# 300-Year NH3 Inventory in Europe: Uncovering Combustion's Recent Share

Alexis Lamothe, Pete Akers, Patrick Ginot, Sarah Albertin, Anja Eichler, Adrien Gilbert, Nicolas Caillon, Elsa Gautier, Sophie Darfeuil, Joël Savarino

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# 300-Year NH3 Inventory in Europe: Uncovering Combustion's Recent Share

**Alexis Lamothe**

alexis.lamothe@univ-grenoble-alpes.fr

Institut des Géosciences de l'Environnement https://orcid.org/0009-0002-0031-2058

**Pete Akers**

Trinity College Dublin

**Patrick Ginot**

Grenoble Institute of Engineering and Management

**Sarah Albertin**

Institut des Géosciences de l'Environnement

**Anja Eichler**

Paul Scherrer Institut https://orcid.org/0000-0003-0206-7463

**Adrien Gilbert**

University of Grenoble Alpes, CNRS, IRD, IGE https://orcid.org/0000-0001-9009-5143

**Nicolas Caillon**

Institut des Géosciences de l'Environnement

**Elsa Gautier**

Institut des Géosciences de l'Environnement

**Sophie Darfeuil**

Institut des Géosciences de l'Environnement https://orcid.org/0000-0003-3307-2548

**Joël Savarino**

IGE https://orcid.org/0000-0002-6708-9623

## Physical Sciences - Article

**Keywords:**

**Posted Date:** April 19th, 2024

**DOI:** https://doi.org/10.21203/rs.3.rs-3550976/v1

**License:** © This work is licensed under a Creative Commons Attribution 4.0 International License.
Read Full License
**Additional Declarations:** There is NO Competing Interest.
1 **300-Year NH₃ Inventory in Europe: Uncovering Combustion's Recent Share**

2 Alexis Lamothe¹,*, Pete D. Akers², Patrick Ginot¹, Sarah Albertin³,¹, Anja Eichler⁴,⁵, Adrien

3 Gilbert¹, Nicolas Caillon¹, Elsa Gautier¹, Sophie Darfeuil¹, Joel Savarino¹,.*

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

5 *Institute of Engineering and Management Univ. Grenoble Alpes

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

7 Ireland

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

9 ⁴Laboratory of Environmental Chemistry, Paul Scherrer Institute, 5232 Villigen PSI,

10 Switzerland

11 ⁵Oeschger Centre for Climate Change Research, University of Bern, 3012 Bern, Switzerland

12 *Correspondence to: Alexis Lamothe (alexis.lamothe@univ-grenoble-alpes.fr) or Joel

13 Savarino (joel.savarino@cnrs.fr)

14 Today, anthropogenic emissions of ammonia (NH₃) surpass natural emissions, causing
15 profound impacts on radiative forcing, ecosystems, biodiversity, and human health¹.

16 Accurate estimates of emissions are imperative for effective mitigation. Currently,
17 official NH₃ inventories attribute around 95 % of European anthropogenic NH₃
18 emissions to agriculture-related activities², but a growing body of evidence suggests that

19 these NH₃ inventories may be inaccurate³. Here, we report the first glacially archived
20 nitrogen isotope record for ammonium (the deposited form of NH₃) from a Mont-Blanc

21 ice core, offering an unprecedented glimpse into NH₃ emissions spanning Western
22 Europe's pre-industrial era to the present day. This 300-year record reveals that

23 anthropogenic NH₃ emissions reflect the evolution of European economies, with the
24 emergence of intensive agriculture and fossil fuel combustion. Most importantly, we
25 show that, over the period 2010-2016, agriculture-related activities accounted for, at

26 most, (78 ± 17) % of anthropogenic NH₃ emissions, with the remainder being
27 attributable to fossil fuel and biomass combustions. This less-than-assumed relative
28 contribution from agricultural emissions strongly argue for a re-evaluation of NH₃

29 inventories and unequivocally put forth that anthropogenic NH₃ emissions cannot be
30 reduced without limiting combustion-related activities.
Nitrogen stable isotope of ammonium

a new proxy in ice core tracing:

Emission sources

<figure><img src="image_2.png" /></figure>

Agriculture and combustion
emissions influence isotopes

Atmospheric reactivity

<figure><img src="image_3.png" /></figure>

Particle growth and formation
alter the isotopic composition

Deposition

<figure><img src="image_4.png" /></figure>

No reemission of NH₃ from the ice
which does not modify the isotopes

and used in a Mont-Blanc ice core to revise European ammonia sources

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

<figure><img src="image_6.png" /></figure>

31

32 *Graphical abstract*

33 **Main**

34 In recent centuries, the agricultural and industrial development of societies has disrupted the
35 nitrogen cycle, with cascading effects⁴. Ammonia (NH₃), whose emissions are intricately
36 linked to this cycle, exerts profound impacts on climate, ecosystems, biodiversity and human
37 health. Notably, with its role in particle formation and stabilisation⁵⁻⁷, NH₃ contributes to
38 atmospheric pollution, which is presented as the most severe environmental issue threatening
39 humanity⁸. Despite the implementation of abatement policies, stubbornly high emissions
40 leave room for questioning their effectiveness⁹.

41 Looking to its history, before 1860, NH₃ primarily originated from natural emissions, such as
42 biomass fires and metabolic processes of soil microbes and plant, with minor anthropogenic
43 emissions largely from animal waste¹⁰. During the 20th century, NH₃ synthesis through the
44 Haber-Bosch process led to anthropogenic sources surpassing naturally-emitted NH₃¹⁰. To
45 achieve the meaningful reductions in NH₃ emissions set by environmental policies¹¹,
46 governments need trustful inventories, which remain challenging to establish considering the
47 non-point sources of NH₃ emissions and its high reactivity.
On global and regional scales, current inventories estimate agriculture to account for the
majority of NH₃ emissions. For instance, in Western Europe, according to the European
Monitoring and Evaluation Programme (EMEP), 52 % and 43 % of NH₃ anthropogenic
emissions originate from fertilised soils and livestock management, respectively, leaving only
5 % to other sources, including traffic and industry¹¹.

This alleged dominance of agriculture in anthropogenic NH₃ emissions has been recently questioned. For instance, satellite measurements revealed discrepancies surroundings NH₃ hotspots¹², and an aerosol isotope compilation of 11 sites across Europe reported a strikingly large share ((44 ± 19) %) from industry and vehicle in anthropogenic NH₃ emissions for 1974-2017³. Although there remains a dearth of evidence that extends beyond local investigations, ammonium (NH₄⁺; the deposited form of NH₃) concentrations measured in Alpine ice cores were found not to match the expected emission trends inferred from inventories⁹, suggesting that the 'bottom-up' methodology of NH₃ emission inventorying is flawed over Western Europe.

Here, we present a 300-year record of NH₃ sources retrieved from a Mont Blanc ice core drilled at Col Du Dôme (CDD, 4250 m asl) that points to higher combustion-related NH₃ emissions across Western Europe. This regional history of NH₃ source apportionment and reaction pathways is based on nitrogen stable isotope data in ammonium (δ¹⁵N(NH₄⁺)) preserved in ice core. This work not only challenges the accuracy of NH₃ emission inventories, but also sheds light on aerosol chemistry, essential to better quantify radiative forcing¹³, and offers critical insight into the changes in atmospheric NH₃ reactivity for Western Europe. Consequently, our findings emphasise the need for a pragmatic NH₃ mitigation policy considering emission sources beyond agriculture, including, but not limited to, industry, transportation, and biomass burning. By providing a new estimate of NH₃ source proportions, our work contributes to the ongoing effort of improving air quality and preserving livelihoods for all living species, today and in the future.

## CDD16 ice core

High-resolution chemical composition, including NH₄⁺ concentration ([NH₄⁺]), was measured on a 126 m ice core from the CDD glacier using a continuous flow analysis system. This core was dated through ion concentration seasonality¹⁴, known Saharan dust events, the 1963 ³H peak from nuclear bomb testing, and a 3D thermal regime model coupled with firn densification¹⁵ (details in Methods and Extended Figure 1). We calculated an age model
80 uncertainty of ±2 years for 1980-2016, ±5 years for 1890-1980, and ±30 years before 1890.

