Mercury (Hg) atmospheric emissions in Europe peaked in the mid-20th century and have since declined due to environmental policies, reducing atmospheric deposition to ecosystems. However, regional disparities have emerged as climate change can remobilize Hg reservoirs, complicating efforts to assess environmental recovery. This study investigates Hg accumulation rates in lacustrine sediments from two neighboring high-altitude alpine lakes in the French Alps: Grand Lake (GDL), a rain-fed lake reflecting regional atmospheric deposition, and Eychauda Lake (EYC), a proglacial lake supplied by glacial meltwater and derived sediment. Sediment chronologies for both lakes were established using short-lived radionuclides, allowing for the reconstruction of Hg deposition over the past century. Comparison of the Hg records from both lakes revealed contrasting trends: GDL responds to regional anthropogenic emissions, while EYC shows a still increasing Hg accumulation rate over recent decades. This discrepancy and its correlation with erosion proxies suggest that glacier melting has released Hg stored in the ice, masking the expected trend of reduced regional emissions. These findings illustrate the importance of accounting for climate-mediated remobilization of Hg when evaluating the effectiveness of environmental policies as this process can impose a climate change-driven penalty that delays ecosystem recovery.

Theme 5 examines the impacts of human activities in Antarctica, including exploring human impacts such as scientific operations, tourism, shipping, local and global pollution, and krill fisheries. interact with climate change to inform environmental policy within and beyond the Antarctic Treaty System. Local pressures generate chemical and plastic pollution, black carbon, underwater noise, wildlife disturbance, invasive species, and antimicrobial resistance, affecting ecosystem resilience. The theme seeks to quantify and monitor anthropogenic pressures through standardized, internationally coordinated protocols spanning chemical, biological, ecological, and cultural dimensions during InSync. Key topics include pollutants and plastics, underwater soundscapes, tourism impacts, fishery ecosystem effects and management, environmental DNA and antimicrobial resistance, marine pollution impacts and climate feedback, and the cultural legacies of historic expeditions. Approaches combine harmonized sampling, long-term monitoring, remote sensing, in situ observations, and modelling to identify hotspots and cumulative impacts. By integrating open-access data across disciplines, Theme 5 advances predictive risk assessment, science-based mitigation, and conservation strategies to detect anthropogenic signatures and strengthen the protection of Antarctic ecosystems.

The melting rate of glaciers in Europe has accelerated continuously since the 1980s and continues today at an unprecedented pace. Such rapid and sustained glacier retreat has important implications for the release of legacy contaminants stored in ice. Since the onset of the Industrial Revolution, atmospheric deposition has led to the accumulation of pollutants, including mercury (Hg)—a highly toxic element with well-documented impacts on ecosystems and human health—within glaciers. Ongoing climate-driven melting can remobilize these long-term contaminant reservoirs.To investigate this process, we examined Hg accumulation rates (Hg AR) in sediments from two neighboring high-altitude lakes in the French Alps. One lake represents a reference system receiving only atmospheric inputs, while the other is influenced by both atmospheric deposition and meltwater from a shrinking glacier. Comparing the two sedimentary records allowed us to isolate the signal associated with cryospheric change.In the reference lake, Hg AR is controlled by regional atmospheric Hg emissions and follows the expected anthropogenic pattern, with maxima during World War II and in the 1970s, followed by a steady decline in recent decades. In contrast, the glacier-fed lake shows a steadily increasing Hg AR from the early 1900s to the present, with a doubling of accumulation rates over the past several decades.Using estimates of glacier volume loss over the last 50 years together with Hg concentrations in glacier ice and cryoconite reported in the literature, we demonstrate that the recent acceleration of Hg AR is consistent with enhanced Hg release driven by glacier shrinkage. These results indicate that glacier melt represents an additional and climate-sensitive source of legacy Hg to downstream aquatic systems, compounding the environmental impacts of cryospheric change alongside pressures such as freshwater scarcity.

