Black carbon (BC) and iron oxide (FeO x ) aerosols represent important contributors to shortwave atmospheric heating and have been associated with adverse human health outcomes. Accurate measurement of BC and FeO x is crucial for understanding their roles in climate forcing and air quality. However, observational dataparticularly for anthropogenic FeO x remain limited. In this study, we use a modified Single Particle Soot Photometer to measure atmospheric concentrations of refractory BC (rBC) and FeO x particles in urban (Atlanta, GA) and rural (Boone, NC) environments in the southeastern U.S. We identify likely emission sources and estimate light absorption properties to assess the relative contributions of rBC and FeO x to atmospheric heating. Our findings indicate that rBC and FeO x concentrations and absorption are substantially higher in the urban environment, reflecting significant anthropogenic contributions to pollutant emissions. Assuming external mixing, we estimate that FeO x contributes 1.3% of total absorption by rBC at the urban site and approximately 0.6% at the rural site. These results highlight the spatial variability of light-absorbing aerosol components and the importance of localized assessments to better understand their impacts on air quality and climate.

Abstract Paleoclimate reconstructions of wildfire using ice cores often rely on high‐resolution aerosol records of refractory black carbon (rBC) or ammonium (NH 4 + ). The interpretation of such records, however, can be hindered by uncertainties related to long‐range atmospheric transport, spatial distribution and strength of emissions, as well as glaciochemical noise introduced to the record during and after deposition. Here, we use arrays of parallel ice cores collected from two sites in Greenland, each with differing characteristics including accumulation rate, elevation, and meteorology, to assess the spatial variability and site representativeness of their rBC and NH 4 + records. The higher accumulation site, Summit, was found to preserve a more coherent signal of both aerosols across multiple ice core records and was more representative of the total site signal than the lower accumulation site, Tunu. Representativeness analyses indicate that a single core at Summit is as representative of the theoretical total site signal as a combined three core composite record from Tunu for rBC and NH 4 + at annual resolution. This work suggests that high temporal resolution records of these aerosols using a single core from either site have considerable uncertainties, highlighting the need for careful consideration of the effects of site conditions on aerosol representativeness for wildfire reconstruction.

Precise synchronization of paleoclimate records is essential for inferring the dynamics and past evolution of the climate system. For the last glacial period, the time scales of ice cores from the Greenland and Antarctic ice sheets have been synchronized by the use of cosmogenic radionuclides, atmospheric gas concentrations, and traces of large volcanic eruptions. Here we identify the sulfate deposition signatures of the same 300 volcanic eruptions in different Greenland and Antarctic ice cores to obtain an inter-hemispheric volcanic ice-core synchronization of the entire last glacial period and the early Holocene (10-110 ka). Compared to earlier bipolar volcanic synchronizations, we close a gap in the period 16.5-24.5 ka and extend the synchronization to cover the 10-12 ka and 60-110 ka intervals. Furthermore, we increase the density of bipolar match points and make updates and corrections of the existing bipolar and unipolar synchronizations. The volcanic synchronization is in agreement with existing bipolar synchronizations from independent 10Be and methane matching. The bipolar volcanic synchronization allows us to determine the precise phasing of interhemispheric abrupt climate events throughout the last glacial period, particularly those associated with Dansgaard-Oeschger (D-O) events. Our improved synchronization and extended time period allow us to show that at the time of the D-O warming transitions, the average Antarctic temperature reaches a maximum within decades after the Greenland temperature maximum. This rapid Antarctic warming is superimposed on the well-known millennial-scale thermal bipolar-seesaw warming in Antarctica commonly attributed to oceanic heat transport and confirms earlier work that the abrupt change observed in Greenland is associated with a direct atmospheric circulation change at a global scale. The exception to this pattern occurs for the EDML ice-coring site located in the Atlantic sector of Antarctica, potentially related to sea-ice conditions in the Weddell Sea. Comparison to state-of-the-art climate model simulations shows excellent agreement in the overall bipolar climate phasing at the warming transitions and allows for analysis of the climate-system behavior at those transitions. The model simulations suggest that the abrupt Antarctic warming response observed is connected with an interhemispheric atmospheric response involving a global scale reorganization of the zonal mean atmospheric circulation. The abrupt D-O surface warming signal in the Northern Hemisphere is teleconnected into an abrupt Antarctic surface warming through changes in the Southern Hemisphere eddy-driven jet and anomalous circulation changes in the associated Ferrel and Polar cells.