81 Following a solid phase extraction method suitable for low concentrated NH<sub>4</sub><sup>+</sup> samples<sup>16</sup>,
82 samples for δ<sup>15</sup>N(NH<sub>4</sub><sup>+</sup>) were analysed at subannual resolution for the upper 110 m (after
83 1928) and a ≈ 2 year (25 cm) resolution for the remaining 16 m.

84 The 10-year mean δ<sup>15</sup>N varies between (-4.9 ± 1.0) ‰ and (4.8 ± 0.7) ‰ over the entire
85 record. From 1800, δ<sup>15</sup>N steadily rises to its maximum value in 1875 followed by a decrease
86 back to a minimal value in 1970, and subsequently increases back to ≈ 0 ‰ (Figure 1). This
87 pattern in δ<sup>15</sup>N does not follow the variations in [NH<sub>4</sub><sup>+</sup>], which increase sharply from the
88 1960s before reaching a maximum around 2000–2005 (Extended Figure 2). Interpreting the
89 ice core δ<sup>15</sup>N record is not straightforward and requires the consideration of isotopic
90 fractionations that can occur in the atmosphere or in the ice, from NH<sub>3</sub> emissions to NH<sub>4</sub><sup>+</sup>
91 burial.

92 **Isotopic composition**

93 NH₃ can originate from different sources that each have their own specific isotopic
94 signature<sup>17-20</sup>. As isotopic signatures often overlap between specific sources, we have
95 aggregated all sources into three more isotopically-distinct groups based on source type: a)
96 volatile (v-NH₃, from agriculture fertiliser, manure, and waste) with a mean δ<sup>15</sup>N values of (-
97 25.0 ± 5.5) ‰, b) combustion (c-NH₃, from vehicles, industry, and biomass burning), with a
98 mean δ<sup>15</sup>N values of (-4.8 ± 4.9) ‰, c) natural sources with currently unknown δ<sup>15</sup>N values
99 (Extended Table 1). Thus, the δ<sup>15</sup>N(NH₃) values will reflect the mean of the three sources'
100 δ<sup>15</sup>N values weighted by the share of the respective contributions. The main caveat of this
101 approach concerns the isotopic signatures of emission sources, which, in the lack of other
102 evidence, are assumed to have remained relatively constant over time.

103 However, the δ<sup>15</sup>N values of emitted NH<sub>3</sub> can be altered though NH<sub>3</sub> neutralisation, aerosol
104 growth, and transport before being archived in ice as NH<sub>4</sub><sup>+</sup> (Figure 1a). These processes need
105 to be studied and eventually corrected for in order to reconstruct the original isotopic
106 composition of the NH<sub>3</sub> sources from δ<sup>15</sup>N(NH<sub>4</sub><sup>+</sup>). Post-depositional isotopic fractionation in
107 the snowpack was shown to be negligible at CDD, as no evidence of NH<sub>3</sub> revolatilisation
108 from snow was observed<sup>16</sup>.

109 Aerosol transport has been theoretically suggested to preferentially deposit <sup>15</sup>N with a (4.2 ±
110 1.7) ‰ isotopic fractionation constant<sup>21</sup>, i.e., δ<sup>15</sup>N(NH<sub>3</sub>) emitted should be 4.2 ‰ lower than
111 the measured δ<sup>15</sup>N(NH<sub>4</sub><sup>+</sup>). However, atmospheric studies reveal that δ<sup>15</sup>N(NH<sub>3</sub>) are actually
(17.6 ± 7.8) ‰ lower³,²². As the underlying reason of this isotopic offset is not known, two inventories will be proposed. Acidity (H⁺) is involved in the equilibrium between NH₃ and NH₄⁺ during aerosol growth (Reactions R. 1 and R. 2)⁵,²³⁻²⁵ which is associated with an isotopic fractionation of +31 ‰²⁴. In addition, the kinetic isotopic fractionation effect expected to occur during NH₃ neutralisation by HNO₃ and H₂SO₄ has been quantified to yield -28 ‰ (Reaction R. 3)²⁶. The higher the neutralisation factor ([NH₄⁺]/([NO₃⁻] + 2 × [SO₄²⁻])), the more NH₃ is likely to remain in the atmosphere, leading to N fractionation. Furthermore, this relation demonstrates that [NH₄⁺] in ice cores is not an absolute proxy of NH₃ concentrations in the atmosphere. Thus, we estimated NH₃ atmospheric concentration based on [NH₃]ₜec = [NO₃⁻] + 2 × [SO₄²⁻] (Extended Figure 2).

$$ \begin{cases} \text{NH}_3(\text{gas}) \leftrightarrow \text{NH}_3(\text{aq}) \\ \text{NH}_3(\text{aq}) + \text{H}^+ \leftrightarrow \text{NH}_4^+ \end{cases} \qquad (\text{R.1 and R2; } \varepsilon = +31 \, \text{‰}) $$

$$ \text{NH}_3(\text{gas}) + \text{H}^+(\text{from H}_2\text{SO}_4/\text{HNO}_3) \rightarrow \text{NH}_4^+ \qquad (\text{R. 3; } \varepsilon = -28 \, \text{‰}) $$

To correct the δ¹⁵N(NH₄⁺) recorded in the CDD ice core from the two latter isotopic fractionation effects listed above we used thermodynamic Rayleigh distillations²⁵ modelled on nitrate studies²⁷ (see Method). Overall, we found that transport is the overwhelming fractionation effect.

<figure><img src="image_7.png" /></figure>

<figure><img src="image_8.png" /></figure>
Figure 1: a) scheme of isotopic fractionations b) measured (light blue) and the corrected δ<sup>15</sup>N values after 17.6 ‰ transport isotopic fractionation (light green), NH<sub>4</sub><sup>+</sup> stability in aerosols (dark olive), and neutralisation (dark blue) corrections. Coloured shaded backgrounds represent absolute uncertainties from measurements and isotopic fractionation.

### NH<sub>3</sub> inventory history

This section discusses the first 300-year history of NH<sub>3</sub> sources over Western Europe, corrected for isotopic fractionations from transport, aerosol stability, and neutralisation. Assuming that the minimum [NH<sub>3</sub>]<sub>rec</sub> in the CDD ice core (40 ppb in 1800) reflects the natural emissions of NH<sub>3</sub> and that it remains constant through time, the share of natural emissions in 1860 and 1990 agree between ice core (76 ‰ and 12 ‰) and previous studies (65 ‰ and 19 ‰)<sup>10</sup>. Using the pre-industrial isotopic composition as the isotopic signature of natural sources ((-18.6 ± 1.3) ‰), an inverting isotopic mass balance provides the δ<sup>15</sup>N(NH<sub>3</sub>) associated to anthropogenic NH<sub>3</sub> emission sources (Figure 2a).

<figure><img src="image_9.png" /></figure>

Figure 2: top: Average δ<sup>15</sup>N(NH<sub>3</sub>) for the whole sources (blue) and anthropogenic sources only (green) (‰). Bottom: 300 years of relative NH<sub>3</sub> emissions (ppb) with the volatile (green), combustion (grey), and natural sources (yellow) (left). Zoom of the 2015 NH<sub>3</sub> inventory for anthropogenic sources only in percentage to allow the comparison between the ice core (left) and the EMEP (right) inventories with absolute uncertainty (hatches).
During the 19th century, c-NH₃ increase and are estimated to account for (18 ± 5) % of the total NH₃ emissions over Western Europe. At this time, biomass burning is the only major combustion source, whose δ¹⁵N(NH₃) signature remains uncertain¹⁹,²⁸,²⁹. With a constant NH₃ natural background, a Keeling plot³⁰ (Extended Figure 4) underlines an isotopic signature of the additional NH₃ source of (+4.4 ± 5.4) ‰, close to the (-4.8 ± 4.9) ‰ attributed to the combustion category. This gives confidence to the proposed NH₃ inventories, and illustrates the possible future applications of the Keeling plot method in assessing past NH₃ isotopic signatures.

