An updated review of atmospheric mercury

The atmosphere is a key component of the biogeochemical cycle of mercury, acting as a reservoir, transport mechanism, and facilitator of chemical reactions. The chemical and physical behavior of atmospheric mercury determines how, when, and where emitted mercury pollution impacts ecosystems. In this review, we provide current information about what is known and what remains uncertain regarding mercury in the atmosphere. We discuss new ambient, laboratory, and theoretical information about the chemistry of mercury in various atmospheric media. We review what is known about mercury in and on solid- and liquid-phase aerosols. We present recent findings related to wet and dry deposition and spatial and temporal trends in atmospheric mercury concentrations. We also review atmospheric measurement methods that are in wide use and those that are currently under development.

A New Perspective is Required to Understand the Role of Forest Ecosystems in Global Mercury Cycle: A Review

Mercury (Hg) is one of the most toxic heavy metal pollutants, which can be easily transmitted and enriched through the food chain, posing severe threat to human beings. Forest ecosystems are one of the most active environments for biogeochemical cycles of Hg. It is essential to research on Hg cycling in the forest ecosystem, which contributes to a comprehensive understanding of global biogeochemical cycle of Hg. However, there is still a lack of consensus on whether the forest ecosystem is a "source" or "sink" of Hg in the global Hg cycle so far. Therefore, it is necessary to elucidate the current state of knowledge on Hg deposition, transformation and fate in the forest ecosystem, especially the existing puzzles or issues encountered by scientists worldwide. This review highlights the complexity and uncertainties of Hg cycling in forest ecosystems. It is proposed that a new perspective is required to further understand the role of forest ecosystems in global Hg cycle based on a sufficient understanding of Hg exchange fluxes at the interface of air-soil and air-plant, Hg deposition flux through litterfall, and accurate construction of Hg mass balance system.

Source Attribution for Mercury Deposition with an Updated Atmospheric Mercury Emission Inventory in the Pearl River Delta Region, China

We used CMAQ-Hg to simulate mercury pollution and identify main sources in the Pearl River Delta (PRD) with updated local emission inventory and latest regional and global emissions. The total anthropogenic mercury emissions in the PRD for 2014 were 11,939.6 kg. Power plants and industrial boilers were dominant sectors, responsible for 29.4 and 22.7%. We first compared model predictions and observations and the results showed a good performance. Then five scenarios with power plants (PP), municipal solid waste incineration (MSWI), industrial point sources (IP), natural sources (NAT), and boundary conditions (BCs) zeroed out separately were simulated and compared with the base case. BCs was responsible for over 30% of annual average mercury concentration and total deposition while NAT contributed around 15%. Among the anthropogenic sources, IP (22.9%) was dominant with a contribution over 20.0% and PP (18.9%) and MSWI (11.2%) ranked second and third. Results also showed that power plants were the most important emission sources in the central PRD, where the ultra-low emission for thermal power units need to be strengthened. In the northern and western PRD, cement and metal productions were priorities for mercury control. The fast growth of municipal solid waste incineration were also a key factor in the core areas. In addition, a coordinated regional mercury emission control was important for effectively controlling pollution. In the future, mercury emissions will decrease as control measures are strengthened, more attention should be paid to mercury deposition around the large point sources as high levels of pollution are observed.

Adding four-dimensional data assimilation by analysis nudging to the Model for Prediction Across Scales - Atmosphere (version 4.0)

The Model for Prediction Across Scales - Atmosphere (MPAS-A) has been modified to allow four-dimensional data assimilation (FDDA) by the nudging of temperature, humidity, and wind toward target values predefined on the MPAS-A computational mesh. The addition of nudging allows MPAS-A to be used as a global-scale meteorological driver for retrospective air quality modeling. The technique of "analysis nudging" developed for the Penn State/National Center for Atmospheric Research (NCAR) Mesoscale Model, and later applied in the Weather Research and Forecasting model, is implemented in MPAS-A with adaptations for its polygonal Voronoi mesh. Reference fields generated from 1°×1° National Centers for Environmental Prediction (NCEP) FNL (Final) Operational Global Analysis data were used to constrain MPAS-A simulations on a 92-25km variable-resolution mesh with refinement centered over the contiguous United States. Test simulations were conducted for January and July 2013 with and without FDDA, and compared to reference fields and near-surface meteorological observations. The results demonstrate that MPAS-A with analysis nudging has high fidelity to the reference data while still maintaining conservation of mass as in the unmodified model. The results also show that application of FDDA constrains model errors relative to 2m temperature, 2m water vapor mixing ratio, and 10m wind speed such that they continue to be at or below the magnitudes found at the start of each test period.