Temporal Dynamics of Atmospheric Microbial Communities in the Alps: Insights from 11-Years of High-Altitude SamplingMarie Labat Saint Vincent1; Patrik Winiger2; Julian Weng2; Stephan C. Schuster3; Christoph Hueglin4; Sophie Darfeuil1; Pauline Bros-Rolere1; Patrick Ginot1; Claudia Mohr2,5; Jean-Luc Jaffrezo1; Imad El-Haddad2; Aurélien Dommergue1; Catherine Larose11: Institut des Géosciences de l’Environnement (IGE) CNRS, UGA, IRD, INRAE, Grenoble INP, 38058, Grenoble CEDEX, France2 : PSI Center for Energy and Environmental Sciences (PSI-CEES), Paul Scherrer Institute, Villigen, 5232, Switzerland3 : Singapore Centre for Environmental Life Sciences Engineering (SCELSE), Nanyang Technological University Singapore, Singapore, Singapore4 : Swiss Federal Laboratories for Materials Science and Technology (EMPA), Dübendorf, 8600, Switzerland5 : Department of Environmental Systems Science, ETH Zurich, 8092 Zürich, Switzerland Atmospheric microbial communities play a significant role in biogeochemical cycles and serve as a key source of microorganisms deposited onto glacial surfaces, where they may be preserved for millennia. Understanding the dynamics of these communities and their responses to environmental factors is critical for assessing their transfer and preservation in glacial archives. In this study, we leverage a unique dataset collected by the Swiss National Air Pollution Monitoring Network (NABEL), consisting of 11 years (2010–2021) of atmospheric particulate samples from Jungfraujoch (3500m a.s.l., Switzerland). This sampling site provides a rare opportunity to unravel atmospheric microbial community dynamics at high altitude as well as providing information on the pool of microorganisms that can potentially be deposited onto glaciers in the Alps.DNA extraction and quantitative PCR (qPCR) were performed on atmospheric filters collected every four days (>1000 samples) to quantify microbial abundance. These data allow us to investigate the temporal trends in abundance in the Alpine atmosphere over more than a decade, highlighting seasonal variations over time. Additionally, correlations with geo-physico-chemical environmental parameters, like temperature, pollution events, and atmospheric composition, were carried out to identify key factors driving these dynamics.This time series represents one of the most comprehensive temporal datasets of atmospheric microbial dynamics at high altitude available. In addition to providing a unique opportunity to characterize the drivers of microbial communities in the atmosphere over longer time scales, this data also represents an important step towards understanding the processes governing microbial deposition and preservation in Alpine ice. This work lays the foundation to the broader goal of validating ice cores as reliable archives of past atmospheric microbial diversity and environmental conditions.

Atmospheric mercury depletion events (AMDEs) are a unique phenomenon in polar mercury cycling, involving intense atmospheric mercury oxidation and deposition that amplify marine mercury sinks. The Southern Ocean, a critical hotspot for environmental mercury exposure, exhibits AMDEs whose characteristics and distribution patterns remain unclear. This study presents the observational dataset of atmospheric gaseous elemental mercury (GEM) across the circum-Antarctic Southern Ocean. We observed pronounced summertime AMDEs that drove high spatial heterogeneity in circumpolar GEM distribution. By combining Generalized Additive Models (GAM) simulations with multi-platform observational evidence, we demonstrate that the Antarctic continental outflow largely shapes the circumpolar distribution of summertime AMDEs, concentrating these events within the convergence zones of katabatic winds. This work highlights the critical role of land-sea coupling effects in Antarctic atmospheric chemistry, and underscores the need to systematically assess the implications of these processes for mercury bioavailability across Southern Ocean ecosystems. Air flowing out from the Antarctic continent largely determines where summer atmospheric mercury depletion events (AMDEs) occur around Antarctica, making them most common in areas where Antarctic cold, downslope winds meet.

Air-sea exchange of gaseous elemental mercury (Hg0) is a major component of the global mercury (Hg) biogeochemical cycle but remains poorly understood due to sparse in situ measurements. Here, we used long-term atmospheric Hg0 (Hgair0) observations combined with air mass back trajectories at four ground-based monitoring sites to study Hg0 air-sea exchange. The trajectories showed that all four sites sample mainly marine air masses. At all sites, we observed a gradual increase in mean Hgair0 concentration with air mass recent residence time in the marine boundary layer (MBL), followed by a steady state. The pattern is consistent with the thin-film gas exchange model, which predicts net Hg0 emissions from the surface ocean until the Hgair0 concentration normalised by Henry's law constant matches the surface ocean dissolved Hg0 (Hgaq0) concentration. This provides strong evidence that ocean Hg0 emissions directly influence Hgair0 concentrations at these sites. Using the observed relationship between Hgair0 concentrations and air mass recent MBL residence time, we estimated mean surface ocean Hgaq0 concentrations of 4-7 pgL-1 for the North Atlantic and Arctic oceans (AA) and 4 pgL-1 for the Southern, South Atlantic and south Indian oceans (SSI). Estimated ocean Hg0 emission fluxes ranged between 0.57-0.86 and 0.60-0.87 ngm-2h-1 for the AA and SSI, respectively, with a global extrapolated mean flux of around 2270 tyr-1 (1600-2900 tyr-1). This study demonstrates the applicability of long-term, ground-based Hgair0 observations in constraining Hg0 air-sea exchange.