From 1875 to 1930, the anthropogenic NH₃ emissions exceed natural ones, mainly explained by increasing emissions from volatile sources. With a quasi-constant [NH₃]rec from combustion, the Keeling plot reflects an additional source of (-17.6 ± 0.7) ‰ that matches the reported signature for manure²⁹. After 1930, this Keeling plot additional source value decreases to (-22.7 ± 0.8) ‰ indicating the use of new fertilizer techniques from the Haber-Bosch process, with a low isotopic signature. Overall, from 1900 to 1970, the increase of [NH₃]rec is mainly driven by agriculture emissions (+760 %) with its contribution increasing from (36 ± 7) % to (88 ± 21) %. Our Western Europe-focused estimate is much higher than the 1.4 times increase proposed for global agricultural NH₃ emissions³¹, highlighting the importance of regional inventories and the substantial evolution of Western European society and economy in this period.

From 1970, the [NH₃]rec increase is solely explained by c-NH₃ emissions increasing from (28 ± 55) ppb in 1970 to (104 ± 39) ppb in 2000. The large implementation of catalytic converters after 1975³² and rise in car ownership³³ likely explains this c-NH₃ steep rise, as demonstrated by a (-6.6 ± 3.5) ‰ additional source signature (Extended Figure 4). Indeed, the reaction of carbon monoxide with water in catalytic converters leads to dihydrogen formation, which in turn reacts with NOₓ and forms NH₃³⁴. The anthropogenic source share reaches a maximum with 88 % of total NH₃ emissions in 1990's.

Since 2000, the NH₃ abatement policies have substantially reduced anthropogenic NH₃ emissions, representing 78 % of the total emissions for 2010-2015. This decline resulted from reduced emissions in both agriculture (-39 %) and combustion (-70 %). However, our isotopic approach estimates that only (78 ± 17) % of anthropogenic NH₃ originates from agriculture, which show consistency with local aerosol studies³. This value could be even lower using the theoretical 4.2 ‰ deposition correction factor. However, the alternative 4.2 ‰ estimate seems
virtually implausible as it would result in a 100 % share of c-NH₃. Our upper limit of (78 ± 17) % for agricultural emissions in Western Europe implies that EMEP inventories overestimate agriculture's contribution (95 %) between 2010 and 2016. This difference is even more significant between 2000 and 2010, with an isotopically estimated agricultural contribution of (68 ± 12) % compared to 94 % in the EMEP inventory. This disparity emphasises inadequacies in regional emission inventories, indicating greater emission of NH₃ through combustion than previously anticipated.

## 187 Implications

Our δ¹⁵N(NH₄⁺) record from Mont Blanc offers 300 years of NH₃ emission source variations that reflect the economic development of Western Europe. It not only shows that anthropogenic emissions have surpassed natural emissions since the late 19th century, but also quantifies the share of agricultural and industrial activities.

Besides, this unique proxy provides novel prospects for exploring atmospheric chemistry via ice cores. As an example, it has emphasized the necessity to account for neutralisation correction in order to better understand [NH₄⁺] measurements in ice cores to reconstruct NH₃ emissions.

Importantly, our work highlights the necessity of revising regional NH₃ inventories over Western Europe, with reason to believe that this could be extended elsewhere³. Indeed, our findings pinpoint that combustion (vehicular, industrial, and/or biomass) contributes substantially more to NH₃ emissions than current official inventories suggest. This revelation highlights the limitations of solely targeting agricultural sources for NH₃ mitigation efforts, while neglecting the substantial emissions from combustion.

As the world crossed the theoretical, yet dangerous, planetary boundary of the nitrogen cycle in 1970³⁵⁻³⁸, and given the local, regional, and global consequences of reactive nitrogen species, we strongly advocate for extending the planetary boundary of the nitrogen cycle to encompass all nitrogen emission sources, including combustion-related NH₃. Failure to act could have serious implications for a sustainable development of agriculture and society.

Our study extends the inaccuracy of emission inventories to a vast, heavily industrialised, and densely populated region. Previous inventories fatally led to biased, incomplete, and inadequate NH₃ regulation policies. The scientific community should therefore conduct a comprehensive re-evaluation of NH₃ sources. This evaluation should prompt the swift
211 establishment of policies targeting industrial, vehicular, and biomass combustion-related NH₃

212 emissions through technological adaptations³⁹ and changes in practices, as it has been done

213 for agriculture. Together, a critical step can be taken towards mitigating air pollution,

214 preventing NH₃ from becoming the next CO₂⁴⁰.

215
216 **References**

217 1. Sutton, M. A. et al. Summary for policy makers. in *The European Nitrogen*

218 *Assessment: Sources, Effects and Policy Perspectives* (eds. Sutton, M. A. et al.) xxiv–xxxiv

219 (Cambridge University Press, 2011). doi:10.1017/CBO9780511976988.002.

220 2. Crippa, M., Guizzardi, D., Muntean, M., Schaaf, E. & Oreggioni, G. EDGAR v5.0

221 Global Air Pollutant Emissions. (2019).

222 3. Chen, Z.-L. et al. Significant contributions of combustion-related sources to ammonia

223 emissions. *Nat Commun* **13**, 7710 (2022).

224 4. Galloway, J. N. et al. The Nitrogen Cascade. *BioScience* **53**, 341–356 (2003).

225 5. Wang, M. et al. Synergistic HNO3–H2SO4–NH3 upper tropospheric particle

226 formation. *Nature* **605**, 483–489 (2022).

227 6. Wang, M. et al. Rapid growth of new atmospheric particles by nitric acid and

228 ammonia condensation. *Nature* **581**, 184–189 (2020).

229 7. Lehtipalo, K. et al. Multicomponent new particle formation from sulfuric acid,

230 ammonia, and biogenic vapors. *Science Advances* **4**, eaau5363 (2018).

231 8. World Health Organization. *WHO global air quality guidelines: particulate matter*

232 (PM2.5 and PM10), ozone, nitrogen dioxide, sulfur dioxide and carbon monoxide.

233 https://www.who.int/publications-detail-redirect/9789240034228 (2021).

234 9. Eichler, A. et al. Consistent histories of anthropogenic western European air pollution

235 preserved in different Alpine ice cores. *The Cryosphere* **17**, 2119–2137 (2023).

236 10. Galloway, J. N. et al. Nitrogen Cycles: Past, Present, and Future. *Biogeochemistry* **70**,

237 153–226 (2004).

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

239

240 (Air Convention). https://www.eea.europa.eu/publications/lrtap-1990-2019 (2021).

241 12. Van Damme, M. et al. Industrial and agricultural ammonia point sources exposed.

242 *Nature* **564**, 99–103 (2018).

243 13. Szopa, S. et al. Short-Lived Climate Forcers. *Climate Change 2021: The Physical*

244 *Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the*
245 *Intergovernmental Panel on Climate Change* 817–922 (2021)

246 doi:10.1017/9781009157896.008.

247 14. Preunkert, S. et al. Impact of subsurface crevassing on the depth-age relationship of

248 high-alpine ice cores extracted at Col du D&ocirc;me between 1994 and 2012. *The*

249 *Cryosphere Discussions* 1–21 (2023) doi:10.5194/tc-2022-259.

250 15. Gilbert, A., Gagliardini, O., Vincent, C. & Wagnon, P. A 3-D thermal regime model

251 suitable for cold accumulation zones of polythermal mountain glaciers. *Journal of*

252 *Geophysical Research: Earth Surface* **119**, 1876–1893 (2014).

253 16. Lamothe, A. et al. An extraction method for nitrogen isotope measurement of

254 ammonium in a low-concentration environment. *Atmospheric Measurement Techniques* **16**,

255 4015–4030 (2023).

256 17. Walters, W. W. et al. Constraining Ammonia Emissions in Vehicle Plumes Utilizing

257 Nitrogen Stable Isotopes. *Atmospheric Chemistry and Physics* (2020).

258 18. Walters, W. W. et al. Quantifying the importance of vehicle ammonia emissions in an

259 urban area of northeastern USA utilizing nitrogen isotopes. *Atmospheric Chemistry and*

260 *Physics* **22**, 13431–13448 (2022).

261 19. Cao, Y. et al. How aerosol pH responds to nitrate to sulfate ratio of fine-mode

262 particulate. *Environ Sci Pollut Res* **27**, 35031–35039 (2020).