Thunderstorms Increase Mercury Wet Deposition

Mercury (Hg) wet deposition, transfer from the atmosphere to Earth's surface by precipitation, in the United States is highest in locations and seasons with frequent deep convective thunderstorms, but it has never been demonstrated whether the connection is causal or simple coincidence. We use rainwater samples from over 800 individual precipitation events to show that thunderstorms increase Hg concentrations by 50% relative to weak convective or stratiform events of equal precipitation depth. Radar and satellite observations reveal that strong convection reaching the upper troposphere (where high atmospheric concentrations of soluble, oxidized mercury species (Hg(II)) are known to reside) produces the highest Hg concentrations in rain. As a result, precipitation meteorology, especially thunderstorm frequency and total rainfall, explains differences in Hg deposition between study sites located in the eastern United States. Assessing the fate of atmospheric mercury thus requires bridging the scales of global transport and convective precipitation.

Impact of climate change on mercury concentrations and deposition in the eastern United States

The global-regional climate-air pollution modeling system (GRE-CAPS) was applied over the eastern United States to study the impact of climate change on the concentration and deposition of atmospheric mercury. Summer and winter periods (300 days for each) were simulated, and the present-day model predictions (2000s) were compared to the future ones (2050s) assuming constant emissions. Climate change affects Hg(2+) concentrations in both periods. On average, atmospheric Hg(2+) levels are predicted to increase in the future by 3% in summer and 5% in winter respectively due to enhanced oxidation of Hg(0) under higher temperatures. The predicted concentration change of Hg(2+) was found to vary significantly in space due to regional-scale changes in precipitation, ranging from -30% to 30% during summer and -20% to 40% during winter. Particulate mercury, Hg(p) has a similar spatial response to climate change as Hg(2+), while Hg(0) levels are not predicted to change significantly. In both periods, the response of mercury deposition to climate change varies spatially with an average predicted increase of 6% during summer and 4% during winter. During summer, deposition increases are predicted mostly in the western parts of the domain while mercury deposition is predicted to decrease in the Northeast and also in many areas in the Midwest and Southeast. During winter mercury deposition is predicted to change from -30% to 50% mainly due to the changes in rainfall and the corresponding changes in wet deposition.

Source apportionment of atmospheric mercury pollution in China using the GEOS-Chem model

China is the largest atmospheric mercury (Hg) emitter in the world. Its Hg emissions and environmental impacts need to be evaluated. In this study, China's Hg emission inventory is updated to 2007 and applied in the GEOS-Chem model to simulate the Hg concentrations and depositions in China. Results indicate that simulations agree well with observed background Hg concentrations. The anthropogenic sources contributed 35-50% of THg concentration and 50-70% of total deposition in polluted regions. Sensitivity analysis was performed to assess the impacts of mercury emissions from power plants, non-ferrous metal smelters and cement plants. It is found that power plants are the most important emission sources in the North China, the Yangtze River Delta (YRD) and the Pearl River Delta (PRD) while the contribution of non-ferrous metal smelters is most significant in the Southwest China. The impacts of cement plants are significant in the YRD, PRD and Central China.

