A., Achermann, B., Billen, G., Bleeker, A., Bouwman, A. F., de Vries, W., Dragosits, U., Döring, U., Fernall, D., Geupel, M., Herolstab, J., Johnes, P., Le Gall, A. C., Monni, S., Nevečeřal, R., Orlandini, L., Prud’homme, M., Reuter, H. I., Simpson, D., ... Winiwarter, W. (2011). Integrating nitrogen fluxes at the European scale. In M. A. Sutton, C. M. Howard, J. W. Erisman, G. Billen, A. Bleeker, P. Grennfelt, H. van Grinsven, & B. Grizzetti (Éds.), *The European Nitrogen Assessment : Sources, Effects and Policy Perspectives* (p. 345-376). Cambridge University Press. https://doi.org/10.1017/CBO9780511976988.019

567 Li, Z., Hastings, M. G., Walters, W. W., Tian, L., Clemens, S. C., Song, L., Shao, L., & Fang, Y. (2020).

568 Isotopic evidence that recent agriculture overprints climate variability in nitrogen deposition to the Tibetan Plateau. *Environment International*, 138, 105614.

570 https://doi.org/10.1016/j.envint.2020.105614

571 Mason, R. E., Craine, J. M., Lany, N. K., Jonard, M., Ollinger, S. V., Groffman, P. M., Fulweiler, R. W., Angerer, J., Read, Q. D., Reich, P. B., Templer, P. H., & Elmore, A. J. (2022). Evidence, causes, and consequences of declining nitrogen availability in terrestrial ecosystems. *Science*.

574 https://doi.org/10.1126/science.abh3767
575 Miller, D. J., Wojtal, P. K., Clark, S. C., & Hastings, M. G. (2017). Vehicle NOx emission plume isotopic

576 signatures : Spatial variability across the eastern United States. *Journal of Geophysical*

577 *Research: Atmospheres*, 122(8), 4698-4717. https://doi.org/10.1002/2016JD025877

578 Morin, S., Savarino, J., Frey, M. M., Domine, F., Jacobi, H.-W., Kaleschke, L., & Martins, J. M. F. (2009).

579 Comprehensive isotopic composition of atmospheric nitrate in the Atlantic Ocean boundary

580 layer from 65°S to 79°N. *Journal of Geophysical Research: Atmospheres*, 114(D5).

581 https://doi.org/10.1029/2008JD010696

582 O'Rourke, P. R., Smith, S. J., Mott, A., Ahsan, H., McDuffie, E. E., Crippa, M., Klimont, Z., McDonald, B.,

583 Wang, S., Nicholson, M. B., Feng, L., & Hoesly, R. M. (2021). *CEDS v_2021_02_05 Release*

584 *Emission Data* (v_2021_02_05) [jeu de données]. Zenodo.

585 https://doi.org/10.5281/zenodo.4509372

586 Preunkert, S., Bohleber, P., Legrand, M., Fischer, H., Gilbert, A., Erhardt, T., Purtschert, R., Zipf, L.,

587 Waldner, A., & McConnell, J. R. (2023). Impact of subsurface crevassing on the depth-age

588 relationship of high-alpine ice cores extracted at Col du D&ocirc;me between 1994 and 2012.

589 *The Cryosphere Discussions*, 1-21. https://doi.org/10.5194/tc-2022-259

590 Preunkert, S., & Legrand, M. (2013). Towards a quasi-complete reconstruction of past atmospheric

591 aerosol load and composition (organic and inorganic) over Europe since 1920 inferred from

592 Alpine ice cores. *Climate of the Past*, 9(4), 1403-1416. https://doi.org/10.5194/cp-9-1403-

593 2013

594 Preunkert, S., Legrand, M., Jourdain, B., & Dombrowski-Etchevers, I. (2007). Acidic gases (HCOOH,

595 CH3COOH, HNO3, HCl, and SO2) and related aerosol species at a high mountain Alpine site

596 (4360 m elevation) in Europe. *Journal of Geophysical Research: Atmospheres*, 112(D23).

597 https://doi.org/10.1029/2006JD008225

598 Preunkert, S., Wagenbach, D., & Legrand, M. (2003). A seasonally resolved alpine ice core record of

599 nitrate : Comparison with anthropogenic inventories and estimation of preindustrial
600 emissions of NO in Europe. *Journal of Geophysical Research: Atmospheres*, 108(D21).