In order to complement the picture of the atmospheric water cycle in the Southern Ocean, we have continuously monitored water vapor isotopes since January 2020 on Amsterdam Island in the Indian Ocean. We present here the first 2-year long water vapor isotopic record at this site. We show that the water vapor isotopic composition largely follows the water vapor mixing ratio, as expected in marine boundary layers. However, we detect 11 periods of a few days where there is a strong loss of correlation between water vapor δ18O and water vapor mixing ratio as well as abrupt negative excursions of water vapor δ18O. These excursions often occur toward the end of precipitation events. Six of these events show a decrease in gaseous elemental mercury, suggesting subsidence of air from a higher altitude. Our study aims to further explore the mechanism driving these negative excursions in water vapor δ18O. We used two different models to provide a data–model comparison over this 2-year period. While the European Centre Hamburg model (ECHAM6-wiso) at 0.9° was able to reproduce most of the sharp negative water vapor δ18O excursions, hence validating the physics process and isotopic implementation in this model, the Laboratoire de Météorologie Dynamique Zoom model (LMDZ-iso) at 2° (3°) resolution was only able to reproduce seven (one) of the negative excursions, highlighting the possible influence of the model resolution for the study of such abrupt isotopic events. Based on our detailed model–data comparison, we conclude that the most plausible explanations for such isotopic excursions are rain–vapor interactions associated with subsidence at the rear of a precipitation event.

The Global Mercury Observation System (GMOS) network, initially a five-year project (2010-2015) funded by the European Commission, continued as a GEO Flagship program to support the Global Observation System for Mercury (GOS4M). GMOS was envisioned as a coordinated global observing system to monitor atmospheric mercury (Hg) on a global scale, to support and evaluate the effective implementation of the Minamata Convention on Mercury (MCM). Twenty-eight ground-based stations have participated in monitoring activities, following GMOS sampling protocols and related data quality control management. The GMOS network provides representative coverage of all latitudes, from the Northern Hemisphere to the Southern Hemisphere including the Arctic Circle, Antarctica, and the Tropical Zone. This work presents atmospheric Hg data, available as Total Gaseous Mercury (TGM) or Gaseous Elemental Mercury (GEM) concentrations, recorded within the GMOS network from 2011 to 2020. TGM/GEM concentrations were analysed in terms of their variability along latitudinal areas, considering their comparability, temporal trends and patterns. The main results confirmed a clear gradient of TGM/GEM concentrations between the northern (1.58 +/- 0.31 ng/m(3)) and southern (0.97 +/- 0.14 ng/m(3)) hemispheres. Decreasing trends in TGM/GEM levels were found to be strongly significant only for selected remote stations with at least 5 years of data coverage. Seasonality in atmospheric TGM/GEM concentrations was observed to increase with latitude and is greater at inland sites than at coastal sites.

Terrestrial volcanism is known to emit mercury (Hg) into the atmosphere. However, despite many years of investigation, its net impact on the atmospheric Hg budget remains insufficiently constrained, in part because the transformations of Hg in volcanic plumes as they age and mix with background air are poorly understood. Here we report the observation of complete gaseous elemental mercury (GEM) depletion events in dilute and moderately aged (~3-7 hours) volcanic plumes from Piton de la Fournaise on Réunion Island. While it has been suggested that co-emitted bromine could, once photochemically activated, deplete GEM in a volcanic plume, we measured low bromine concentrations in both the gas- and particle phase and observed complete GEM depletion even before sunrise, ruling out a leading role of bromine chemistry here. Instead, we hypothesize that the GEM depletions were mainly caused by gas-particle interactions with sulfate-rich volcanic particles (mostly of submicron size), abundantly present in the dilute plume. We consider heterogeneous GEM oxidation and GEM uptake by particles as plausible manifestations of such a process and derive empirical rate constants. By extrapolation, we estimate that volcanic aerosols may scavenge 210 Mg y-1 (67 – 480 Mg y-1) of Hg from the atmosphere globally, acting effectively as atmospheric mercury sink. While this estimate is subject to large uncertainties, it highlights that Hg transformations in aging volcanic plumes must be better understood to determine the net impact of volcanism on the atmospheric Hg budget and Hg deposition pathways.

