Ian M. Hedgecock

Mercury and photochemistry in the marine boundary layer-modelling studies suggest the in situ production of reactive gas phase mercury

Abstract Following a modelling investigation of the role of the ambient aerosol in the cycling—that is the transport, transformation and deposition—of mercury in the atmosphere, the precise part played by the sea salt component of the marine aerosol in the remote marine boundary layer has been studied using a combination of models to describe the photolytic, gas phase and aqueous phase and heterogeneous chemistry of the marine boundary layer, in conjunction with inter phase mass transport and mercury chemistry. The role of the ocean in the emission of elemental mercury is, as yet, not entirely understood, but certainly the speciation of mercury deposited to the ocean surface is important as regards its re-emission. Models of mercury chemistry to date have tended to focus on cloud chemistry, and with good reason, as precipitation accounts for a large part of the global mercury deposition pattern; however, the composition of the marine aerosol is entirely different from that of cloud or fog droplets and the modelling studies here show that it plays a more local role being partially responsible for the gas phase speciation of mercury. The role of photochemical processes is investigated and particular attention is paid to halogen chemistry, as the chloride ion has been shown previously to have a notable effect on the concentration of oxidised mercury associated with particles, or better, solution droplets. The role of the sea salt component of the marine aerosol in the production of gas phase oxidised mercury species is described qualitatively and quantitatively.

Our current understanding of major chemical and physical processes affecting mercury dynamics in the atmosphere and at the air-water/terrestrial interfaces

The predictions of atmospheric chemical models are limited by the accuracy of our understanding of the basic physical and chemical processes that underlie the models. In this work we review the current state of our knowledge of the chemical processes that transform atmospheric mercury species via gas and aqueous phase reactions and the physical processes of deposition. We concur with the conclusions of other recent reviews that our understanding of the basic chemistry that controls mercury is incomplete and the experimental data either limited or nonexistent. In spite of this recent experimental and theoretical studies of mercury reaction kinetics have clarified some issues. Observations in Polar Regions suggest that Hg0 can undergo fast oxidation in the presence of elevated levels of bromine compounds. Both experimental and theoretical studies suggest that the recombination of Hg0 with Br atoms is sufficiently fast to initiate this oxidation process. However there is a large uncertainty in the value of the rate coefficient for this recombination reaction and in the fate of the reaction product, HgBr. Most global mercury models incorporate reactions of Hg0 with OH and O3. Based on the most recent high level ab-initio calculations of the stability of HgO it appears that neither of these reactions is likely to play a significant role in mercury oxidation. The most important aqueous oxidation for Hg0 appears to be reaction with O3 however that there has only been one determination of the Hg + O3 reaction rate constant in the aqueous phase. Aqueous phase reduction of oxidized mercury via reaction with HO2 is the only significant reduction reaction in current models but now seems unlikely to be significant. Again this suggests that the chemistry controlling mercury transformation in current models requires significant modification.

Observational and modeling constraints on global anthropogenic enrichment of mercury.

Centuries of anthropogenic releases have resulted in a global legacy of mercury (Hg) contamination. Here we use a global model to quantify the impact of uncertainty in Hg atmospheric emissions and cycling on anthropogenic enrichment and discuss implications for future Hg levels. The plausibility of sensitivity simulations is evaluated against multiple independent lines of observation, including natural archives and direct measurements of present-day environmental Hg concentrations. It has been previously reported that pre-industrial enrichment recorded in sediment and peat disagree by more than a factor of 10. We find this difference is largely erroneous and caused by comparing peat and sediment against different reference time periods. After correcting this inconsistency, median enrichment in Hg accumulation since pre-industrial 1760 to 1880 is a factor of 4.3 for peat and 3.0 for sediment. Pre-industrial accumulation in peat and sediment is a factor of ∼ 5 greater than the precolonial era (3000 BC to 1550 AD). Model scenarios that omit atmospheric emissions of Hg from early mining are inconsistent with observational constraints on the present-day atmospheric, oceanic, and soil Hg reservoirs, as well as the magnitude of enrichment in archives. Future reductions in anthropogenic emissions will initiate a decline in atmospheric concentrations within 1 year, but stabilization of subsurface and deep ocean Hg levels requires aggressive controls. These findings are robust to the ranges of uncertainty in past emissions and Hg cycling.

Dynamic processes of mercury over the Mediterranean region: results from the Mediterranean Atmospheric Mercury Cycle System (MAMCS) project

The Mediterranean Atmospheric Mercury Cycle System (MAMCS) project was performed between 1998 and 2000 and involved the collaboration of universities and research institutes from Europe, Israel and Turkey. The main goal of MAMCS was to investigate dynamic processes affecting the cycle of mercury in the Mediterranean atmosphere by combining ad hoc field measurements and modelling tasks. To study the fate of Hg in the Mediterranean Basin an updated emission inventory was compiled for Europe and the countries bordering the Mediterranean Sea. Models were developed to describe the individual atmospheric processes which influence the chemical and physical characteristics of atmospheric Hg, and these were coupled to meteorological models to examine the dispersion and deposition of Hg species in the Mediterranean Basin. One intercomparison and four two-week measurement campaigns were carried out over a three-year period. The work presented here describes the results in general terms but focuses on the areas where definite conclusions were unforthcoming and thus highlights those aspects where, in spite of advances made in the understanding of Hg cycling, further work is necessary in order to be able to predict confidently Hg and Hg compound concentration fields and deposition patterns.