601 https://doi.org/10.1029/2003JD003475

602 Preunkert, S., Wagenbach, D., Legrand, M., & Vincent, C. (2000). Col du Dôme (Mt Blanc Massif,

603 French Alps) suitability for ice-core studies in relation with past atmospheric chemistry over

604 Europe. *Tellus B: Chemical and Physical Meteorology*, 52(3), 993-1012.

605 https://doi.org/10.3402/tellusb.v52i3.17081

606 Röthlisberger, R., Bigler, M., Hutterli, M., Sommer, S., Stauffer, B., Junghans, H. G., & Wagenbach, D.

607 (2000). Technique for Continuous High-Resolution Analysis of Trace Substances in Firn and

608 Ice Cores. *Environmental Science & Technology*, 34(2), 338-342.

609 https://doi.org/10.1021/es9907055

610 Shi, G., Chai, J., Zhu, Z., Hu, Z., Chen, Z., Yu, J., Ma, T., Ma, H., An, C., Jiang, S., Tang, X., & Hastings, M.

611 G. (2019). Isotope Fractionation of Nitrate During Volatilization in Snow : A Field Investigation

612 in Antarctica. *Geophysical Research Letters*, 46(6), 3287-3297.

613 https://doi.org/10.1029/2019GL081968

614 Soledad Ureta-Zañartu, M. (2022). How chemistry shaped societies : The case of the Atacama Desert

615 in Chile. *ChemTexts*, 9(1), 1. https://doi.org/10.1007/s40828-022-00172-3

616 Song, W., Liu, X.-Y., Hu, C.-C., Chen, G.-Y., Liu, X.-J., Walters, W. W., Michalski, G., & Liu, C.-Q. (2021).

617 Important contributions of non-fossil fuel nitrogen oxides emissions. *Nature*

618 *Communications*, 12(1), 243. https://doi.org/10.1038/s41467-020-20356-0

619 Vega, C. P., Pohjola, V. A., Samyn, D., Pettersson, R., Isaksson, E., Björkman, M. P., Martma, T., Marca,

620 A., & Kaiser, J. (2015). First ice core records of NO3– stable isotopes from Lomonosovfonna,

621 Svalbard. *Journal of Geophysical Research: Atmospheres*, 120(1), 313-330.

622 https://doi.org/10.1002/2013JD020930

623 Vicars, W. C., Morin, S., Savarino, J., Wagner, N. L., Erbland, J., Vince, E., Martins, J. M. F., Lerner, B.

624 M., Quinn, P. K., Coffman, D. J., Williams, E. J., & Brown, S. S. (2013). Spatial and diurnal

625 variability in reactive nitrogen oxide chemistry as reflected in the isotopic composition of
<table>
  <tr>
    <td>626</td>
    <td>atmospheric nitrate: Results from the CalNex 2010 field study. Journal of Geophysical Research: Atmospheres, 118(18), 10,567-10,588. https://doi.org/10.1002/jgrd.50680</td>
  </tr>
  <tr>
    <td>627</td>
    <td>Walters, W. W., Goodwin, S. R., & Michalski, G. (2015). Nitrogen Stable Isotope Composition (δ15N) of Vehicle-Emitted NOx. Environmental Science & Technology, 49(4), 2278-2285.</td>
  </tr>
  <tr>
    <td>630</td>
    <td>https://doi.org/10.1021/es505580v</td>
  </tr>
  <tr>
    <td>631</td>
    <td>Walters, W. W., Simonini, D. S., & Michalski, G. (2016). Nitrogen isotope exchange between NO and NO<sub>2</sub> and its implications for δ<sup>15</sup>N variations in tropospheric NO<sub>x</sub> and atmospheric nitrate. Geophysical Research Letters, 43(1), 440-448. https://doi.org/10.1002/2015GL066438</td>
  </tr>
  <tr>
    <td>634</td>
    <td></td>
  </tr>
</table>
# 1000 years of Nitrogen Oxide Sources in Western Europe: Evidence from Nitrogen Stable Isotopes (δ¹⁵N) of Nitrate in a Mont Blanc Ice Core

Alexis Lamothe¹, Pete D. Akers², Sarah Albertin³,¹, Nicolas Caillon¹, Sophie Darfeuil¹, Elsa Gautier¹, Patrick Ginot¹, Shohei Hattori⁴,⁵, Joël Savarino¹

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¹Univ. Grenoble Alpes, CNRS, IRD, INRAE, Grenoble INP*, IGE, 38000 Grenoble, France

*Institute of Engineering and Management Univ. Grenoble Alpes

²Discipline of Geography, School of Natural Sciences, Trinity College Dublin, Dublin, Ireland

³LATMOS/IPSL, Sorbonne Université, UVSQ, CNRS, 75005 Paris, France

⁴International Center for Isotope Effects Research (ICIER), Nanjing University, Nanjing, 210023, China