The Minamata Convention, a global and legally binding treaty that entered into force in 2017, aims to protect human health and the environment from harmful mercury (Hg) effects by reducing anthropogenic Hg emissions and environmental levels. The Conference of the Parties is to periodically evaluate the Convention’s effectiveness, starting in 2023, using existing monitoring data and observed trends. Monitoring atmospheric Hg levels has been proposed as a key indicator. However, data gaps exist, especially in the Southern Hemisphere. Here, we present over a decade of atmospheric Hg monitoring data at Amsterdam Island (37.80°S, 77.55°E), in the remote southern Indian Ocean. Datasets include gaseous elemental and oxidised Hg species ambient air concentrations from either active/continuous or passive/discrete acquisition methods, and annual total Hg wet deposition fluxes. These datasets are made available to the community to support policy-making and further scientific advancements.

Deployment times of at least two months and dedicated efforts to limit contamination during transport and storage are recommended to obtain reliable results when using passive samplers for gaseous elemental mercury in the remote atmosphere.

The recent characterization of antibiotic resistance genes (ARGs) in clouds evidenced that the atmosphere actively partakes in the global spreading of antibiotic resistance worldwide. Indeed, the outdoor atmosphere continuously receives large quantities of particles of biological origins, emitted from both anthropogenic or natural sources at the near Earth's surface. Nonetheless, our understanding of the composition of the atmospheric resistome, especially at mid-altitude (i.e. above 1000 m a.s.l.), remains largely limited. The atmosphere is vast and highly dynamic, so that the diversity and abundance of ARGs are expected to fluctuate both spatially and temporally. In this work, the abundance and diversity of ARGs were assessed in atmospheric aerosol samples collected weekly between July 2016 and August 2017 at the mountain site of puy de Dôme (1465 m a.s.l., central France). Our results evidence the presence of 33 different subtypes of ARGs in atmospheric aerosols, out of 34 assessed, whose total concentration fluctuated seasonally from 59 to 1.1 × 105 copies m-3 of air. These were heavily dominated by genes from the quinolone resistance family, notably the qepA gene encoding efflux pump mechanisms, which represented >95 % of total ARGs concentration. Its abundance positively correlated with that of bacteria affiliated with the genera Kineococcus, Neorhizobium, Devosia or Massilia, ubiquitous in soils. This, along with the high abundance of Sphingomonas species, points toward a large contribution of natural sources to the airborne ARGs. Nonetheless, the increased contribution of macrolide resistance (notably the erm35 gene) during winter suggests a sporadic diffusion of ARGs from human activities. Our observations depict the atmosphere as an important vector of ARGs from terrestrial sources. Therefore, monitoring ARGs in airborne microorganisms appears necessary to fully understand the dynamics of antimicrobial resistances in the environment and mitigate the threats they may represent.

ECHAM6wiso and LMDZ6iso simulations, and python script analyzing model outputs, associated with the article : Amaelle Landais, Cécile Agosta, Françoise Vimeux, Olivier Magand, Cyrielle Solis, Alexandre Cauquoin, Niels Dutrievoz, Camille Risi, Christophe Leroy Dos Santos, Elise Fourré, Olivier Cattani, Bénédicte Minster, Frédéric Prié, Mathieu Casado, Aurélien Dommergue, Yann Bertrand, and Martin Werner (submitted to Atmospheric Chemistry and Physics, 2023) Modeling abrupt excursions in water vapor isotopic variability during cold fronts at the Pointe Benedicte observatory in Amsterdam Island. If you use the data or the python script, please cite the last version of this article available on https://www.egusphere.net/ or https://acp.copernicus.org/. Please also cite the articles related to the model simulations: Risi, C., Bony, S., Vimeux, F., and Jouzel, J.: Water-stable isotopes in the LMDZ4 general circulation model: Model evaluation for present-day and past climates and applications to climatic interpretations of tropical isotopic records, Journal of Geophysical Research Atmospheres, 115, https://doi.org/10.1029/2009JD013255, 2010. Cauquoin, A. and Werner, M.: High-Resolution Nudged Isotope Modeling With ECHAM6-Wiso: Impacts of Updated Model Physics and ERA5 Reanalysis Data, Journal of Advances in Modeling Earth Systems, 13, e2021MS002532, https://doi.org/10.1029/2021MS002532, 2021. Cauquoin, A., Werner, M., and Lohmann, G.: Water isotopes -- climate relationships for the mid-Holocene and preindustrial period simulated with an isotope-enabled version of MPI-ESM, Climate of the Past, 15, 1913–1937, https://doi.org/10.5194/cp-15-1913-2019, 2019.