Global change and mercury cycling: challenges for implementing a global mercury treaty

The Minamata Convention aims to protect human health and the environment from anthropogenic emissions and releases of mercury. In the present study, the provisions of the Minamata Convention are examined to assess their influence on global biogeochemical cycling of Hg. Although the convention's scope covers all major categories of atmospheric emissions, the degree to which it will affect future emissions trajectories remains unclear. A box model analysis shows that future global biogeochemical cycling under projected technological provisions would result mainly in avoided increases and that estimated differences in atmospheric concentrations resulting from policies would be on the order of 1% to 2% per year. Present experience suggests that scientific knowledge is not currently sufficient to attribute causes to changes of this magnitude. Enhancements to capacity to measure the effectiveness of the Minamata Convention are suggested, including both measurement and modeling.

How well do environmental archives of atmospheric mercury deposition in the Arctic reproduce rates and trends depicted by atmospheric models and measurements?

This review compares the reconstruction of atmospheric Hg deposition rates and historical trends over recent decades in the Arctic, inferred from Hg profiles in natural archives such as lake and marine sediments, peat bogs and glacial firn (permanent snowpack), against those predicted by three state-of-the-art atmospheric models based on global Hg emission inventories from 1990 onwards. Model veracity was first tested against atmospheric Hg measurements. Most of the natural archive and atmospheric data came from the Canadian-Greenland sectors of the Arctic, whereas spatial coverage was poor in other regions. In general, for the Canadian-Greenland Arctic, models provided good agreement with atmospheric gaseous elemental Hg (GEM) concentrations and trends measured instrumentally. However, there are few instrumented deposition data with which to test the model estimates of Hg deposition, and these data suggest models over-estimated deposition fluxes under Arctic conditions. Reconstructed GEM data from glacial firn on Greenland Summit showed the best agreement with the known decline in global Hg emissions after about 1980, and were corroborated by archived aerosol filter data from Resolute, Nunavut. The relatively stable or slowly declining firn and model GEM trends after 1990 were also corroborated by real-time instrument measurements at Alert, Nunavut, after 1995. However, Hg fluxes and trends in northern Canadian lake sediments and a southern Greenland peat bog did not exhibit good agreement with model predictions of atmospheric deposition since 1990, the Greenland firn GEM record, direct GEM measurements, or trends in global emissions since 1980. Various explanations are proposed to account for these discrepancies between atmosphere and archives, including problems with the accuracy of archive chronologies, climate-driven changes in Hg transfer rates from air to catchments, waters and subsequently into sediments, and post-depositional diagenesis in peat bogs. However, no general consensus in the scientific community has been achieved.

Investigation of mercury wet deposition physicochemistry in the Ohio River Valley through automated sequential sampling

Intra-storm variability and soluble fractionation was explored for summer-time rain events in Steubenville, Ohio to evaluate the physical processes controlling mercury (Hg) in wet deposition in this industrialized region. Comprehensive precipitation sample collection was conducted from July through September 2006 using three different methods to evaluate both soluble and insoluble fractions as well as scavenging and washout properties of Hg and a suite of trace elements. Real-time filtration of event total precipitation revealed that 61±17% (mean±standard deviation) of Hg in wet deposition was in a soluble form. Comparison of total and dissolved element concentrations (solubility fractionation) showed the following order of decreasing solubility: S>Na>Se>Ca>Mg>Hg>As>Mn>V>Cr>Fe>La≈Ce ranging from 95% (S) to 4% (Ce). To examine removal mechanisms occurring during the course of a precipitation event, discrete, sequential sub-event precipitation samples were collected. Results indicated that Hg had lower "scavenging coefficients" (the rate of Hg concentration decrease throughout the events) than the majority of elements analyzed, indicating that either (i) Hg is incorporated into rain via gas phase inclusion or particulate nucleation within cloud, or (ii) Hg is available in the boundary layer for scavenging, even in the latter stages of precipitation. The Hg scavenging coefficient (-0.39) was low compared to S (-0.73), a co-pollutant of Hg. When compared to an upwind, regionally representative site, the scavenging coefficient of Hg for the locally influenced precipitation was 25% lower. This observation suggests that a continuous feed of soluble Hg was the reason for the low scavenging coefficient. Overall, this investigation of Hg wet deposition in Steubenville indicates that the physical and chemical properties of Hg emissions are driving the elevated deposition rates observed near point sources.