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⁵Frontiers Science Center for Critical Earth Material Cycling, State Key Laboratory for Mineral Deposits Research, School of Earth Sciences and Engineering, Nanjing University, Nanjing 210023, China

Corresponding author: Alexis Lamothe (lamothe@cerege.fr) or Joël Savarino (joel.savarino@cnrs.fr)

<figure><img src="image_8.png" /><figcaption>**Figure S1: Seasonality of δ¹⁵N(NO₃⁻)**</figcaption></figure>

15

In comparison to previous studies, no substantial difference is observed for δ¹⁵N(NO₃⁻) between summer and winter in the CDD16 ice. This figure presents the seasonal distribution of δ¹⁵N(NO₃⁻) values in CDD16-C01 for summer (orange) and winter (blue) with first and third quartile (black dashes) and median (bold coloured dashes)
<figure><img src="image_9.png" /><figcaption>**Figure S2: Post-deposition investigation**</figcaption></figure>

Glacially-archived nitrate is reported to be associated with post-depositional processes that can be investigated in the topmost few centimeters. This figure presents the measurements of nitrate concentration and stable isotopes in the topmost 100 centimeters in CDD16-C01. Nitrate concentration (mauve, /ppb) and nitrogen stable isotope (green, /‰) (top). Nitrate oxygen stable isotopes with the $^{17}$O anomaly (red, /‰) and $^{18}$O (blue, /‰) (bottom). Shaded zones correspond to the measurement uncertainty (1σ).

<figure><img src="image_10.png" /><figcaption>**Figure S3: Atmospheric fractionation investigation**</figcaption></figure>

To investigate the implication of atmospheric acidity on nitrate isotopic fractionation, we compare the $\delta^{15}$N(NO$_3$$^-)$ signal with acidity trend, and we apply the Geng et al. model (2014). At the top, are presented the variations of $\delta^{15}$N(NO$_3$$^-)$ (green, /‰) and acidity (red, /mol m$^{-3}$) across the last two centuries retrieved from the ice. At the bottom, we compare the isotopic composition $\delta^{15}$N(NO$_3$$^-)$ measured in the ice (green, /‰) with the modeled $\delta^{15}$N(NO$_3$$^-)$ trend based on acidity variations following Geng et al. (2014) (blue, /‰).
<table><caption>Table S1: main anthropogenic source δ¹⁵N(NOₓ) signatures</caption><thead><tr><td>Source</td><td>δ¹⁵N mean source signature</td><td>Reference</td></tr></thead><tbody><tr><td>Biomass burning</td><td>(-1.3 ± 4.3) ‰</td><td>(Zong et al., 2022)</td></tr><tr><td>Coal</td><td>(17.3 ± 8.3) ‰</td><td>(Felix et al., 2012)</td></tr><tr><td>Gas</td><td>(-17.8 ± 3.0) ‰</td><td>(Walters et al., 2015)</td></tr><tr><td>Vehicles</td><td>(-5.5 ± 3.5) ‰</td><td>(Miller et al., 2017)</td></tr><tr><td>Fertilized soils</td><td>(-35 ± 10) ‰</td><td>(Elliott et al., 2019)</td></tr><tr><td>Animal waste</td><td>(-18.8 ± 10.3) ‰</td><td>(Fang and Michalski, 2022)</td></tr></tbody></table>