Mercury (Hg) fate and transport research requires more effort to obtain a deep knowledge of its biogeochemical cycle, particularly in the Southern Hemisphere and Tropics that are still missing of distributed monitoring sites.Continuous monitoring of atmospheric Hg concentrations and trend worldwide is relevant for the effectiveness evaluation of the Minamata Convention on Mercury (MCM) actions. In this context, Gaseous Elemental Mercury (GEM) and total mercury (THg) in precipitations were monitored from 2013 to 2019 at the Amsterdam Island Observatory (AMS - 37°48′S, 77°34′E) to provide insights into the Hg pathway in the remote southern Indian Ocean, also considering ancillary dataset of Rn-222, CO2, CO, and CH4. GEM average concentration was 1.06 ± 0.07 ng m−3, with a slight increase during the austral winter due to both higher wind speed over the surface ocean and contributions from southern Africa. In wet depositions, THg average concentration was 2.39 ± 1.17 ng L−1, whereas the annual flux averaged 2.04 ± 0.80 μg m−2 year−1. In general, both GEM and Volume-Weighted Mean Concentration (VWMC) of THg did not show an increasing/decreasing trend over the seven-year period, suggesting a substantial lack of evolution about emission of Hg reaching AMS.Air masses Cluster Analysis and Potential Source Contribution Function showed that oceanic evasion was the main Hg contributor at AMS, while further contributions were attributable to long-range transport events from southern Africa, particularly when the occurrence of El Niño increased the frequency of wildfires.

Atmospheric mercury (Hg) observations in the lower free troposphere (LFT) can give important insights into Hg redox chemistry and can help constrain Hg background concentrations on a regional level. Relatively continuous sampling of LFT air, inaccessible to most ground-based stations, can be achieved at high-altitude observatories. However, such high-altitude observatories are rare, especially in the Southern Hemisphere (SH), and atmospheric Hg in the SH LFT is unconstrained. To fill this gap, we continuously measured gaseous elemental mercury (GEM; hourly) and reactive mercury (RM; integrated over ∼ 6–14 d) for 9 months at Maïdo mountain observatory (2160 m a.s.l.) on remote Réunion Island (21.1∘ S, 55.5∘ E) in the tropical Indian Ocean. GEM exhibits a marked diurnal variation characterized by a midday peak (mean: 0.95 ng m−3; SD: 0.08 ng m−3) and a nighttime low (mean: 0.78 ng m−3; SD: 0.11 ng m−3). We find that this diurnal variation is likely driven by the interplay of important GEM photo-reemission from the islands' vegetated surfaces (i.e. vegetation + soil) during daylight hours (8–22 ng m−2 h−1), boundary layer influences during the day, and predominant LFT influences at night. We estimate GEM in the LFT based on nighttime observations in particularly dry air masses and find a notable seasonal variation, with LFT GEM being lowest from December to March (mean 0.66 ng m−3; SD: 0.07 ng m−3) and highest from September to November (mean: 0.79 ng m−3; SD: 0.09 ng m−3). Such a clear GEM seasonality contrasts with the weak seasonal variation reported for the SH marine boundary layer but is in line with modeling results, highlighting the added value of continuous Hg observations in the LFT. Maïdo RM is 10.6 pg m−3 (SD: 5.9 pg m−3) on average, but RM in the cloud-free LFT might be about twice as high, as weekly–biweekly sampled RM observations are likely diluted by low-RM contributions from the boundary layer and clouds.