263 20. Walters, W. W., Chai, J. & Hastings, M. G. Theoretical Phase Resolved Ammonia–

264 Ammonium Nitrogen Equilibrium Isotope Exchange Fractionations: Applications for

265 Tracking Atmospheric Ammonia Gas-to-Particle Conversion. *ACS Earth and Space*

266 *Chemistry* **3**, 79–89 (2019).

267 21. Guo, H., Weber, R. J. & Nenes, A. High levels of ammonia do not raise fine particle

268 pH sufficiently to yield nitrogen oxide-dominated sulfate production. *Sci Rep* **7**, 12109

269 (2017).

270 22. Pan, Y. et al. Fossil Fuel Combustion-Related Emissions Dominate Atmospheric

271 Ammonia Sources during Severe Haze Episodes: Evidence from ¹⁵N-Stable Isotope in Size-

272 Resolved Aerosol Ammonium. *Environmental Science & Technology* **50**, 8049–8056 (2016).

273 23. Geng, L. et al. Nitrogen isotopes in ice core nitrate linked to anthropogenic

274 atmospheric acidity change. *Proceedings of the National Academy of Sciences* **111**, (2014).
275 24. Kawashima, H. & Kurahashi, T. Inorganic ion and nitrogen isotopic compositions of atmospheric aerosols at Yurihonjo, Japan: Implications for nitrogen sources. *Atmospheric Environment* **45**, 6309–6316 (2011).

278 25. Kawashima, H., Yoshida, O. & Suto, N. Ion-exchange resin and denitrification pretreatment for determining δ15N-NH4+, δ15N-NO3–, and δ18O-NO3– values. *Rapid Communications in Mass Spectrometry* **35**, e9027 (2021).

281 26. Feng, X. *et al*. Biomass burning is a non-negligible source for ammonium during winter haze episodes in rural North China: Evidence from high time resolution 15N-stable isotope. *Journal of Geophysical Research: Atmospheres* **n/a**, e2022JD038012 (2023).

284 27. Keeling, C. D. The concentration and isotopic abundances of carbon dioxide in rural and marine air. *Geochimica et Cosmochimica Acta* **24**, 277–298 (1961).

286 28. Hoesly, R. M. *et al*. Historical (1750–2014) anthropogenic emissions of reactive gases and aerosols from the Community Emissions Data System (CEDS). *Geoscientific Model Development* **11**, (2018).

289 29. Farrauto, R. J. & Heck, R. M. Catalytic converters: state of the art and perspectives. *Catalysis Today* **51**, 351–360 (1999).

291 30. Oldenziel, R. & Bruhèze, A. A. de la. Contested Spaces: Bicycle Lanes in Urban Europe, 1900-1995. *Transfers* **1**, 29–49 (2011).

293 31. Livingston, C., Rieger, P. & Winer, A. Ammonia emissions from a representative in-use fleet of light and medium-duty vehicles in the California South Coast Air Basin. *Atmospheric Environment* **43**, 3326–3333 (2009).

296 32. Sandström, V. *et al*. Disparate history of transgressing planetary boundaries for nutrients. *Global Environmental Change* **78**, 102628 (2023).

298 33. Richardson, K. *et al*. Earth beyond six of nine planetary boundaries. *Science Advances* **9**, eadh2458 (2023).

300 34. Steffen, W. *et al*. Planetary boundaries: Guiding human development on a changing planet. *Science* **347**, (2015).

302 35. Rockström, J. *et al*. A safe operating space for humanity. *Nature* **461**, 472–475 (2009).
303

36. Snyder, B. E. R. et al. A ligand insertion mechanism for cooperative NH3 capture in metal-organic frameworks. *Nature* **613**, 287–291 (2023).

304

305

37. Battye, W., Aneja, V. P. & Schlesinger, W. H. Is nitrogen the next carbon? *Earth's Future* **5**, 894–904 (2017).

306

307

308

## Methods

309

### Col Du Dôme (CDD)

310

The CDD16 ice core was drilled in September 2016 on the Col Du Dôme (CDD) glacier

311

(45°50′31.39″N, 6°50′50.70″E; 4250 m asl) in the Mont-Blanc massif. The CDD is a cold and

312

high accumulation glacier (up to 2.5 m w e yr⁻¹)¹ that has been shown to record Western

313

European air pollution history² and whose physical characteristics had previously been

314

reported¹,³.

315

This drilling was organized as part of the Ice Memory programme, during which three ice

316

cores were extracted, one for ongoing analysis, the other two for long-term storage. The

317

drilling was done with an electromechanical drill in dry hole. The cores have a diameter of

318

102 mm and are stored in one-meter long sections in isothermal boxes until they are

319

transported to Grenoble, France, for storage. A comparison with emission inventories² and the

320

Lagrangian particle dispersion model FLEXPART⁴ demonstrated that CDD is mainly

321

sensitive to emissions from countries surrounding the Alps: in decreasing order of sensitivity,

322

Switzerland, France, Italy, Spain, Germany (and ex West Germany before 1989), Belgium,

323

and the United Kingdom.

324

### Concentration measurements

325

The ice-core main chemical composition has been measured using the PANDA Continuous

326

Flow Analysis (CFA) system at IGE (https://panda.osug.fr/, Grenoble). Several instruments

327

coupled to the CFA allow a high-resolution multitracer analysis of some anions and cations,

328

black carbon, dust, and trace metals on a single ice bar (34 × 34 mm²), as well as collecting

329

samples for off-line ion chromatography coupled with a Mass Spectrometer (IC-MS). [NH₄⁺]

330

and [NO₃⁻] were measured through an online-colorimetry module developed at IGE based on

331

previous work⁵. Elemental [²³Na], [²⁴Mg], [²⁷Al], [⁴⁴Ca], and [³²S] concentrations measured

332

with an online ICP-MS (iCAP TQ, ThermoFischer) are assumed to be representative of the

333

inorganic [Na⁺], [Mg²⁺], [Al³⁺], [Ca²⁺], and [SO₄²⁻] concentrations, respectively. The
quantification of [Cl⁻] was performed by calculating [Cl⁻]=1.61 × [Na⁺]⁶ since the discrete
measurements were not done along the full ice core. The resulting ion balance showed good
correspondence with previous studies (Extended Figure 7)²,⁷.

337 *Dating the ice core*

338 The characteristic summer to winter variations in ion concentrations² allow a precise dating of
339 the ice core for the top 120 m. Saharan dust events (SDEs) and the 1963 ³H peak are used as
340 known time horizons and lead to a dating uncertainty of 2 and 5 years for the periods 1980-
341 2016 and 1890-1980, respectively. SDEs are determined by comparing CFA terrigenous
342 concentrations (Al, Ca, Fe, Sr) with satellite Aerosol Optical Thickness (AOT 550 nm from
343 MERRA-2 model using the NASA Giovanni tool) from 1980 to 2016 and literature¹,⁸. Tritium
344 activity was determined by liquid scintillation counting (LSC) at the Paul Scherrer Institute
345 (Villigen, Switzerland). Using the outer ice core parts, 10 g of ice samples were melted and
346 suspended with 10 mL of a scintillator cocktail and placed in a TriCarb 2770 SLL/BGO
347 counter (Packard SA, Meriden, II, USA). The counting time per sample was 2 × 600 min with
348 a blank value of 0.55 counts per minute (cpm) and a detection limit of 1.7 Bq kg⁻¹. 22 samples
349 covering a depth range between 100.8 and 106 m were analysed. Maximum tritium activity
350 was obtained at 103 m depth (82.11 m w e) and assigned to the year 1963. Although sites with
351 non-continuous time scale should be avoided, four potential time discontinuities have been
352 identified based on the seasonality of the δD signal at 72.5 m w e, 83.3 m w e, 85.9 m w.e.,
353 and 92.4 m w e (Extended Figure 8) and may be explained, albeit not verified, by melting
354 events or glacier crevasses, as it has been recently hypothesised for one of this time jump⁹.
355 Thanks to time horizons, we were able to estimate the dating before and after these
356 discontinuities. As a great uncertainty in the dating exists inside these discontinuities, data
357 were just removed. Therefore, these events do not affect the overall dating and only lead to
358 the removal of data for 3 years, 4 years, and 5 years for the first three respective time
359 discontinuities, while the last one is where the dating based on seasonal variations had been
360 stopped.