Light absorbing impurities, such as black carbon (BC) and mineral dust (MD) particles, are important constituents of seasonal snowpacks, especially when they are present near the surface where they can influence snow albedo and energy balance. Such impurities can be directly deposited or accumulate on the snow surface during melt processes. While soluble chemical species are known to rapidly flush from snowpacks during melt, the behavior of BC and MD particles, which mostly consist of insoluble species, is not well constrained. Previous work has shown that BC particles can either be concentrated at the snow surface or flushed from the snowpack under different conditions. Here we develop a comprehensive data set of BC and MD particles within and eluting from a seasonal Sierra Nevada, CA, USA snowpack by combining timeseries of snow profile measurements with time-resolved characterization of meltwater captured by snow lysimeters. Together, the snow and lysimeter measurements indicate that BC and MD particles are retained until the final few days of snowmelt, when they are rapidly flushed from the remaining, shallow snow column. During the final melt, BC and MD concentrations and mass fluxes closely parallel melt rates, indicating that snowpack properties and melt rates are the main controls on how these impurities behave in a given snowpack.

Polar ice cores and historical records evidence a large-magnitude volcanic eruption in 1831 CE. This event was estimated to have injected similar to 13 Tg of sulfur (S) into the stratosphere which produced various atmospheric optical phenomena and led to Northern Hemisphere climate cooling of similar to 1 degrees C. The source of this volcanic event remains enigmatic, though one hypothesis has linked it to a modest phreatomagmatic eruption of Ferdinandea in the Strait of Sicily, which may have emitted additional S through magma-crust interactions with evaporite rocks. Here, we undertake a high-resolution multiproxy geochemical analysis of ice-core archives spanning the 1831 CE volcanic event. S isotopes confirm a major Northern Hemisphere stratospheric eruption but, importantly, rule out significant contributions from external evaporite S. In multiple ice cores, we identify cryptotephra layers of low K andesite-dacite glass shards occurring in summer 1831 CE and immediately prior to the stratospheric S fallout. This tephra matches the chemistry of the youngest Plinian eruption of Zavaritskii, a remote nested caldera on Simushir Island (Kurils). Radiocarbon ages confirm a recent (<300 y) eruption of Zavaritskii, and erupted volume estimates are consistent with a magnitude 5 to 6 event. The reconstructed radiative forcing of Zavaritskii (-2 +/- 1 W m(-2)) is comparable to the 1991 CE Pinatubo eruption and can readily account for the climate cooling in 1831-1833 CE. These data provide compelling evidence that Zavaritskii was the source of the 1831 CE mystery eruption and solve a confounding case of multiple closely spaced observed and unobserved volcanic eruptions.

Black carbon from biomass and fossil fuel burning is an important aerosol in the climate system. Understanding its historical variation is crucial to constrain current anthropogenic and wildfire impacts on the atmosphere. Patagonia was proposed previously as a major source region for the late 13th century black carbon increase observed in an ice core from the northern Antarctic Peninsula but not in continental Antarctic ice cores. Here, we reconstruct regional black carbon trends using high-resolution measurements of refractory black carbon (rBC) in two Patagonian lake-sediment cores spanning the last two millennia and compare the results with other records of fire activity in the region. Our new rBC reconstruction, which is consistent with macroscopic charcoal data from the same sites as well as regional charcoal data, indicates low fire activity in this region of Patagonia over the past 2000 years, with no major, long-lasting and systematic increase from the 13th century onwards that goes significantly beyond values detected earlier in these records. The consistently low rBC deposition at these sites suggests that Patagonian emissions did not contribute to the observed late 13th century rBC increases in ice cores from the Antarctic Peninsula. Moreover, the low amounts of rBC deposition throughout the Industrial Period suggests that Patagonian rBC records primarily reflect emissions from regional biomass burning and not fossil fuel combustion.

