Patrick Ginot

Bromine explosion events (BEs) are an important source of reactive bromine in the Arctic boundary layer and contribute to episodic ozone depletion in spring. While bromide-rich sea ice and snow are generally regarded as the primary source, the extent to which atmospheric deposition of bromine can establish and replenish a snowpack bromide reservoir remains poorly constrained. We combine simulations with the ECHAM/MESSy Atmospheric Chemistry model (EMAC), field observations, and laboratory analyses of snow samples collected around Ny-Ålesund (Svalbard) in April 2024 to investigate the role of atmospheric bromine deposition for BEs. To connect atmospheric deposition fluxes with observed snow chemistry, we developed the Snow Model with Evolving Layers (SMELs), a simple offline snow model that tracks bromide accumulation within a dynamically evolving snowpack. EMAC simulations indicate that bromine dry deposition from very short-lived substances (VSLS) and sea-salt aerosol (SSA) dehalogenation can establish snow bromide reservoirs sufficient to sustain typical Arctic springtime BrO abundances. Bromine species partitioning upon deposition suggests that a lack of HBr deposition may suppress heterogeneous Br₂ production during fall. SMELs reproduces the observed vertical bromide enhancement and depletion patterns and demonstrates that finite snowpit sampling can introduce substantial biases. The strong agreement between modeled and observed normalized bromide profiles suggests that bromine deposition and post-depositional processing operate on regional scales around Ny-Ålesund. Our results identify atmospheric bromine deposition as an important seasonal bromide reservoir and highlight the role of bromine speciation and snowpack processes in controlling Arctic bromine activation.

Ammonia (NH3) is the main alkaline gas in the atmosphere and a key regulator of aerosol formation, nitrogen deposition, and air quality. Yet the relationship between NH3 emissions and deposited ammonium (NH4+) in remote archives, remains poorly constrained. Here, we present the first ice-core record of δ15N(NH4+), covering 1000 years of European atmosphere. Ice-core δ15N(NH4+) is systematically enriched relative to inventory-based δ15N(NH3), consistent with atmospheric isotopic fractionation between emitted NH3 and deposited NH4+. The 10-year seasonally resolved record shows lower δ15N(NH4+) in winter than in summer, which is consistent with NH3–NH4+ partitioning integrated during transport, although no unique pathway can be identified as other mechanisms remain insufficiently constrained. Between 1900 and 1970, lower δ15N(NH4+) broadly coincides with higher acidity and lower neutralisation factor, consistent with increasing proportion of NH4+ through NH3–NH4+ partitioning, although cloud processing and other atmospheric processes cannot be separated. After 1980, this relationship breaks down, with a divergence between ice-core NH4+ and NH3 emission inventories, the isotope–concentration relationship disappears, and a decrease in apparent isotopic fractionation. Boundary layer observations suggest that a large underestimation of NH3 emissions is unlikely. This shift therefore indicates a change in the source-to-deposition atmospheric processes, although the responsible pathway remains unresolved. Ice-core δ15N(NH4+) therefore provides a long-term view of the source-to-deposition regime shifts, a perspective increasingly needed as Europe moves toward a less acidic, more NH3-rich atmosphere.

Bromine explosion events (BEs) are an important source of reactive bromine in the Arctic boundary layer and contribute to episodic ozone depletion in spring. While bromide-rich sea ice and snow are generally regarded as the primary source, the extent to which atmospheric deposition of bromine can establish and replenish a snowpack bromide reservoir remains poorly constrained. We combine simulations with the ECHAM/MESSy Atmospheric Chemistry model (EMAC), field observations, and laboratory analyses of snow samples collected around Ny-Ålesund (Svalbard) in April 2024 to investigate the role of atmospheric bromine deposition for BEs. To connect atmospheric deposition fluxes with observed snow chemistry, we developed the Snow Model with Evolving Layers (SMELs), a simple offline snow model that tracks bromide accumulation within a dynamically evolving snowpack. EMAC simulations indicate that bromine dry deposition from very short-lived substances (VSLS) and sea-salt aerosol (SSA) dehalogenation can establish snow bromide reservoirs sufficient to sustain typical Arctic springtime BrO abundances. Bromine species partitioning upon deposition suggests that a lack of HBr deposition may suppress heterogeneous Br2 production during fall. SMELs reproduces the observed vertical bromide enhancement and depletion patterns and demonstrates that finite snowpit sampling can introduce substantial biases. The strong agreement between modeled and observed normalized bromide profiles suggests that bromine deposition and post-depositional processing operate on regional scales around Ny-Ålesund. Our results identify atmospheric bromine deposition as an important seasonal bromide reservoir and highlight the role of bromine speciation and snowpack processes in controlling Arctic bromine activation.

