The Marginal Ice Zone as a dominant source region of atmospheric mercury during central Arctic summertime

Mercury (Hg) is a highly toxic heavy metal, posing a global pollution threat due to its long-range atmospheric transport. Atmospheric Hg exists in three forms: gaseous elemental mercury (GEM or Hg(0)), and gaseous or particulate divalent Hg (Hg(II) and Hg(p)). GEM, comprising >90% of atmospheric Hg, has a long atmospheric residence time (0.5–1 year) due to its high volatility and low reactivity. Divalent Hg has a shorter lifetime and is removed via deposition. The Arctic is a critical component of the Northern Hemisphere Hg cycle and a sensitive area for Hg exposure. High Hg levels in Arctic populations stem from their traditional diets and long food chains. Long-range atmospheric transport, riverine input, and coastal erosion are primary Hg sources to the Arctic. A fraction of these emissions can accumulate in seawater, plants, and ice, later being re-emitted. Atmospheric Hg transport, transformation, and deposition significantly influence the Arctic Hg cycle. Since the discovery of springtime atmospheric Hg depletion events (AMDEs), numerous studies have investigated Arctic atmospheric Hg at coastal stations, revealing a springtime minimum (driven by AMDEs) followed by a summertime GEM maximum exceeding hemispheric background levels. The origin of this summertime maximum remains a key research question. Previous studies proposed long-range transport from Asia, oceanic Hg evasion, and cryospheric re-emissions as potential sources. However, the relative contributions of these sources are still uncertain, particularly in the central Arctic Ocean where data has been limited. This study uses data from the MOSAIC expedition to address this gap, focusing on GEM concentrations and identifying their sources in the central Arctic Ocean.

Several studies have attempted to explain the summertime maximum in Arctic atmospheric mercury. Some attributed it to long-range transport of Asian air masses, highlighting the importance of anthropogenic emissions from lower latitudes. Other research suggested that enhanced summertime GEM concentrations could be linked to oceanic evasion from the Arctic Ocean, possibly fueled by terrestrial inputs such as rivers and coastal erosion. This hypothesis was supported by observations and modeling studies using the GEOS-Chem model. More recent isotopic work has challenged this view, suggesting that re-emissions from the Arctic cryosphere play a dominant role. However, the precise mechanism and magnitude of cryospheric re-emission remain uncertain, with studies showing varying estimates of Hg photoreduction loss from snow. This disparity underscores the complexity of factors influencing Hg emission from snowpack and sea ice, including solar radiation, Hg speciation, halide and particulate matter concentrations, temperature, snowfall, and upward latent heat flux. The existing understanding of the Arctic Hg cycle is largely based on coastal observations, potentially leading to an incomplete picture of the processes controlling the cycle over the entire Arctic Ocean. Regional differences in sea-ice properties (dynamic deformation, thermal processes) and chemical composition can influence redox reactions and snow re-emission, impacting air-sea exchange and atmospheric Hg concentrations. This study aims to address this limitation by providing a detailed analysis of in situ observations of GEM in the central Arctic.

This study analyzes summertime (June 9–September 30, 2020) GEM observations from the MOSAIC expedition in the central Arctic Ocean (78.34–90°N, 40.93°W–175.7°E). GEM concentrations were continuously measured using a Tekran™ 2537B cold vapor atomic fluorescence spectroscopic (CVAFS) Hg analyzer on the *Polarstern*. The inlet system removed moisture and particles. Regular calibrations ensured accuracy. Independent GEM measurements were also performed in the University of Colorado sea-laboratory container. Ancillary data, including dimethylsulfide (DMS), carbon monoxide (CO), and sulfur dioxide (SO2) from the MOSAIC expedition, were used. Surface ocean chlorophyll *a* (Chla) was measured daily. Meteorological data (wind speed, atmospheric pressure, air temperature, shortwave radiation) and sea-ice fraction were obtained from the ship and GEOS-FP data. Air mass back-trajectories (168 h) were generated using the HYSPLIT model. A generalized additive model (GAM) was employed to evaluate the relative importance of various factors influencing GEM variability. The GAM included parameters representing long-range transport (CO, trajectory distance), local oceanic emissions (open-water fraction), and meteorological factors (wind speed, pressure, temperature). Model validation was performed using 5-fold cross-validation. A potential source contribution function (PSCF) analysis, combined with HYSPLIT backward trajectories and GDAS meteorological data, identified potential source regions of GEM.

GEM concentrations in the central Arctic Ocean during the MOSAIC expedition ranged from 1.02 to 2.99 ng/m³, averaging 1.54 ± 0.27 ng/m³. The average GEM concentrations from June to August were similar to those observed at nearby coastal stations (Alert, Zeppelin, Villum), indicating comparable summertime GEM levels in the central Arctic and coastal regions. Monthly averaged GEM concentrations peaked in July (1.80 ± 0.32 ng/m³), followed by June, August, and September, consistent with patterns observed at coastal stations. The highest GEM concentration (2.99 ng/m³) coincided with high solar radiation but low CO and SO2 levels, suggesting a natural origin rather than anthropogenic influence. The GAM analysis revealed that oceanic evasion was the dominant source of GEM variability (>50%), while long-range transport contributed minimally (<2%). Meteorological factors (temperature, pressure, wind speed) explained another 37% of GEM variability. PSCF analysis confirmed that oceanic evasion was primarily confined to the MIZ. A positive correlation between GEM and Chla indicated a link between biological activity (phytoplankton blooms) and GEM production in the MIZ. The melting of sea ice in the MIZ facilitates air-sea gas exchange. A back-of-the-envelope calculation estimated a Hg evasion flux of 56 ng m⁻²day⁻¹ in the MIZ, exceeding fluxes in the open ocean and continental shelf areas.

This study demonstrates that oceanic evasion in the MIZ, combined with regional transport, is the primary source of atmospheric GEM in the central Arctic during summer, explaining the observed summertime maximum. The findings support the idea of a transition from a mercury sink in spring (AMDEs) to a source in summer. The MIZ's high biological activity (phytoplankton blooms) and efficient air-sea gas exchange due to ice melt appear to drive the high Hg evasion flux. The vertical stratification of the upper ocean in the central Arctic, in contrast to the well-mixed conditions of the MIZ, could account for the limited oceanic evasion north of the MIZ. The results align with previous observations at Arctic coastal stations but provide a more refined understanding by highlighting the specific role of the MIZ. The significant contribution of the MIZ to atmospheric Hg in the central Arctic necessitates a closer look at how the MIZ impacts the global mercury cycle.

This study provides strong evidence that the Arctic Marginal Ice Zone (MIZ) is a dominant source of atmospheric mercury during the summertime. Oceanic evasion in the MIZ, driven by biological activity and ice melt, is the main contributor to the observed summertime GEM maximum. This contrasts with minimal contributions from long-range transport. Given the rapid Arctic warming and expansion of the MIZ, oceanic Hg evasion is expected to increase further, strengthening the central Arctic's role as a summer Hg source. Future research should focus on more precise quantification of Hg evasion fluxes in the MIZ and improved understanding of the interplay between biological processes, sea-ice dynamics, and Hg cycling.

While this study provides valuable insights, some limitations exist. The back-of-the-envelope calculation of Hg evasion flux in the MIZ is a simplified estimate; more comprehensive flux measurements are needed. The analysis is based on a single year's data from the MOSAIC expedition. Longer-term observations are required to fully assess the interannual variability and long-term trends. The GAM model relies on selected predictor variables, and other factors might influence GEM concentrations. Future studies incorporating more comprehensive datasets and advanced statistical methods will enhance our understanding.