Sulfur dioxide (SO2) is an air pollutant which can have harmful effects on both human health and the environment. Furthermore, SO2 also contributes to climate change — SO2 emissions form sulfate aerosols that act as cloud condensation nuclei, increasing cloud formation and decreasing solar radiation reaching the surface. An accurate knowledge of past SO2 emissions is therefore essential to quantify and model the associated global climate forcing. Current bottom-up SO2 emission inventories used for historical Earth System Modeling (ESM) are poorly constrained by observations prior to the late 20th century. Here we revisit and evaluate the historical SO2 emission inventories of the last 150 years used in the Coupled Model Intercomparison Project Phase 6 (CMIP6). Our emission reconstruction is based on an inversion technique employing an array of ice core records of deposited sulfur and atmospheric transport/deposition modeling. The inversion technique minimizes discrepancies between the spatial-temporal patterns of emission inventories and the observed deposition at the ice core sites. We find substantial differences between reconstructed SO2 emissions and existing bottom-up inventories which do not fully capture the spatial-temporal emission patterns. Our results imply that changes to existing historical emission inventories might be necessary in order to ensure an accurate modeling of the Earth’s climate sensitivity within future ESM simulations.

Climate-driven changes in high-elevation forest distribution and reductions in snow and ice cover have major implications for ecosystems and global water security. In the Greater Yellowstone Ecosystem of the Rocky Mountains (United States), recent melting of a high- elevation (3,091 m asl) ice patch exposed a mature stand of whitebark pine (Pious albicaulis) trees, located similar to 180 m in elevation above modern treeline, that date to the develop tree-ring- based temperature estimates for the upper-elevation climate conditions that resulted in ancient forest establishment and growth and the subsequent regional ice- patch growth and downslope shift of treeline. Results suggest that mid-Holocene forest establishment and growth occurred under warm-season (May-Oct) mean temperatures of 6.2 degrees C (+/- 0.2 degrees C), until a multicentury cooling anomaly suppressed temperatures below 5.8 degrees C, resulting in stand mortality by c. 5,440 y BP. Transient climate model simulations indicate that regional cooling was driven by changes in summer insolation and Northern Hemisphere volcanism. The initial cooling event was followed centuries later (c. 5,100 y BP) by sustained Icelandic volcanic eruptions that forced a centennial-scale 1.0 degrees C summer cooling anomaly and led to rapid ice-patch growth and preservation of the trees. With recent warming (c. 2000-2020 CE), warm-season temperatures now equal and will soon exceed those of the mid-Holocene period of high treeline. It is likely that perennial ice cover will again disappear from the region, and treeline may expand upslope so long as plant-available moisture and disturbance are not limiting.

Ancient texts and archaeological evidence indicate substantial lead exposure during antiquity that potentially impacted human health. Although lead exposure routes were many and included the use of glazed tablewares, paints, cosmetics, and even intentional ingestion, the most significant for the nonelite, rural majority of the population may have been through background air pollution from mining and smelting of silver and lead ores that underpinned the Roman economy. Here, we determined potential health effects of this air pollution using Arctic ice core measurements of Roman-era lead pollution, atmospheric modeling, and modern epidemiology-based relationships between air concentrations, blood lead levels (BLLs), and cognitive decline. Findings suggest air lead concentrations exceeded 150 ng/m 3 near metallurgical emission sources, with average enhancements of >1.0 ng/m 3 over Europe during the Pax Romana apogee of the Roman Empire. The result was blood lead enhancements in young children of about 2.4 µg/dl above an estimated Neolithic background of 1.0 µg/dl, leading to widespread cognitive decline including a 2.5-to-3 point reduction in intelligence quotient throughout the Roman Empire.