The central regions of the East Antarctic ice sheet contains some of the oldest ice on earth, due to low snow accumulation rates and consistently cold conditions. One consequence of low accumulation however is that the little snow amount which is deposited at irregular times is more strongly affected by erosion and re-deposition by wind, inducing local mixing and loss of snowfall events in the snowpack. This discontinuous deposition leads to highly disturbed snow layers, limiting the interpretation of climate records from ice cores in these regions to time scales larger than decades. In order to interpret climate records at higher temporal resolution it is crucial to assess and quantify the local patterns of snow accumulation leading to stratigraphic noise. Here, we reconstruct the spatial and temporal variability of snow accumulation in Central Antarctica using chemical composition and physical properties in 35 snow profiles in a 50 m long snow trench at Dome C. We show that a high resolution alignment of the chemistry profiles is a suitable method for inter-annual dating of the single trench profiles, allowing the reconstruction of accumulation time series with a 1 year resolution over the last 20 years. This reconstruction shows annually-varying past surface configurations, with up to 10 % of the surface subject to annual accumulation hiatus. More persistent multi-year scale patterns are also evidenced, causing difference in snow age of up to 4 years at similar depth in neighboring profiles, highlighting the complex dynamics of snow accumulation in central Antarctica.

In the face of natural and anthropogenic emissions, the habitability of Earth’s atmosphere is maintained thanks to atmospheric oxidants – photochemically produced reactive species which destroy toxic pollutants, remove greenhouse gases and maintain chemical stability. Understanding the chemical dynamics of the atmosphere is hence crucial for accurate predictions of air quality and radiative forcing in a changing climate, as identified in IPCC AR6. Because the species involved are often highly reactive, studying historic atmopsheric chemistry is challenging: consequently, there is no consensus on the magnitude or the sign of the relationship between atmospheric oxidative capacity and global climate.One avenue for past atmospheric chemistry reconstructions is isotopic analysis of nitrate – an oxidation product of atmospheric NOx – archived in non-polar ice cores. The N and O isotope compositions of ice core nitrate depend on past oxidation reactions and past NOx sources, while newly-accessible nitrate clumped isotopes may provide complementary information on nitrate formation pathways. However, useful signals can be obscured by isotopic fractionation during nitrate transport, deposition and burial, while seasonal variations in atmospheric chemistry or snow accumulation can bias ice core records. These difficulties often make ice core nitrate isotope interpretations non-unique, limiting their utility as investigative tools for past atmospheric chemistry.To investigate the processes controlling nitrate isotopes archived in non-polar ice cores, we collected firn cores and weekly high-volume atmospheric samples at high altitude sites in the Mont-Blanc massif (France/Italy) and the Cordillera Oriental (Bolivia). Using the newly-adapted Electrospray-Orbitrap mass spectrometer, we investigated the seasonality of atmospheric nitrate isotope ratios δ15N, δ18O and Δ17O, and clumped isotopes Δ15N18O and Δ18O18O, and compared atmospheric isotopic signals to those in contemporaneously-deposited firn over an accumulation season. We find substantial seasonal isotopic variability in atmospheric nitrate, which is partially preserved in firn core records. However, several isotopic disagreements could reflect syn- or post-depositional isotopic fractionation processes, and the isotopic seasonality should be carefully considered when intepreting ice core records where accumulation is seasonal.