361 A three-dimensional thermo-mechanical model¹⁰ is used to confirm the dating on the first 120
362 m and date approximatively the deepest 5 meters. The model solves the Stokes equation using
363 a compressible firn rheology that is equivalent to the Glenn’s flow law when the density
364 reaches the ice density¹⁰. It has been calibrated using present-days observations of surface
365 velocities and borehole temperature measurements. The age-depth relationship is obtained by
backtracking particles at different depths along the borehole and toward the surface using the modelled 3D velocity field. The uncertainty of the dating increases with depth as velocities errors lead to larger dating variation for longer particle trajectories. Assuming 20 % errors on the modelled velocity field, we calculate a plausible age range reported in the Extended Figure 1.

### $\delta^{15}N(NH_4^+)$ measurements

Nitrogen stable isotopes of $NH_4^+$ have been measured following a solid phase extraction method¹¹. This protocol allows to control contamination below 13.5 nmol and correct its impact in the isotopic composition. Briefly, a ≈ 50 nmol $NH_4^+$ sample (estimated from the CFA measurement) is melted and loaded on a cationic AG 50W resin that has been converted to the $Na^+$ form. A quantitative elution is performed by the addition of 2 × 2.5 mL of 1 M NaCl. Eluted $NH_4^+$ is then converted into $N_2O$ and finally $N_2$ by a two-step chemical reaction¹²,¹³. The isotopic composition of the resulting gas ($N_2$) can finally be measured with a Finnigan MAT-253 IRMS¹⁴.

Using the $NH_4^+$ seasonal variations and due to size limitations (50 nmol $NH_4^+$), $\delta^{15}N$ was measured subannually for the uppermost 103 m of ice (i.e., from 1960 to 2016), annually for the section between 103 m and 113 m (i.e., from 1895 to 1960), and with a 25 cm resolution for the remaining length. All averages were calculated with mass balance.

### Isotopic fractionation corrections

As shown in Figure 1.a, from $NH_3$ emission to the burial of $NH_4^+$ isotopic fractionations occur during aerosol nucleation, aerosol growth, and transport that must be corrected. These corrections are made in the opposite direction to the chronology. Therefore, the first isotopic fractionation to be corrected from $\delta^{15}N(NH_4^+)$ in the ice is that linked to transport, which would give a $\delta^{15}N(NH_4^+)_{aer}$ value of $NH_4^+$ in the aerosol. This $\delta^{15}N(NH_4^+)_{aer}$ value will then be corrected from the isotopic fractionation that occurs during the aerosol growth for which a Rayleigh distillation is developed. This correction leads to the $\delta^{15}N(NH_4^+)_{rem}$ value for $NH_4^+$ that remains in the aerosol (unlike $NH_3$) which is finally corrected from the isotopic fractionation that occurs during aerosol nucleation. This final correction allows to obtain the $\delta^{15}N(NH_3)$ value of the sources that emitted $NH_3$.

Between the emission of $NH_3$ and the burial of $NH_4^+$ in the ice, the last one to take place is transport. We can consider that the preferential deposition of $^{15}NH_4^+$ is an irreversible process
in an open system. Thus, according to the rules of isotopy¹⁵, a correction with a constant isotopic fractionation coefficient is applied. As mentioned in the manuscript, due to lack of studies on this point, two fractionation coefficients are considered, (4.2 ± 4.5) ‰ and (17.6 ± 7.8) ‰, the first referring to the dry deposition¹⁶ and the second being the average difference measured between δ¹⁵N(NH₃) and δ¹⁵N(NH₄⁺) in aerosol studies¹⁷. An estimate using the 4.2 ‰ correction factor would result in an even higher estimate of c-NH₃ sources. Equation 1 shows the correction applied for the 17.6 ‰ scenario:

$$404 \qquad \delta^{15}N(NH_4^+)_{\text{aer}} = \delta^{15}N(NH_4^+)_{\text{ice}} - (17.6 \pm 7.8) (\text{/‰}) \qquad (\text{Eq. 1})$$

The isotopic exchange between NH₃(g) and NH₄⁺(p) is known to be associated with an equilibrium isotopic fractionation of +31 ‰¹⁸ while the partition of NH₃ between gas- and condensate-phase is pH dependent. As a result, δ¹⁵N(NH₄⁺)ₐer is expected to be dependent on the pH. Our observations are in agreement with this, as δ¹⁵N(NH₄⁺)ₐer is anti-correlated with H⁺ concentrations ([H⁺] = (-0.66 ± 0.32) × δ¹⁵N(NH₄⁺)ₐer - (10.6 ± 5.7); R² = 0.32; p = 0.013; Extended Figure 3). As acidity controls the partitioning the isotopic exchange between NH₃(g) and NH₄⁺(p) (Reactions R1 and R2 in the manuscript) is sensitive to pH with lower pH favouring ¹⁵N in NH₄⁺. We can use thermodynamic to quantify this exchange as function of the pH, as we briefly describe here based on previous studies¹⁹.

Reactions R1 and R2 are associated with the following equations:

$$415 \qquad \mathcal{H}_{NH_3} = \frac{[NH_3]}{P_{NH_3}} \qquad (\text{Eq. 2})$$

$$416 \qquad \frac{1}{K_a} = \frac{[NH_4^+]}{[NH_3] \times [H^+]} \qquad (\text{Eq. 3})$$

where $\mathcal{H}_{NH_3}$ is the Henry's constant, $P_{NH_3}$ is the partial pressure of NH₃(g) to be differentiated with [NH₃], the concentration of NH₃ in the particle, and $K_a$ is the acid dissociation constant.

The total amount of dissolved NH₃, noted NH₄⁺tot, i.e. the total amount of NH₄⁺ and NH₃ in the particle phase, can then be deduced:

$$421 \qquad [NH_4^+_{tot}] = [NH_4^+] + [NH_3] = \mathcal{H}_{NH_3} P_{NH_3} \left(1 + \frac{[H^+]}{K_a}\right) \qquad (\text{Eq. 4})$$

The fraction $NH_4^+_{tot}$ is then written:

$$423 \qquad f \approx \frac{[H^+] \mathcal{H}_{NH_3}^* W_i R T}{1 + [H^+] \mathcal{H}_{NH_3}^* W_i R T} \qquad (\text{Eq. 5})$$
424 With

$$
425 \quad \ln(\mathcal{H}_{\mathrm{NH}_3}^*) = \frac{\mathcal{H}_{\mathrm{NH}_3}}{\kappa_a} = 25.393 - 10373.6 \left(\frac{1}{T_r} - \frac{1}{T}\right) + 4.131 \left(\frac{T_r}{T} - 1 - \ln\left(\frac{T_r}{T}\right)\right) \quad (\text{Eq. 6}^{20})
$$

426 where T<sub>r</sub> is the reference temperature of 298.15 K<sup>20</sup> and W<sub>i</sub> is the water content.

427 Using the Rayleigh distillation equations, similar to what is used for nitrate<sup>21</sup>, it is possible to
428 establish the isotopic contents in the aerosol (δ<sup>15</sup>N<sub>rem</sub>) and gas (evaporated, δ<sup>15</sup>N<sub>evap</sub>) phases:

$$
429 \quad \delta^{15}N_{rem} = (1 + \delta^{15}N_0)f^{\epsilon} - 1 \quad (\text{Eq. 7})
$$

$$
430 \quad \delta^{15}N_{evap} = (1 + \delta^{15}N_0) \frac{1-f^{\epsilon+1}}{1-f} - 1 \quad (\text{Eq. 8})
$$

431 where ε is the isotopic fractionation constant and δ<sup>15</sup>N<sub>0</sub> is the isotopic composition of the
432 source (total ammonium NH<sub>4</sub><sup>+</sup><sub>tot</sub>), f is the fraction of total ammonium NH<sub>4</sub><sup>+</sup><sub>tot</sub> expressed in
433 Eq 5.