Volcanic supereruptions are considered among the few drivers of global and existential catastrophes, with recent hypotheses suggesting massive volcanic stratospheric sulfate injection could instigate major shifts in global climate. The absence of supereruptions during recent history as well as large uncertainties on eruption ages limits understanding of the climatic risk they impose. Polar ice cores have well-resolved continuous age models, record past temperature, and contain volcanic sulfate and cryptotephra deposits which can be geochemically fingerprinted to determine eruption timing and improve stratospheric sulfur loading estimates. Here, we provide an age of 79,500 years for the Atitl & aacute;n Los Chocoyos supereruption, one of the largest Quaternary eruptions, by identifying tephra shards in ice cores from both Greenland and Antarctica. This ice core age is supported by a revised marine sediment core stratigraphy age for the Los Chocoyos ash layer. Through comparison with well-dated ice-core temperature proxy records, our study suggests that despite being one of the largest sulfur emissions recorded in ice cores, the Los Chocoyos supereruption did not trigger a millennial-scale cold period.

Anthropogenic nitrogen oxide (NOx = NO + NO2) emissions have increased since the Industrial Revolution as a result of fossil fuel burning, contributing to increasing atmospheric acidity and changes to the oxidative capacity of the atmosphere. Oxidation of NOx leads to the formation of atmospheric nitrate both in the gas phase (HNO3(g)) and aerosol phase (p-NO3–), which may then be removed from the atmosphere via wet and dry deposition. Ice core records of nitrate may thus be used to infer past changes in atmospheric NOx concentrations and atmospheric acidity given high enough accumulation rates to prevent substantial post-depositional photolytic loss from the snowpack. Increasing trends innitrate concentrations over the 20th century have been observed in ice core records throughout the Northern Hemisphere including Greenland and the North Pacific. However, two ice cores (1980 NW Col and 2002 PR Col ice cores) retrieved from the summit plateau (5,334 m a.s.l.) of Mt. Logan, the second tallest mountain in North America located in the glaciated region of the St. Elias Mountains in southwest Yukon, revealed no long-term trend in acid chemistry. This lack of sensitivity to increasing atmospheric acidity was largely attributed to the high elevation of the site within the free troposphere and the efficient scrubbing of atmospheric pollutants during transit across the Pacific. Here, we present a nitrate record from the new 2022 Mt. Logan ice core since 1912 CE (~256 m depth). Reconstructed accumulation at the site is extremely high with an average rate of 2.97 m weq a-1 from 1912 to 2020, implying excellent preservation of volatile species coupled with low average temperatures (-26.9°C). The nitrate record suggests a statistically significant (p < 0.01) increasing trend since 1912 CE, in contrast to both the NW Col and PR Col records. The record agrees with other Northern Hemisphere ice core nitrate records including Summit (Greenland; r = 0.49, p < 0.01, 1912–2006), Begguya (Alaska; r = 0.44, p < 0.01, 1912–2012), and Eclipse (Yukon; r = 0.30, p < 0.01, 1912–2001). These results indicate that the highest elevation regions of the North Pacific, such as Mt. Logan, are indeed sensitive to anthropogenic NOx emissions, with ice cores providing rare insight into mid-tropospheric acid chemistry where preservation is adequate.

Abstract Rapid warming and human exploitation threaten boreal forests. Understanding links among vegetation, climate, and people in this vast biome requires highly resolved long‐term records that integrate regional inputs. We developed an 850‐year pollen‐based record of supraregional vegetation change using a southern Greenland ice core and atmospheric modeling that identified the boreal and mixed‐conifer forests of eastern Canada as the dominant pollen source regions. Conifer pollen increased ∼1400 CE at the onset of the cooler and drier Little Ice Age. A subsequent decline began ∼1650 CE and a statistically significant pollen change after 1760 CE suggests ecological consequences of the Little Ice Age cooling and initial human exploitation that persisted until recent decades. These supraregional changes are broadly consistent with local records and demonstrate intensification of human impacts on northern forests, suggesting a shift from a climate‐modulated to an increasingly human‐controlled system during recent centuries.