In this study we tested under field conditions the hypothesis that electrical phenomena may influence chemical composition of snow. The field experiment was conducted at the Akademik Vernadsky Station within the framework of the State Program of Scientific Research of Ukraine in Antarctica during two winter seasons in 2022 and 2023, using a newly designed trap for charged snow. This instrument was constructed to selectively attract charged snow particles from the blowing-snow flux and was deployed during blowing snow events The experiments were performed in winter to ensure that chemical modifications were not affected by photochemical reactions. A similar field experiment was conducted in the Arctic at Ny-Ålesund, Svalbard. Here, we focus on the chemical composition of the snow samples collected by the trap, which were analyzed using ion chromatography and compared with the composition of the background blowing snow.We found that samples of charged snow were significantly more concentrated than background snow, which is attributed to the sublimation during conditions of blowing snow events. When the measured ion concentrations were compared with the expected concentration ranges and ratios characteristic of sea salt, the charged snow samples were, however, depleted in chloride, with this difference far exceeding the measurement uncertainty. The dependence of chloride “losses” on the fraction of sublimated water indicated a strong change and revealed the presence of a threshold at approximately 80 % sublimated water, beyond which these losses—interpreted as emissions to the atmosphere—increased sharply. Products of chlorine free-radical reactions in the atmosphere have been reported by numerous authors; however, the mechanism responsible for initiating these reactions remains uncertain. The present experiment provides evidence that the charging of snow may serve as such a triggering process. The presence of a similar threshold at 70–80 % sublimated water, after which ion losses increase sharply, was also observed for Br⁻, SO₄²⁻, and Mg²⁺. However, the presence of local sources renders these relationships less significant. The change in the composition at such high sublimation fractions may indicate emission of these ions due to overcoming of the Rayleigh limit indicating that electrical charging affects chemical processes in snow.In contrast, the experiments conducted in the coastal marine environment at Ny-Alesund indicate that ion ratios characteristic of sea salt were preserved in the charged snow samples, demonstrating that no or only limited chemical transformations took place. The presence of sea ice appears to be critical for the manifestation of chemical effects. When sea ice is present, snow particle charging during blizzards occurs primarily due to frictional and sublimation-driven processes. In the absence of an ice surface, charging is most likely driven by the sorption of marine aerosols maintaining the expected sea salt ratios in the charged snow.

Nitrogen stable isotopes ( δ ¹⁵ N) of ice core nitrate (NO ₃ ⁻ ) are often subject to diverse interpretations associated with changes in nitrogen oxide (NO _x ) sourcing, atmospheric reactions, and/or post‐depositional processes. Here, an ice core from Mont Blanc (French Alps) was analyzed to investigate the δ ¹⁵ N(NO ₃ ⁻ ) record over the past 1,000 years. Atmospheric isotopic fractionation, including gas‐particle partitioning, introduces a limited and quantifiable effect on δ ¹⁵ N(NO ₃ ⁻ )—up to 2.4 ‰ in extreme cases. Thus, the glacially‐archived NO ₃ ⁻ reliably reflects a record of Western European NO _x emissions by comparing δ ¹⁵ N and concentrations of NO ₃ ⁻ , and isotope–concentration mixing relationships (Keeling plot) with historical emission inventories. An increase in δ ¹⁵ N(NO ₃ ⁻ ) in the 1800s reflects the dominance of NO _x emissions from coal combustion. During the 20th century, the δ ¹⁵ N(NO ₃ ⁻ ) value substantially decreased, from (4.7 ± 1.5) ‰ in 1900 to (−1.9 ± 1.4) ‰ in 1990, and Keeling plot interpretation attributes this decrease to increasing oil combustion emissions. Between 1750 and 2016, the ice core's δ ¹⁵ N(NO ₃ ⁻ ) record generally aligns with existing NO _x inventories for Western Europe. However, during the early 20th‐century, low ice core δ ¹⁵ N(NO ₃ ⁻ ) values suggest that the inventories may have underestimated NO _x emissions resulting from agriculture. Since 2000, the decreasing NO ₃ ⁻ concentrations and δ ¹⁵ N(NO ₃ ⁻ ) values highlight the success of mitigation policies in reducing fossil fuel‐induced NO _x emissions, albeit with a delay of 20 years relative to emission inventories, that can be attributed to gas‐particle partitioning and mis‐quantification of NO _x sources. This work reaffirms the value of alpine ice cores for understanding aerosol sources.