434 The calculation is made with [H⁺] induced from the ice core, *H*<sub>NH₃</sub>*<sup>\*</sup>* calculated from Eq 6<sup>20</sup>, T
435 from Chamonix temperature record (kindly provided by Météo-France) from 1934 to 2016,
436 W<sub>i</sub> = 1 mol m<sup>-3</sup>, and with a 31 ‰ isotopic fractionation constant<sup>18</sup>. Although Chamonix and
437 CDD's temperature are different, we assume the trend to be similar throughout the 20<sup>th</sup>
438 century and to not have any impact on the modelled δ<sup>15</sup>N trend. A constant water content is
439 the most important assumption as aerosol composition is known to affect water content<sup>22</sup>.
440 However, to our best knowledge, the evolution of water content has not be reported; we
441 therefore used the same hypothesis as that found in other studies<sup>21</sup>.

442 The last correction to be considered is documented to be a kinetic isotopic fractionation that
443 occurs during aerosol nucleation<sup>18</sup> with an isotopic constant of -28 ‰<sup>23</sup>. This isotopic
444 fractionation is also evidenced by the correlation found between δ<sup>15</sup>N(NH<sub>4</sub><sup>+</sup>)<sub>aer</sub> corrected from
445 such effect (δ<sup>15</sup>N(NH<sub>4</sub><sup>+</sup>)<sub>rem</sub>) and the neutralisation factor N H<sub>4</sub><sup>+</sup> N O<sub>3</sub><sup>-</sup> + 2×S O<sub>4</sub><sup>2-</sup> (= (0.020 ± 0.002) ×
446 δ<sup>15</sup>N(NH<sub>4</sub><sup>+</sup>)<sub>rem</sub> + (0.85 ± 0.05); R<sup>2</sup> = 0.32; p = <0.01) (Extended Figure 3).

447 We propose to adapt the Rayleigh distillation (Eq 7 and 8) to the nucleation step (i.e., the NH₃
448 neutralisation by HNO₃ or H₂SO₄). For this purpose, we propose to use thermodynamics to
449 take into account the neutralisation factor in the fraction of total NH₄⁺. The neutralisation of
450 NH₃ by HNO₃ or H₂SO₄ is expressed as follow:

$$
451 \quad \mathrm{NH_3} + \mathrm{AH} \rightarrow \mathrm{NH_4^+} + \mathrm{A^-} \quad (\mathrm{R.} \ 3)
$$
With AH is an acid and A⁻ its conjugated base. Although some similarities exist with the previous isotopic fractionation correction (Eq. 2 to Eq. 8), reaction R.3 is irreversible and occurs in an open system. At t = 0, before any neutralisation occurs, [NH₃] is what has been emitted by the sources (noted NH₃,source), while during the reaction, a certain amount is converted into NH₄⁺ and the rest remains as NH₃ (noted NH₃,rem) We therefore obtain the following mass balance:

$$ [NH_{3,source}] = [NH_{4}^{+}] + [NH_{3,rem}] \quad (\text{Eq. 9}) $$

While the neutralisation factor is defined as $N = \frac{NH_4^+}{NO_3^- + 2 \times SO_4^{2-}}$, we can also express it depending on NH₃,rem. Indeed, the neutralisation factor defines the fraction of NH₃,source which is converted into NH₄⁺. Therefore, N can be expressed as $N = NH_{3,rem}/NH_4^+$. This expression is implemented to the expression of the fraction of NH₄⁺, f:

$$ f = \frac{\frac{NH_4^+}{NH_{3,source}+NH_4^++NH_{3,rem}}}{\frac{NH_4^+}{NH_{3,source}+NH_4^+(1+N)}} \quad (\text{Eq. 10}) $$

Equation 5 is therefore adapted into:

$$ f \approx \frac{[H^+] \mathcal{H}^*_{NH_3} W_i R T(1+N)}{1 + [H^+] \mathcal{H}^*_{NH_3} W_i R T(1+N)} \quad (\text{Eq. 11}) $$

The Rayleigh distillation correction (Eq 7 and 8) is then applied using the ammonium fraction calculated from Eq. 11 to obtain the isotopic composition of NH₃,source.

## Uncertainty calculations

**Uncertainty associated with [NH₄⁺]:** The measurement of the chemical concentrations is subject to minor uncertainties. As no duplicate measurements are possible with ice cores, it is generally agreed that the uncertainty is around 5 % of the measurement⁶. This uncertainty estimation seems conservative since the comparison of off-line IC-MS and CFA concentrations measurements are ± 2 % different.

**Uncertainty associated with δ¹⁵N(NH₄⁺):** The δ¹⁵N(NH₄⁺) isotopic uncertainties is higher than for the concentration. In the paper presenting the methodology to measure δ¹⁵N(NH₄⁺) in ice cores, the authors make available a code for correcting blank levels, which can also be used to calculate the deviation of the isotopic measurement from the measurement of international isotopic standards¹¹,²⁴. The 10-year δ¹⁵N(NH₄⁺) values had therefore uncertainties ranging between 0.6 ‰ and 1.8 ‰.
480 Uncertainty propagation during isotopic fractionation correction: Uncertainty was propagated during the three steps of isotopic fractionation correction. As mentioned in the main manuscript, the highest source of uncertainty concerns the transport-related isotopic fractionation. As the phenomenon is not understood, two scenarios have been performed: one with a (17.6 ± 7.8) ‰ isotopic fractionation correction leading to higher contribution of agriculture emission sources, and one with a (4.2 ± 4.5) ‰ isotopic fractionation correction leading to higher contribution of combustion-related emission sources. Errors during the neutralisation and aerosol growth steps were propagated following the Rayleigh distillation equation (Eq. 7), considering uncertainties inf δ<sup>15</sup>N, [H<sup>+</sup>], and neutralisation factor.

489 δ<sup>15</sup>N signatures

490 Emission inventories induced from stable isotopes are sensitive to isotopic source signatures<sup>25</sup>. We have used the most up-to-date isotopic signatures (Extended Table 1). A bootstrapping statistical method allowed to calculate mean isotopic signatures of (-4.8 ± 4.9) ‰ and (-25.0 ± 5.5) ‰ associated with c-NH<sub>3</sub> and v-NH<sub>3</sub>, respectively.

494 For volatile-NH₃ sources, we considered agriculture emissions from fertilisers and the emissions from human and animal (i.e., manure) wastes due to the overlapping isotopic signatures. Recently, it was shown the necessity to correct the reported isotopic signatures depending on the collecting technologies²⁵. The v-NH₃ mean isotopic signature reported in Extended Table 1 is calculated as the mean between fertiliser isotopic signatures (±28.3 ± 5.8) ‰²⁶ and (±32.0 ± 4.6) ‰²⁷) and waste isotopic signatures (±17.6 ± 5.6) ‰²⁶ and (±22.1 ± 5.0) ‰²⁷).

501 For c-NH₃ sources, we only considered two emission categories referred in the literature as fuel combustion dissociating the traffic from the other types of combustion. We did not consider biomass burning isotopic signatures since a lot of discrepancy exist<sup>28-31</sup>. The c-NH₃ mean isotopic signature reported in Extended Table 1 is calculated as the mean between non-traffic combustion isotopic signature (±8.2 ± 5.5) ‰²⁶, and (-10.6 ± 5.3) ‰²⁷) and traffic isotopic signature ((1.8 ± 3.2) ‰²⁶, and (-2.2 ± 2.5) ‰²⁷).

507 For stable isotopes, the Keeling plot method is a powerful method to obtain the isotopic signature of a single source added to a constant background³². The y-intercept that results from the linear regression of δ¹⁵N(NH₃) as a function of 1/[NH₃] is the isotopic signature of the additional source. Therefore, for periods during which one NH₃ anthropogenic source drives the overall evolution of NH₃ emissions on its own, we assume that the Keeling plot
method of this period would provide the approximate value of this source isotopic signature.
Thus, the 1800-1875 period is characterised by the addition of the source whom isotopic
signature is (4.5 ± 5.4) ‰ (Extended Figure 4). This value fits with increasing NH₃ emissions
from coal combustion and biomass burning. Similarly, the additional source of the 1875-1920
period has an isotopic signature of (-19.3 ± 0.7) ‰, in agreement with the increasing usage of
manure. For the period 1920-1970, the increasing use of fertilisers further decreases the
isotopic signature of the additional source with a (-22.7 ± 0.8) ‰ value. Between 1970 and
2000, as discussed in the manuscript, the NH₃ emissions are primarily driven by c-NH₃
sources in agreement with the (-3.9 ± 3.5) ‰ value of the Keeling plot.