The Eldgj & aacute; eruption is the largest basalt lava flood of the Common Era. It has been linked to a major ice-core sulfur (S) spike in 939-940 CE and Northern Hemisphere summer cooling in 940 CE. Despite its magnitude and potential climate impacts, uncertainties remain concerning the eruption timeline, atmospheric dispersal of emitted volatiles, and coincident volcanism in Iceland and elsewhere. Here, we present a comprehensive analysis of Greenland ice-cores from 936 to 943 CE, revealing a complex volatile record and cryptotephra with numerous geochemical populations. Transitional alkali basalt tephra matching Eldgj & aacute; are found in 939-940 CE, while tholeiitic basalt shards present in 936/937 CE and 940/941 CE are compatible with contemporaneous Icelandic eruptions from Gr & iacute;msv & ouml;tn and B & aacute;r & eth;arbunga-Vei & eth;iv & ouml;tn systems (including V-Sv tephra). We also find four silicic tephra populations, one of which we link to the Jala Pumice of Ceboruco (Mexico) at 941 +/- 1 CE. Triple S isotopes, Delta 33S, spanning 936-940 CE are indicative of upper tropospheric/lower stratospheric transport of aerosol sourced from the Icelandic fissure eruptions. However, anomalous Delta 33S (down to -0.4 parts per thousand) in 940-941 CE evidence stratospheric aerosol transport consistent with summer surface cooling revealed by tree-ring reconstructions. Tephra associated with the anomalous Delta 33S have a variety of compositions, complicating the attribution of climate cooling to Eldgj & aacute; alone. Nevertheless, our study confirms a major S emission from Eldgj & aacute; in 939-940 CE and implicates Eldgj & aacute; and a cluster of eruptions as triggers of summer cooling, severe winters, and privations in similar to 940 CE. The eruption of Eldgj & aacute; in the tenth century is the largest lava flood in the history of Iceland. Although Eldgj & aacute; emitted immense volumes of ash, lava, and gas, the exact timing and duration of this eruption, as well as its environmental and climatic impact remain unclear. Here, we provide a comprehensive chemical analysis of Greenland ice-core records spanning the period 936-943 CE. We identify volcanic ash from at least three different Icelandic eruptions and confirm that there was a major ash and gas emission from Eldgj & aacute; in 939 CE. Using tree ring temperature estimates we find strong evidence for Northern Hemisphere climate cooling in the summer of 940 CE. However, the variety of volcanic ash identified in the ice-cores shows that several Icelandic and Northern Hemisphere arc volcanoes were also erupting in this period. While Eldgj & aacute; remains the prime candidate, these additional eruptions complicate the attribution of reported climate and societal changes to Eldgj & aacute; alone. Ultimately, our study sheds new light on a cluster of volcanic eruptions between 936 and 943 CE and highlights the challenges of disentangling the individual contributions of multiple eruptions on the environment and climate. New analyses of Greenland ice-core records of volcanism between 936 and 943 CE Icelandic eruptions from Gr & iacute;msv & ouml;tn and B & aacute;r & eth;arbunga-Vei & eth;iv & ouml;tn detected between 936 and 941 CE, and major Eldgj & aacute; emission in 939-940 CE Various silicic eruptions identified, including the Jala Pumice (Mexico), providing new and valuable trans-continental tephra isochrons

Estimating fire emissions prior to the satellite era is challenging because observations are limited, leading to large uncertainties in the calculated aerosol climate forcing following the preindustrial era. This challenge further limits the ability of climate models to accurately project future climate change. Here, we reconstruct a gridded dataset of global biomass burning emissions from 1750 to 2010 using inverse analysis that leveraged a global array of 31 ice core records of black carbon deposition fluxes, two different historical emission inventories as a priori estimates, and emission-deposition sensitivities simulated by the atmospheric chemical transport model GEOS-Chem. The reconstructed emissions exhibit greater temporal variabilities which are more consistent with paleoclimate proxies. Our ice core constrained emissions reduced the uncertainties in simulated cloud condensation nuclei and aerosol radiative forcing associated with the discrepancy in preindustrial biomass burning emissions. The derived emissions can also be used in studies of ocean and terrestrial biogeochemistry. Two new gridded, model-ready historical biomass burning emission datasets (BB4CMIPpost and LPJ-LMfirepost) are developed by inverse modeling that leveraged 31 ice core records, existing emissions as a priori, and chemical transport model simulations.