The central regions of the East Antarctic ice sheet contains some of the oldest ice on earth, due to low snow accumulation rates and consistently cold conditions. One consequence of low accumulation however is that the little snow amount which is deposited at irregular times is more strongly affected by erosion and re-deposition by wind, inducing local mixing and loss of snowfall events in the snowpack. This discontinuous deposition leads to highly disturbed snow layers, limiting the interpretation of climate records from ice cores in these regions to time scales larger than decades. In order to interpret climate records at higher temporal resolution it is crucial to assess and quantify the local patterns of snow accumulation leading to stratigraphic noise. Here, we reconstruct the spatial and temporal variability of snow accumulation in Central Antarctica using chemical composition and physical properties in 35 vertical profiles sampled in a 50 m long snow trench at Dome C. We show that a high resolution alignment of the chemistry profiles is a suitable method for inter-annual dating of the single trench profiles, allowing the reconstruction of accumulation time series with a 1 year resolution over the last 20 years. This reconstruction shows annually-varying past surface configurations, with about 10 % of the surface subject to accumulation hiatus. More persistent patterns with timescales of years are also evidenced, causing difference in snow age of up to 4 years at similar depth in neighboring profiles, highlighting the complex dynamics of the snow accumulation in central Antarctica.

The central regions of the East Antarctic ice sheet contains some of the oldest ice on earth, due to low snow accumulation rates and consistently cold conditions. One consequence of low accumulation however is that the little snow amount which is deposited at irregular times is more strongly affected by erosion and re-deposition by wind, inducing local mixing and loss of snowfall events in the snowpack. This discontinuous deposition leads to highly disturbed snow layers, limiting the interpretation of climate records from ice cores in these regions to time scales larger than decades. In order to interpret climate records at higher temporal resolution it is crucial to assess and quantify the local patterns of snow accumulation leading to stratigraphic noise. Here, we reconstruct the spatial and temporal variability of snow accumulation in Central Antarctica using chemical composition and physical properties in 35 vertical profiles sampled in a 50 m long snow trench at Dome C. We show that a high resolution alignment of the chemistry profiles is a suitable method for inter-annual dating of the single trench profiles, allowing the reconstruction of accumulation time series with a 1 year resolution over the last 20 years. This reconstruction shows annually-varying past surface configurations, with about 10 % of the surface subject to accumulation hiatus. More persistent patterns with timescales of years are also evidenced, causing difference in snow age of up to 4 years at similar depth in neighboring profiles, highlighting the complex dynamics of the snow accumulation in central Antarctica.

Black carbon (BC) is a key atmospheric forcer due to its interaction with solar radiation and clouds. However, accurately quantifying and understanding the impact of atmospheric aging on BC properties and radiative forcing remains a major challenge. To address this, the AIDA (Atmospheric Interactions and Dynamics in the Atmosphere) aRCtic Transport Experiment (ARCTEx) project simulated BC aging under quasi-realistic Arctic conditions in the AIDA chamber. Four distinct scenarios were simulated based on reanalysis data, representing summer and winter conditions at both low and high altitudes, to capture the variability in BC aging processes in the presence of nitrate and organic matter precursors during Arctic transport. In the first part of the paper, we define the meteorological conditions characterizing northward transport under different scenarios and describe the technical solutions to simulate 5 d transport in the AIDA chamber. In the second part of the work, we assess the evolution of fundamental properties, including density, morphology, and mixing state, as observed during the aging process. The ARCTEx project demonstrates that large facilities such as AIDA can successfully reproduce environmental conditions, enabling a gradual aging process that closely follows the natural timescales observed in the atmosphere. Our experiments revealed that temperature strongly influences the aging timescale and the evolution of BC's diameter, effective density, and coating thickness. Low-altitude scenarios exhibited rapid aging, resulting in fully coated, compact BC particles within 39–98 h, corresponding to 50 and 80° N, respectively. In contrast, high-altitude transport was characterized by slow aging, with limited coating and compaction, even after 115 h of simulation. These findings provide valuable insights into the temporal evolution of BC properties during Arctic transport. In forthcoming work, we will report the implications of this evolution for climate-relevant properties such as light absorption and activation as cloud droplets and ice crystals. Together, these studies aim to enhance the representation of BC aging in climate models, reducing uncertainties in Arctic radiative-forcing estimates.