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Abstract Tropospheric reactive halogens influence the oxidizing capacity of the atmosphere. Although previous studies have shown that iodine exerts the strongest impact on tropospheric ozone concentrations compared to chlorine and bromine, the impact of aerosol iodide recycling back to the gas phase on oxidants has not been estimated prior to this work. Here, we explicitly represent aerosol iodine speciation in a chemical transport model, including soluble organic iodine (SOI), iodate, and iodide, and allow for interconversion among these species. We find that aerosol iodine speciation, interconversion, and recycling substantially affect modeled oxidant abundances. Across model sensitivity studies, tropospheric O_x(the odd‐oxygen family for ozone) and O_z(the odd‐oxygen family for the HO_xfamily, including OH) burdens vary by up to 10% and 5%, respectively. These ranges arise from uncertainties in heterogeneous iodine chemistry, demonstrating a need for future laboratory experiments and field observations in this area. 

Abstract The Southern Ocean has emerged as a key region for constraining aerosol‐climate interactions due to its relatively low anthropogenic influence. Sulfate is an important aerosol over the Southern Ocean, and models suggest dimethyl sulfide (DMS) is the largest source of sulfate during summer. However, sulfur isotopes of sulfate (δ³⁴S(SO₄²⁻)) in Antarctic ice cores suggest a significant contribution from a previously unexplained non‐DMS source. Here we show that the fractional contribution from passive volcanic degassing (f_volc) explains observed δ³⁴S(SO₄²⁻), and that a global chemical transport model underestimatesf_volcacross Antarctica. Underestimatedf_volcimplies that the model mischaracterizes sulfate sources in this important region. The discrepancy between observed and modeled sulfate sources can be reconciled by increasing passive volcanic sulfur degassing emissions and decreasing DMS emissions. Our results imply that current biases in emissions inventories could bias assessments of aerosol‐cloud interactions in the Southern Ocean region and globally. 

Abstract Ice core nitrate can serve as a valuable tracer for past atmospheric nitrogen oxide (NO_x) and oxidant concentrations. However, photolysis‐driven postdepositional processing can alter ice core nitrate signals and complicate their interpretation. We present a new nitrogen isotope (δ¹⁵N) record of nitrate measured in the West Antarctic Ice Sheet (WAIS) Divide ice core covering the last glacial period and Holocene. The glacialδ¹⁵N(NO₃⁻) is substantially higher than the Holoceneδ¹⁵N(NO₃⁻), with a glacial‐interglacial difference of (26.6 ± 5.7) ‰. All samples exhibit a strong negative correlation betweenδ¹⁵N(NO₃⁻) and the snow accumulation rate, suggesting that postdepositional processing is the primary driver of theδ¹⁵N(NO₃⁻) variability. Photochemical model calculations indicate that changes in the degree of postdepositional processing can fully explain the observed glacial‐interglacialδ¹⁵N(NO₃⁻) difference, with 12.7% and 31.8% of nitrate mass loss during the Holocene and glacial climate, respectively. Comparison with the Greenland GISP2δ¹⁵N(NO₃⁻) record indicates that the glacial level of postdepositional modification was higher at WAIS Divide despite its snow accumulation rate was twice that at Summit, Greenland. This is due to the higher light‐absorbing impurity contents in the GISP2 ice core that reduced postdepositional losses. We also assess the ability of usingδ¹⁵N(NO₃⁻) for past surface mass balance (SMB) reconstruction at WAIS Divide. We find this proxy is reliable for regions/periods with snow accumulation rates lower than 150 kg m⁻²a⁻¹. While exceeding this threshold, the assumption of a constantδ¹⁵N of initially deposited nitrate is no longer valid, leading to significant bias in SMB reconstruction such as WAIS Divide. 

Abstract Tropospheric reactive bromine (Br_y) influences the oxidation capacity of the atmosphere by acting as a sink for ozone and nitrogen oxides. Aerosol acidity plays a crucial role in Br_yabundances 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 Br_ychanges, consistent with the observed relationship between acidity and bromine. The influence of atmospheric acidity on Br_yshould be considered in interpretation of ice‐core bromine trends. 

Abstract Snowpack emissions are recognized as an important source of gas‐phase reactive bromine in the Arctic and are necessary to explain ozone depletion events in spring caused by the catalytic destruction of ozone by halogen radicals. Quantifying bromine emissions from snowpack is essential for interpretation of ice‐core bromine. We present ice‐core bromine records since the pre‐industrial (1750 CE) from six Arctic locations and examine potential post‐depositional loss of snowpack bromine using a global chemical transport model. Trend analysis of the ice‐core records shows that only the high‐latitude coastal Akademii Nauk (AN) ice core from the Russian Arctic preserves significant trends since pre‐industrial times that are consistent with trends in sea ice extent and anthropogenic emissions from source regions. Model simulations suggest that recycling of reactive bromine on the snow skin layer (top 1 mm) results in 9–17% loss of deposited bromine across all six ice‐core locations. Reactive bromine production from below the snow skin layer and within the snow photic zone is potentially more important, but the magnitude of this source is uncertain. Model simulations suggest that the AN core is most likely to preserve an atmospheric signal compared to five Greenland ice cores due to its high latitude location combined with a relatively high snow accumulation rate. Understanding the sources and amount of photochemically reactive snow bromide in the snow photic zone throughout the sunlit period in the high Arctic is essential for interpreting ice‐core bromine, and warrants further lab studies and field observations at inland locations. 