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<table><caption>Table S2: accessible data</caption><thead><tr><td>Year</td><td>[NO<sub>3</sub><sup>-</sup>] /ppb</td><td>δ<sup>15</sup>N(NO<sub>3</sub><sup>-</sup>) /‰</td><td>δ<sup>15</sup>N(NO<sub>3</sub><sup>-</sup>) error /‰</td></tr></thead><tbody><tr><td>2015</td><td>206.0</td><td>-5.19</td><td>0.23</td></tr><tr><td>2010</td><td>368.0</td><td>-3.89</td><td>0.53</td></tr><tr><td>2005</td><td>479.0</td><td>-1.10</td><td>1.30</td></tr><tr><td>2000</td><td>539.1</td><td>-2.23</td><td>0.97</td></tr><tr><td>1995</td><td>456.3</td><td>-0.81</td><td>1.33</td></tr><tr><td>1990</td><td>384.8</td><td>-2.65</td><td>1.02</td></tr><tr><td>1985</td><td>310.2</td><td>-0.12</td><td>0.47</td></tr><tr><td>1980</td><td>208.2</td><td>-2.49</td><td>0.47</td></tr><tr><td>1975</td><td>142.5</td><td>-0.32</td><td>0.52</td></tr><tr><td>1970</td><td>256.3</td><td>-2.30</td><td>0.52</td></tr><tr><td>1965</td><td>193.1</td><td>-3.40</td><td>0.51</td></tr><tr><td>1960</td><td>165.7</td><td>-1.06</td><td>0.52</td></tr><tr><td>1955</td><td>61.1</td><td>-0.25</td><td>0.51</td></tr><tr><td>1950</td><td>70.6</td><td>1.17</td><td>0.52</td></tr><tr><td>1945</td><td>92.1</td><td>0.79</td><td>0.34</td></tr><tr><td>1940</td><td>37.4</td><td>-0.30</td><td>0.33</td></tr><tr><td>1935</td><td>71.9</td><td>2.92</td><td>0.50</td></tr><tr><td>1930</td><td>63.4</td><td>0.95</td><td>0.50</td></tr><tr><td>1925</td><td>57.6</td><td>1.62</td><td>0.35</td></tr><tr><td>1920</td><td>51.9</td><td>2.18</td><td>0.52</td></tr><tr><td>1915</td><td>46.7</td><td>3.20</td><td>0.52</td></tr><tr><td>1910</td><td>56.4</td><td>4.16</td><td>0.52</td></tr></tbody></table>
<table><tr><td>1900</td><td>81.0</td><td>4.75</td><td>0.52</td></tr><tr><td>1875</td><td>87.0</td><td>3.80</td><td>0.50</td></tr><tr><td>1850</td><td>76.9</td><td>2.60</td><td>0.47</td></tr><tr><td>1825</td><td>68.5</td><td>2.37</td><td>0.30</td></tr><tr><td>1800</td><td>98.2</td><td>2.72</td><td>0.37</td></tr><tr><td>1750</td><td>110.9</td><td>3.82</td><td>0.30</td></tr><tr><td>1700</td><td>101.5</td><td>2.75</td><td>0.30</td></tr><tr><td>1600</td><td>96.0</td><td>2.77</td><td>0.30</td></tr><tr><td>1500</td><td>124.3</td><td>4.05</td><td>0.30</td></tr><tr><td>1250</td><td>98.1</td><td>5.13</td><td>0.30</td></tr><tr><td>1100</td><td>117.1</td><td>4.64</td><td>0.30</td></tr></table>

References

Elliott, E., Yu, Z., Cole, A., and Coughlin, J.: Isotopic advances in understanding reactive nitrogen deposition and atmospheric processing, Science of the Total Environment, 662, https://doi.org/10.1016/j.scitotenv.2018.12.177, 2019.

Fang, H. and Michalski, G.: Assessing the roles emission sources and atmospheric processes play in simulating δ¹⁵N of atmospheric NOₓ and NO₃⁻ using CMAQ (version 5.2.1) and SMOKE (version 4.6), Geoscientific Model Development, 15, 4239–4258, https://doi.org/10.5194/gmd-15-4239-2022, 2022.

Felix, J. D., Elliott, E. M., and Shaw, S. L.: Nitrogen Isotopic Composition of Coal-Fired Power Plant NOx: Influence of Emission Controls and Implications for Global Emission Inventories, Environ. Sci. Technol., 46, 3528–3535, https://doi.org/10.1021/es203355v, 2012.

Geng, L., Alexander, B., Cole-Dai, J., Steig, E. J., Savarino, J., Sofen, E. D., and Schauer, A. J.: Nitrogen isotopes in ice core nitrate linked to anthropogenic atmospheric acidity change, Proceedings of the National Academy of Sciences, 111, https://doi.org/10.1073/pnas.1319441111, 2014.

Miller, D. J., Wojtal, P. K., Clark, S. C., and Hastings, M. G.: Vehicle NOx emission plume isotopic signatures: Spatial variability across the eastern United States, Journal of Geophysical Research: Atmospheres, 122, 4698–4717, https://doi.org/10.1002/2016JD025877, 2017.

Walters, W. W., Tharp, B. D., Fang, H., Kozak, B. J., and Michalski, G.: Nitrogen Isotope Composition of Thermally Produced NOx from Various Fossil-Fuel Combustion Sources, Environ. Sci. Technol., 49, 11363–11371, https://doi.org/10.1021/acs.est.5b02769, 2015.

Zong, Z., Shi, X., Sun, Z., Tian, C., Li, J., Fang, Y., Gao, H., and Zhang, G.: Nitrogen isotopic composition of NOx from residential biomass burning and coal combustion in North China, Environmental Pollution, 304, 119238, https://doi.org/10.1016/j.envpol.2022.119238, 2022.