Differential exposure of alpine ospreys to mercury: melting glaciers, hydrology or deposition patterns?

Mercury (Hg) is a global contaminant impacting even remote environments. In alpine watersheds, glacial meltwater is a source of Hg, which accumulated in glaciers during the 1960-1980 cooling cycle. The considerable variation observed for Hg exposure of alpine animals in proximal watersheds could result from differences among those watersheds in Hg loading from glacial meltwater. Alternatively, variation may be the result of hydrology, atmospheric Hg deposition patterns, or food web characteristics. To examine those possibilities, we measured Hg in ospreys (Pandion haliaetus), apex predators in 15 watersheds in western Canada. Mercury levels in feathers of nestlings increased with increasing modeled atmospheric deposition rates and decreased with lake size. In eggs mercury decreased with δ(13)C, an indicator of food web structure, and with pH and elevation. Thus, Hg levels in chicks were strongly associated with local patterns relevant when the chicks were growing (e.g. the period post-snow melt: Hg deposition, lake size) while Hg levels in eggs were weakly associated with local patterns relevant during the snow melt (elevation, δ(13)C), with the remainder of the Hg variation in eggs determined by other factors such as possible Hg accumulation by the adult elsewhere. Modeled atmospheric deposition from prevailing upwind locations including Asia, followed by runoff into small lakes, were related to Hg patterns in osprey, with little apparent role for recent melting of glaciers. Our study highlights the importance of physical patterns to the environmental chemistry of top predators.

Global source-receptor relationships for mercury deposition under present-day and 2050 emissions scenarios

Global policies regulating anthropogenic mercury require an understanding of the relationship between emitted and deposited mercury on intercontinental scales. Here, we examine source-receptor relationships for present-day conditions and four 2050 IPCC scenarios encompassing a range of economic development and environmental regulation projections. We use the GEOS-Chem global model to track mercury from its point of emission through rapid cycling in surface ocean and land reservoirs to its accumulation in longer lived ocean and soil pools. Deposited mercury has a local component (emitted Hg(II), lifetime of 3.7 days against deposition) and a global component (emitted Hg(0), lifetime of 6 months against deposition). Fast recycling of deposited mercury through photoreduction of Hg(II) and re-emission of Hg(0) from surface reservoirs (ice, land, surface ocean) increases the effective lifetime of anthropogenic mercury to 9 months against loss to legacy reservoirs (soil pools and the subsurface ocean). This lifetime is still sufficiently short that source-receptor relationships have a strong hemispheric signature. Asian emissions are the largest source of anthropogenic deposition to all ocean basins, though there is also regional source influence from upwind continents. Current anthropogenic emissions account for only about one-third of mercury deposition to the global ocean with the remainder from natural and legacy sources. However, controls on anthropogenic emissions would have the added benefit of reducing the legacy mercury re-emitted to the atmosphere. Better understanding is needed of the time scales for transfer of mercury from active pools to stable geochemical reservoirs.

Science and strategies to reduce mercury risks: a critical review

Despite decades of scientific research and policy actions to control mercury, exposure to toxic methylmercury continues to pose risks to humans and the environment. This article critically reviews the linkages between scientific advancements and mercury reduction policies aimed at reducing this risk, focusing on the challenges that mercury poses as an issue that crosses both spatial and temporal scales. Scientific aspects of the mercury issue at various spatial and temporal scales are reviewed, and policy examples at global, national and local scale are analysed. Policy activity to date has focused on the mercury problem at a single level of spatial scale, and on near-term timescales. Efforts at the local scale have focused on monitoring levels in fish and addressing local contamination issues; national-scale assessments have addressed emissions from particular sources; and global-scale reports have integrated long-range transport of emissions and commercial trade concerns. However, aspects of the mercury issue that cross the political scale (such as interactions between different forms of mercury) as well as contamination problems with long timescales are at present beyond the reach of current policies. It is argued that these unaddressed aspects of the mercury problem may be more effectively addressed by (1) expanded cross-scale policy coordination on mitigation actions and (2) better incorporating adaptation into policy decision-making to minimize impacts.