Large volcanic eruptions are key time markers in paleoclimatology because they inject large quantities of volcanic fallout (such as sulfuric acids and tephra) into the atmosphere which is then widely distributed and deposited in environmental archives such as ice cores, lakes and peat bogs. They also produce strong climate effects, imprinted in climate archives such as tree-rings. The caldera-forming eruption of Mount Mazama (Crater Lake, Oregon, USA) some 7700 years ago ranks among the largest eruptions of the Holocene but little is known about its exact timing and global-scale climate impacts. Here we use new high-resolution ice-core analyses of volatiles (S, Cl), particle-size distribution, crypto-tephra and sulfur isotopes (33S, 34S), from ice cores in Greenland and Antarctica, to constrain the date, stratospheric sulfur injection, global aerosol distribution and climate forcing of this eruption. We further demonstrate that the climatic effects left distinctive fingerprints in ultra-long tree-ring chronologies from North America and Europe allowing the date of this eruption to be pinned to a specific year, thereby aligning climate proxy records in North America, Greenland and Europe on a common timeline. Using an ensemble of fully-coupled Earth System Model simulations we identify some key regions experiencing large anomalies in temperature and hydro-climate following the Mt. Mazama eruption. These extreme conditions were not only relevant for hunter-gatherer communities and early agricultural societies emerging in Eurasia, that experienced these compounding effects, but they also help us in identifying a global existential risk arising from comparable eruptions in the future.

Growing season temperatures play a crucial role in controlling treeline elevation at regional to global scales. However, understanding of treeline dynamics in response to long-term changes in temperature is limited. In this study, we analyze pollen, plant macrofossils, and charcoal preserved in organic layers within a 10,400-year-old ice patch and in sediment from a 6000-year-old wetland located above present-day treeline in the Beartooth Mountains, Wyoming, to explore the relationship between Holocene climate variability and shifts in treeline elevation. Pollen data indicate a lower-than-present treeline between 9000 and 6200 cal yr BP during the warm, dry summer and cold winter conditions of the early Holocene. Increases in arboreal pollen at 6200 cal yr BP suggest an upslope treeline expansion when summers became cooler and wetter. A possible hiatus in the wetland record at ca. 4200–3000 cal yr BP suggests increased snow and ice cover at high elevations and a lowering of treeline. Treeline position continued to fluctuate with growing season warming and cooling during the late-Holocene. Periods of high fire activity correspond with times of increased woody cover at high elevations. The two records indicate that climate was an important driver of vegetation and treeline change during the Holocene. Early Holocene treeline was governed by moisture limitations, whereas late-Holocene treeline was sensitive to increases in growing season temperatures. Climate projections for the region suggest warmer temperatures could decrease effective growing season moisture at high elevations resulting in a reduction of treeline elevation.

Stratospheric sulfate aerosols from explosive volcanic eruptions reflect incoming solar radiation and cool the planet, leading to the hypothesis that the largest volcanic events triggered millennial-scale cold periods over the last ice age. Here, we identify tephra shards from the Atitlán Los Chocoyos supereruption (LCY), one of the largest Quaternary eruptions, in ice cores from Greenland and Antarctica (dated at 79.5 ± 1.7 ka), and a marine sediment core (linked to a sea level highstand at 80.5 ± 0.9 ka). The large ice core sulfate peak associated with the tephra results in an estimated stratospheric sulfur injection of 226 ± 48 Tg S (1σ) for LCY, consistent with volcanic-induced cooling on a multi-annual scale. However, the well-constrained timing of LCY within the high-resolution temperature proxy records of ice cores proves it was not a trigger of millennial-scale cooling.