521 Method References

522 1. Preunkert, S., Wagenbach, D., Legrand, M. & Vincent, C. Col du Dôme (Mt Blanc Massif, French Alps) suitability for ice-core studies in relation with past atmospheric chemistry over Europe. *Tellus B: Chemical and Physical Meteorology* **52**, 993–1012 (2000).

525 2. Preunkert, S. & Legrand, M. Towards a quasi-complete reconstruction of past atmospheric aerosol load and composition (organic and inorganic) over Europe since 1920 inferred from Alpine ice cores. *Climate of the Past* **9**, 1403–1416 (2013).

528 3. Vincent, C. et al. Strong changes in englacial temperatures despite insignificant changes in ice thickness at Dôme du Goûter glacier (Mont Blanc area). *The Cryosphere* **14**, 925–934 (2020).

531 4. Legrand, M. et al. Cadmium Pollution From Zinc-Smelters up to Fourfold Higher Than Expected in Western Europe in the 1980s as Revealed by Alpine Ice. *Geophysical Research Letters* **47**, (2020).

534 5. Röthlisberger, R. et al. Technique for Continuous High-Resolution Analysis of Trace Substances in Firn and Ice Cores. *Environ. Sci. Technol.* **34**, 338–342 (2000).

536 6. Legrand, M., Preunkert, S., Wagenbach, D. & Fischer, H. Seasonally resolved Alpine and Greenland ice core records of anthropogenic HCl emissions over the 20th century. *Journal of Geophysical Research* **107**, 4139–4153 (2002).

539 7. Legrand, M., Preunkert, S., Wagenbach, D., Cachier, H. & Puxbaum, H. 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**, (2003).
543 8. Clifford, H. M. et al. A 2000 Year Saharan Dust Event Proxy Record from an Ice Core in the European Alps. *Journal of Geophysical Research: Atmospheres* **124**, (2019).

545 9. Preunkert, S. et al. Impact of subsurface crevassing on the depth-age relationship of high-alpine ice cores extracted at Col du D&ocirc;me between 1994 and 2012. *The Cryosphere Discussions* 1-21 (2023) doi:10.5194/tc-2022-259.

548 10. Gilbert, A., Gagliardini, O., Vincent, C. & Wagnon, P. A 3-D thermal regime model suitable for cold accumulation zones of polythermal mountain glaciers. *Journal of Geophysical Research: Earth Surface* **119**, 1876-1893 (2014).

551 11. Lamothe, A. et al. An extraction method for nitrogen isotope measurement of ammonium in a low-concentration environment. *Atmospheric Measurement Techniques* **16**, 4015-4030 (2023).

554 12. Zhang, L., Altabet, M. A., Wu, T. & Hadas, O. Sensitive Measurement of NH4+ 15N/14N (δ15NH4+) at Natural Abundance Levels in Fresh and Saltwaters. *Anal. Chem.* **79**, 5297-5303 (2007).

557 13. McIlvin, M. R. & Altabet, M. A. Chemical Conversion of Nitrate and Nitrite to Nitrous Oxide for Nitrogen and Oxygen Isotopic Analysis in Freshwater and Seawater. *Anal. Chem.* **77**, 5589-5595 (2005).

560 14. Kaiser, J., Hastings, M. G., Houlton, B. Z., Röckmann, T. & Sigman, D. M. Triple Oxygen Isotope Analysis of Nitrate Using the Denitrifier Method and Thermal Decomposition of N₂O. *Analytical Chemistry* **79**, (2007).

563 15. Hayes, J. M. An introduction to isotopic calculations. *Woods Hole Oceanographic Institution, Woods Hole, MA 2543*, (2004).

565 16. Walters, W. W. et al. Characterizing the spatiotemporal nitrogen stable isotopic composition of ammonia in vehicle plumes. *Atmospheric Chemistry and Physics* **20**, 11551-11567 (2020).

568 17. Walters, W. W. et al. Quantifying the importance of vehicle ammonia emissions in an urban area of northeastern USA utilizing nitrogen isotopes. *Atmospheric Chemistry and Physics* **22**, 13431-13448 (2022).

571 18. Walters, W. W., Chai, J. & Hastings, M. G. Theoretical Phase Resolved Ammonia- Ammonium Nitrogen Equilibrium Isotope Exchange Fractionations: Applications for
573 Tracking Atmospheric Ammonia Gas-to-Particle Conversion. *ACS Earth and Space Chemistry* **3**, 79–89 (2019).
574

575 19. Guo, H. *et al*. Fine particle pH and gas-particle phase partitioning of inorganic species in Pasadena, California, during the 2010 CalNex campaign. *Atmospheric Chemistry and Physics* **17**, 5703–5719 (2017).
576
577

578 20. Clegg, S. L., Brimblecombe, P. & Wexler, A. S. Thermodynamic Model of the System H++NH4++SO42--NO3--H2O at Tropospheric Temperatures. *J. Phys. Chem. A* **102**, 2137–2154 (1998).
579
580

581 21. Geng, L. *et al*. Nitrogen isotopes in ice core nitrate linked to anthropogenic atmospheric acidity change. *Proceedings of the National Academy of Sciences* **111**, (2014).
582

583 22. Nenes, A., Pandis, S. N., Weber, R. J. & Russell, A. Aerosol pH and liquid water content determine when particulate matter is sensitive to ammonia and nitrate availability. *Atmospheric Chemistry and Physics* **20**, 3249–3258 (2020).
584
585

586 23. Pan, Y. *et al*. Fossil Fuel Combustion-Related Emissions Dominate Atmospheric Ammonia Sources during Severe Haze Episodes: Evidence from ¹⁵N-Stable Isotope in Size-Resolved Aerosol Ammonium. *Environmental Science & Technology* **50**, 8049–8056 (2016).
587
588

589 24. Lamothe, A. *et al*. NH4_method_in_low_concentrated_environment-sizecorrection_andCalibrationScripts. (2023) doi:10.5281/zenodo.7728983.
590

591 25. Bhattarai, N. *et al*. δ15N-stable isotope analysis of NHx: An overview on analytical measurements, source sampling and its source apportionment. *Front. Environ. Sci. Eng.* **15**, 126 (2021).
592
593

594 26. Kawashima, H. *et al*. Sources identification of ammonium in PM2.5 during monsoon season in Dhaka, Bangladesh. *Science of The Total Environment* **838**, 156433 (2022).
595

596 27. Bhattarai, N. *et al*. Nitrogen isotopes suggest agricultural and non-agricultural sources contribute equally to NH3 and NH4+ in urban Beijing during December 2018. *Environmental Pollution* 121455 (2023) doi:10.1016/j.envpol.2023.121455.
597
598

599 28. Feng, X. *et al*. Biomass burning is a non-negligible source for ammonium during winter haze episodes in rural North China: Evidence from high time resolution 15N-stable isotope. *Journal of Geophysical Research: Atmospheres* n/a, e2022JD038012 (2023).
600
601
29. Kawashima, H., Yoshida, O. & Suto, N. Ion-exchange resin and denitrification pretreatment for determining δ15N-NH4+, δ15N-NO3–, and δ18O-NO3– values. *Rapid Communications in Mass Spectrometry* **35**, e9027 (2021).

30. Chang, Y. *et al*. Assessing Contributions of Agricultural and Nonagricultural Emissions to Atmospheric Ammonia in a Chinese Megacity. *Environmental Science & Technology* **53**, 1822–1833 (2019).

31. Chang, Y., Liu, X., Deng, C., Dore, A. J. & Zhuang, G. Source apportionment of atmospheric ammonia before, during, and after the 2014 APEC summit in Beijing using stable nitrogen isotope signatures. *Atmospheric Chemistry and Physics* **16**, 11635–11647 (2016).

32. Keeling, C. D. The concentration and isotopic abundances of carbon dioxide in rural and marine air. *Geochimica et Cosmochimica Acta* **24**, 277–298 (1961).