The Arctic climate is sensitive to aerosol abundance, making accurate historical records of atmospheric aerosols essential for understanding the recent rapid Arctic warming. We present 235-year ice core records of sulfate (SO42-), nitrate (NO3-), chloride (Cl-) and other impurities from central Greenland, and compare them with emission inventories in the surrounding continents. The comparisons reveal a high correlation between the ice core records and North American emissions, consistent with that North America as the dominant pollution source region to Greenland. Since around 1970, SO42-, NO3- and Cl- exhibit divergent patterns relative to their pre-1970 increasing trends paralleling with anthropogenic emissions. In particular, SO42- declined in step with North American SO2 emissions but decreased more rapidly after ~1990, probably due to combined effects from the enhanced sulfate loss in the source regions due to intensified in-cloud sulfur oxidation and the reduced transport efficiency. Nitrate tracked NOX emissions since ~ 1900 until 1990, but remained high after 1990 when anthropogenic emissions in all source regions decreased. This post-1990 pattern may arise from the increasing natural NOX emissions in the Arctic, while feedbacks of atmospheric chemistry to a changing atmospheric acidity during this period may also contribute through affecting the phase partitioning and then long-range transport of nitrate. Chloride also paralleled with anthropogenic emissions since the 1960s but was further modulated by acid displacement processes, as indicated by the covariation of Cl- excess with reconstructed snow acidity. Our results demonstrate that Arctic aerosols reflect both emission controls and their modulation by atmospheric chemistry and transport. 

Abstract. Tropospheric reactive iodine influences the oxidizing capacity of the atmosphere and serves as an important source of ultra-fine particles. However, the paucity of observations of gas-phase and aerosol iodine, combined with incomplete understanding and representation of iodine chemistry in models, leads to substantial uncertainties in understanding iodine abundance, speciation, and impacts. Motivated by known gaps in previous modeling studies, we introduced speciated aerosol iodine and aerosol iodide recycling to the global chemical transport model, GEOS-Chem. Modeled aerosol iodine is speciated into fine and coarse mode soluble organic iodine (SOI), iodate, and iodide. Aerosol iodide is recycled into the gas phase via heterogeneous chemistry involving halogen nitrates and hypohalous acids to form I2, ICl, and IBr, which represents an additional source of gas-phase iodine to the atmosphere. Iodide dehalogenation doubles the tropospheric burden of reactive iodine (Iy) while reducing model-measurement bias for IO and aerosol iodine. The rate of aerosol iodine conversion to Iy is more than twice as fast as the combined rates of inorganic ocean emissions and the photolysis of organic iodine gases, suggesting that aerosols are important in mediating the abundance and lifetime of tropospheric Iy. The incorporation of SOI and iodate into the model prevents iodide dehalogenation by partitioning iodide into less reactive reservoirs, which has a stabilizing effect for reactive iodine chemistry. These findings have implications for reactive halogen abundances and global oxidant budgets in the troposphere. 

Abstract. Dimethyl sulfide (DMS) is primarily emitted by marine phytoplankton and oxidized in the atmosphere to form methanesulfonic acid (MSA) and sulfate aerosols. Ice cores in regions affected by anthropogenic pollution show an industrial-era decline in MSA, which has previously been interpreted as indicating a decline in phytoplankton abundance. However, a simultaneous increase in DMS-derived sulfate (bioSO4) in a Greenland ice core suggests that pollution-driven oxidant changes caused the decline in MSA by influencing the relative production of MSA versus bioSO4. Here we use GEOS-Chem, a global chemical transport model, and a zero-dimensional box model over three time periods (preindustrial era, peak North Atlantic NOx pollution, and 21st century) to investigate the chemical drivers of industrial-era changes in MSA and bioSO4, and we examine whether four DMS oxidation mechanisms reproduce trends and seasonality in observations. We find that box model and GEOS-Chem simulations can only partially reproduce ice core trends in MSA and bioSO4 and that wide variation in model results reflects sensitivity to DMS oxidation mechanism and oxidant concentrations. Our simulations support the hypothesized increase in DMS oxidation by the nitrate radical over the industrial era, which increases bioSO4 production, but competing factors such as oxidation by BrO result in increased MSA production in some simulations, which is inconsistent with observations. To improve understanding of DMS oxidation, future work should investigate aqueous-phase chemistry, which produces 82 %–99 % of MSA and bioSO4 in our simulations, and constrain atmospheric oxidant concentrations, including the nitrate radical, hydroxyl radical, and reactive halogens. 

An industrial-era drop in Greenland ice core methanesulfonic acid (MSA) is thought to herald a collapse in North Atlantic marine phytoplankton stocks related to a weakening of the Atlantic Meridional Overturning Circulation. In contrast, stable levels of marine biogenic sulfur production contradict this interpretation and point to changes in atmospheric oxidation as a potential cause of the MSA decline. However, the impact of oxidation on MSA production has not been quantified, nor has this hypothesis been rigorously tested. Here we present a multi-century MSA record from the Denali, Alaska, ice core, which shows an MSA decline similar in magnitude but delayed by 93 years relative to the Greenland record. Box model results using updated chemical pathways indicate that oxidation by industrial nitrate radicals has suppressed atmospheric MSA production, explaining most of Denali’s and Greenland’s MSA declines without requiring a change in phytoplankton production. The delayed timing of the North Pacific MSA decline, relative to the North Atlantic, reflects the distinct history of industrialization in upwind regions and is consistent with the Denali and Greenland ice core nitrate records. These results demonstrate that multi-decadal trends in industrial-era Arctic ice core MSA reflect rising anthropogenic pollution rather than declining marine primary production.