Ice cores are powerful archives for reconstructing volcanism and developing tephrochronological frameworks, as they can preserve both the soluble, i.e. aerosols, and non-soluble, i.e. tephra, products of volcanic eruptions. In addition, and particularly over Holocene timescales, high-precision annually resolved chronologies have been developed for these records and permit ages to be assigned to eruptions. The identification of tephra in ice cores in direct association with chemical indicators of volcanism, such as sulphate, can significantly enhance volcanic reconstructions as tephra can be linked to an eruptive source. Such source attributions can provide information on the location of the eruptions, the magnitude of aerosol emissions at the source and help assess any climatic impact. In addition, they can aid the reconstruction of volcanic histories and the assessment of future hazard risk. The tephra record for the interior of East Antarctica over the last 5,500 years is potentially underexploited as a prior focus on visible horizons and exploring the deep ice cores that cover longer time spans has resulted in only one horizon, dated to ~3.5 ka BP, being identified in these records. Here we discuss ongoing tephrochronological investigations of two ice-cores, B53 and B54, retrieved from the interior of the East Antarctic Plateau. High-resolution, sub-annual chemical records have been measured from both cores using a continuous melter system. These data were used to develop a sampling strategy to identify cryptotephra horizons with ice-core sections containing coeval peaks in fine insoluble particles and non-sea-salt sulphur targeted and >50 events were directly sampled. This approach recently has been used to identify cryptotephras in both Greenland and Antarctic ice cores. When glass tephra shards were identified thin sections were created and individual glass shards were geochemically analysed using electron-probe microanalysis to help identify their volcanic source and permit correlations between records. Thus far, more than 10 cryptotephra horizons have been identified and linked to regional sources such as the South Sandwich and South Shetland Islands and the ~3.5 ka BP event has been traced in both cores as a visible layer. More detailed investigations are being conducted on samples from specific volcanic signals of interest that may derive from eruptions of ultra-distal volcanic sources. Such eruptions could have deposited very small glass tephra shards over Antarctica, which poses significant analytical challenges and necessitates the use of innovative approaches for tephra identification and geochemical analysis.

Abstract Tropospheric reactive bromine (Bry) influences the oxidation capacity of the atmosphere by acting as a sink for ozone and nitrogen oxides. Aerosol acidity plays a crucial role in Bry abundances through acid‐catalyzed debromination from sea‐salt‐aerosol, the largest global source. Bromine concentrations in a Russian Arctic ice‐core, Akademii Nauk, show a 3.5‐fold increase from pre‐industrial (PI) to the 1970s (peak acidity, PA), and decreased by half to 1999 (present day, PD). Ice‐core acidity mirrors this trend, showing robust correlation with bromine, especially after 1940 (r = 0.9). Model simulations considering anthropogenic emission changes alone show that atmospheric acidity is the main driver of Bry changes, consistent with the observed relationship between acidity and bromine. The influence of atmospheric acidity on Bry should be considered in interpretation of ice‐core bromine trends.

Bromine in ice cores has been proposed as a qualitative sea ice proxy to produce sea ice reconstructions for the polar regions. Here we report the first statistical validation of this proxy with satellite sea ice observations by combining bromine enrichment (with respect to seawater, Brenr) records from three Greenlandic ice cores (SIGMA-A, NU and RECAP) with satellite sea ice imagery, over three decades. We find that during the 1984-2016 satellite-era, ice core Brenr values are significantly correlated with first-year sea ice formed in the Baffin Bay and Labrador Sea supporting that the gas-phase bromine enrichment processes, preferentially occurring over the sea ice surface, are the main driver for the Brenr signal in ice cores. Moreover, in assessing Brenr's capability to record historical sea ice variability, we compare 20th-century Arctic Sea ice historical and proxy records with our reconstructions, based on an autoregressive-moving-average (ARMA) model, finding overall good agreement. While further enhancements are warranted, including site-specific calibrations and a comprehensive investigation into bromine transport-related concerns, this study presents a new method to quantitatively reconstruct past seasonal sea ice variability through bromine enrichment in ice cores.