## Acknowledgements

We acknowledge the LabEx OSUG@2020 (“Investissements d’avenir” – ANR10 LABX56), the ANR project ANR-15-IDEX-02 (IDEX-UGA), the IceMemory Foundation, and the technical support from the F2G (French National platform for Coring and Drilling, handled by INSU, the MIUR (Ministry of Education, University and Research) for supporting this research. A.L. deeply thanks Anthony Vella, Salomé Koelsch and Selin Bagci for laboratory handling, Bruno Jourdain, Olivier Alemany, Luc Piard, Jérôme Chappellaz, and Anne-Catherine Ohlmann for the drilling, Michel Legrand, Lei Geng, Christian Vincent, and Albane Barbero for scientific advises, and Guillaume Chagnaud, Eliot Jager, Thomas Chauve, Lorenzo Martello, and Gaelle Capitaine for feedbacks on the paper.

## Author contributions

A.L. led the study, performed the laboratory measurements with the help of P.G., S.A., E.G., N.C., and S.D, conducted the scientific interpretations with contribution of P.A., S.A. and J.S., A.L. wrote the first draft with the help of P.A., and all co-authors contributed to the improvement and correction of the paper. J.S. and P.G. supervised A.L. P.G. conducted the CDD drilling with the support of N.C.. A.E. measured ³H levels. A.G. developed the 3D model.
632 **Competing interest declaration**

633 The authors do not declare any situation that may have influence the scientific conclusion of
634 this work.

635 **Data availability**

636 10 year average isotopic measurements and the measurements used in this work are available
637 here (https://zenodo.org/records/10055462, DOI: 10.5281/zenodo.10055462). A complete
638 database of CDD measurements will be later accessible in an open-access dataset still under
639 construction, which will be named CDD16 after the Col du Dôme drilling campaign in 2016.
640 When finalized, the link to this dataset will be found on the PANDA analytical platform
641 website (https://panda.osug.fr/; PANDA, 2023). Until then, one can contact the authors, who
642 will share the data.

643 **Extended data**

644 Extended Figure 1: dating profile

<figure><img src="image_10.png" /></figure>

646 Summary of the dating profile with age (2016 - year) as a function of depth (m w e): the
647 seasonal counting layer dating (blue) with its uncertainty (shaded blue), the 3D model dating
648 (green) with its uncertainty (shaded green), the main Saharan dust events (yellow stars), the
649 1963 ³H peak (purple triangle), and the four temporal discontinuities (black circles)

650 Extended Figure 2: comparison between [NH₄⁺] and [NH₃]
<figure><img src="image_11.png" /></figure>

651

652 Comparison between [NH₄⁺] measured in the ice (dashed green) and [NH₃] corrected (= [NO₃⁻]

653 ] + 2 × [SO₄²⁻]) from neutralisation variations (blue) as a function of the age.

654 Extended Figure 3: acidity and δ¹⁵N correlation

<figure><img src="image_12.png" /></figure>

655

656 A) δ¹⁵N(NH₄⁺) measured in the ice core /‰. B) δ¹⁵N(NH₄⁺)aer corrected from the -17.6 ‰

657 transport isotopic fractionation compared with acidity concentration [H⁺] (dashed red) /ppb.

658 C) δ¹⁵N(NH₄⁺)rem corrected from aerosol growth isotopic fractionation compared with the

659 neutralisation factor $\frac{NH_4^+}{NO_3^- + 2 \times SO_4^{2-}}$ (dashed purple).

660 Extended Figure 4: Keeling plot
<figure><img src="image_13.png" /></figure>

661

Keeling plots tracing δ¹⁵N(NH₃) (‰) as a function of 1/[NH₃] (/ppb⁻¹) allows to obtain the isotopic signature of a source added in a constant background, reading the intercept¹, each dots account for 10 year averages between 1890 and 2015 and the measurement resolution for the remaining period. The central figure displays the Keeling plots for different periods of the CDD16 ice core, the preindustrial values (orange circles), 1800-1875 period (red crosses), 1875-1920 period (green triangles), 1920-1970 period (blue stars), 1970-2000 period (purple squares), and 2000-2015 (red hexagons). The bottom and top figures display zooms of selected periods. No trends were observed for the pre-industrial and 2000-2015 periods.

Extended Figure 5: comparison of acidity
<figure><img src="image_14.png" /></figure>

671

672 Comparison of acidity concentrations measured in different CDD ice cores (/$\mu$mol L$^{-1}$) across the 20th century. Our acidity concentration is calculated from the CFA reconstruction (dashed blue line). Legrand et al 2003 acidity concentration is from the publication<sup>2</sup> (solid yellow line). The reconstructed acidity concentration from all previous CDD publications is calculated with the concentrations of major ions ($NH_4^+$, $Na^+$, $Mg^{2+}$, $Ca^{2+}$, $NO_3^-$, $SO_4^{2-}$, $Cl^-$) from previous publications<sup>3</sup>.

678 Extended Figure 8: an example of a time discontinuity

<figure><img src="image_15.png" /></figure>

679

680 $\delta$D (‰) of melted water from CDD16 as a function of depth (m w e). Three time horizons are added (1963 for the <sup>3</sup>H peak, and 1947 and 1897 with major Saharan Dust Events) as well
as some annual layers identified through annual counting. We clearly notice the discontinuity
85.8 and 86.8 m w e with the disappearance of the seasonality pattern (high values for
summer, and low values for winter).

Extended data Table 1: Table isotopic source values

<table><thead><tr><th>Source</th><th>δ<sup>15</sup>N(NH<sub>3</sub>) /‰</th><th>Reference</th></tr></thead><tbody><tr><td>Traffic combustion</td><td>(2.3±6.7) ‰</td><td>4-7</td></tr><tr><td>Non-traffic fossil fuel combustion</td><td>(-7.7±13) ‰</td><td>4-7</td></tr><tr><td>Biomass burning</td><td>Uncertain. Estimated between -11.8 and 24.3‰</td><td>6,8,9</td></tr><tr><td>Livestock</td><td>(-18.4±10.2) ‰</td><td>4-7</td></tr><tr><td>Fertiliser</td><td>(-30.4±7.9) ‰</td><td>4-7</td></tr></tbody></table>

References

1. Keeling, C. D. The concentration and isotopic abundances of carbon dioxide in rural and marine air. *Geochimica et Cosmochimica Acta* **24**, 277–298 (1961).

2. Legrand, M., Preunkert, S., Wagenbach, D., Cachier, H. & Puxbaum, H. 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**, (2003).

3. Preunkert, S. & Legrand, M. Towards a quasi-complete reconstruction of past atmospheric aerosol load and composition (organic and inorganic) over Europe since 1920 inferred from Alpine ice cores. *Climate of the Past* **9**, 1403–1416 (2013).

4. Bhattarai, N. *et al*. δ15N-stable isotope analysis of NHx: An overview on analytical measurements, source sampling and its source apportionment. *Front. Environ. Sci. Eng.* **15**, 126 (2021).

5. Kawashima, H., Yoshida, O. & Suto, N. Long-Term Source Apportionment of Ammonium in PM2.5 at a Suburban and a Rural Site Using Stable Nitrogen Isotopes. *Environ. Sci. Technol.* (2022) doi:10.1021/acs.est.2c06311.
703 6. Feng, X. et al. Biomass burning is a non-negligible source for ammonium during
704 winter haze episodes in rural North China: Evidence from high time resolution 15N-stable
705 isotope. *Journal of Geophysical Research: Atmospheres* **n/a**, e2022JD038012 (2023).

706 7. Bhattarai, N. et al. Nitrogen isotopes suggest agricultural and non-agricultural sources
707 contribute equally to NH3 and NH4+ in urban Beijing during December 2018. *Environmental
708 Pollution* 121455 (2023) doi:10.1016/j.envpol.2023.121455.

709 8. Kawashima, H. & Kurahashi, T. Inorganic ion and nitrogen isotopic compositions of
710 atmospheric aerosols at Yurihonjo, Japan: Implications for nitrogen sources. *Atmospheric
711 Environment* **45**, 6309–6316 (2011).

712 9. Kawashima, H., Yoshida, O. & Suto, N. Ion-exchange resin and denitrification
713 pretreatment for determining δ15N-NH4+, δ15N-NO3–, and δ18O-NO3– values. *Rapid
714 Communications in Mass Spectrometry* **35**, e9027 (2021).

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