Near-surface mercury and ozone depletion events occur in the lowest part of the atmosphere during Arctic spring. Mercury depletion is the first step in a process that transforms long-lived elemental mercury to more reactive forms within the Arctic that are deposited to the cryosphere, ocean, and other surfaces, which can ultimately get integrated into the Arctic food web. Depletion of both mercury and ozone occur due to the presence of reactive halogen radicals that are released from snow, ice, and aerosols. In this work, we added a detailed description of the Arctic atmospheric mercury cycle to our recently published version of the Weather Research and Forecasting model coupled with Chemistry (WRF-Chem 4.3.3) that includes Arctic bromine and chlorine chemistry and activation/recycling on snow and aerosols. The major advantage of our modelling approach is the online calculation of bromine concentrations and emission/recycling that is required to simulate the hourly and daily variability of Arctic mercury depletion. We used this model to study coupling between reactive cycling of mercury, ozone, and bromine during the Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC) spring season in 2020 and evaluated results compared to land -based, ship-based, and remote sensing observations. The model predicts that elemental mercury oxidation is driven largely by bromine chemistry and that particulate mercury is the major form of oxidized mercury. The model predicts that the majority (74%) of oxidized mercury deposited to land-based snow is re-emitted to the atmosphere as gaseous elemental mercury, while a minor fraction (4%) of oxidized mercury that is deposited to sea ice is re-emitted during spring. Our work demonstrates that hourly differences in bromine/ozone chemistry in the atmosphere must be considered to capture the springtime Arctic mercury cycle, including its integration into the cryosphere and ocean.

Mercury (Hg) stable isotopes have been broadly used to investigate the sources, transformation and deposition of atmospheric Hg during the industrial era thanks to the multiple isotope signatures deriving from mass-dependent (represented by delta Hg-202) and mass-independent fractionation (represented by AxxxHg) in the environment. Less is known about the impact of past climate change on atmospheric Hg deposition and cycling, and whether Hg isotopes covary with past climate. Here, we investigate Hg concentration and Hg isotope signatures in a 6600-year-old ombrotrophic peat record from Amsterdam Island (AMS, 37.8(o)S), and in modern AMS rainfall and gaseous elemental Hg (Hg-0) samples. Results show that Holocene atmospheric Hg deposition and plant Hg uptake covary with dust deposition, and are both lower under a high humidity regime associated with enhanced Southern Westerly Winds. Modern AMS gaseous Hg-0 and rainfall HgII isotope signatures are similar to those in the Northern Hemisphere (NH). Holocene peat delta Hg-199 and A200Hg are significantly correlated (R2 = 0.67, P < 0.001, n = 58), consistently oscillating between the modern Hg-0 and rainfall Hg-II end-members. Peat A200Hg and delta Hg-199 provide evidence of plant uptake of Hg-0 as the dominant pathway of Hg deposition to AMS peatland, with some exceptions during humid periods. In contrast to NH archives generally documenting a modern increase in delta Hg-199, recent peat layers (post-1900CE) from AMS show the lowest delta Hg-199 in the peat profile (-0.42 +/- 0.27 parts per thousand, 1cs, n = 8). This likely reflects a significant change in the post-depositional process on deposited anthropogenic Hg in 20th century (i.e. dark abiotic reduction), enabling more negative delta Hg-199 to be observed in AMS peat. We further find that the oscillations of Hg isotopes are consistent with established Holocene climate variability from dust proxies. We suggest peat Hg isotope records might be a valid rainfall indicator. (C) 2022 The Authors. Published by Elsevier Ltd.

The most efficient way to quantify HgII inputs to ecosystems is to measure wet and dry deposition. Wet deposition of HgII is determined by measuring Hg concentrations and the volume of precipitation. Dry deposition of HgII is determined through direct measurement and/or determined indirectly by measuring air concentrations and using model-generated deposition velocities. Here, data collected using an Aerohead sampler holding cation exchange membranes are summarized, and the utility of this method for understanding dry deposition, and other measurements and processes is discussed. This analysis includes information from publications, and recent data collected at Guadalupe Mountains National Park, Texas, USA, and Amsterdam Island, Southern Indian Ocean. This method primarily measures gaseous HgII and little particulate-bound Hg. The Aerohead method is useful for looking at large-scale trends in deposition, verifying Hg depletion events, calculating dry deposition velocities for compounds with specific chemistry, and identification of sources of HgII. At numerous locations in the western USA, deposition rates were greater at higher elevations due to elevated concentrations associated with long-range transport of atmospheric pollution. When used in tandem with the Reactive Mercury Active System or a dual-channel system, more accurate deposition velocities - that vary as a function of GOM compound chemistry - can be calculated.