Reactive gaseous mercury in the marine boundary layer: modelling and experimental evidence of its formation in the Mediterranean region

Reactive gaseous mercury (RGM) concentrations have been modelled using a photochemical box model of the marine boundary layer (MBL) and compared to measured data obtained during a research cruise. The model has been constrained by using measured concentrations of elemental Hg and ozone, as well as measured temperature and relative humidity. The results show good qualitative agreement both during the rough weather encountered on the first part of the voyage and the second, calmer period of the campaign. Quantitative agreement is obtained using a box height of 100 m during the first leg of the campaign. The modelled and measured results from the second leg agree as far as the nocturnal RGM concentration minima are concerned but underestimate the daytime maxima by a factor of two. The comparison of the modelled with measured results supports the hypothesis that there are daytime mercury oxidation reactions occurring which have not yet been identified.

Global atmospheric cycle of mercury: a model study on the impact of oxidation mechanisms

Mercury (Hg) is a global pollutant since its predominant atmospheric form, elemental Hg, reacts relatively slowly with the more abundant atmospheric oxidants. Comprehensive knowledge on the details of the atmospheric Hg cycle is still lacking, and in particular, there is some uncertainty regarding the atmospherically relevant reduction-oxidation reactions of mercury and its compounds. ECHMERIT is a global online chemical transport model, based on the ECHAM5 global circulation model, with a highly customisable chemistry mechanism designed to facilitate the investigation of both aqueous- and gas-phase atmospheric mercury chemistry. An improved version of the model which includes a new oceanic emission routine has been developed. Results of multiyear model simulations with full atmospheric chemistry have been used to examine the how changes to chemical mechanisms influence the model’s ability to reproduce measured Hg concentrations and deposition flux patterns. The results have also been compared to simple fixed-lifetime tracer simulations to constrain the possible range of atmospheric mercury redox rates. The model provides a new and unique picture of the global cycle of mercury, in that it is online and includes a full atmospheric chemistry module.

Role of the ambient aerosol in the atmospheric processing of semivolatile contaminants: A parameterized numerical model (Gas-Particle Partitioning (GASPAR))

A parameterized description of the ambient aerosol is the basis of a model that treats both gas-particle partitioning and aqueous phase chemical transformations of semivolatile contaminants. Dividing the aerosol population into source, size, hygroscopic, and compositional classes, it is possible to assess the importance of contaminant-aerosol interactions under varying meteorological conditions. Using mercury as a test case, the model provides not only the quantity and speciation of mercury associated with particulate matter for use in dry deposition models and in conjunction with dispersion/meteorological models, but shows conclusively that deliquesced aerosol particles are not simply transporters of adsorbed mercury, but play an active and significant role in the transformation of elemental to oxidized mercury. The sensitivity analysis carried out using a version of the Direct Decoupled Method has shown the transfer of Hg(II) to the gas phase from the aqueous phase to be highly dependent on the chloride ion concentration in the initial parameterization array which describes the ambient aerosol. The chloride ion concentration has a notable effect on the oxidized Hg that is associated with the particle when the chemistry model reaches steady state. The reason for this is clarified by the dependencies of the neutral Hg containing species concentrations on the rates of mass transfer and the initial concentrations. The presence of soot in the aerosol particles is shown to be particularly important in the partitioning of Hg(II) between the gas, aqueous and particulate phases. The implications, given the higher solubility of most oxidized mercury species compared to elemental mercury, are fundamental for the understanding of the cycling and fate of mercury in the environment.

Development and application of a regional-scale atmospheric mercury model based on WRF/Chem: a Mediterranean area investigation

The emission, transport, deposition and eventual fate of mercury (Hg) in the Mediterranean area has been studied using a modified version of the Weather Research and Forecasting model coupled with Chemistry (WRF/Chem). This model version has been developed specifically with the aim to simulate the atmospheric processes determining atmospheric Hg emissions, concentrations and deposition online at high spatial resolution. For this purpose, the gas phase chemistry of Hg and a parametrised representation of atmospheric Hg aqueous chemistry have been added to the regional acid deposition model version 2 chemical mechanism in WRF/Chem. Anthropogenic mercury emissions from the Arctic Monitoring and Assessment Programme included in the emissions preprocessor, mercury evasion from the sea surface and Hg released from biomass burning have also been included. Dry and wet deposition processes for Hg have been implemented. The model has been tested for the whole of 2009 using measurements of total gaseous mercury from the European Monitoring and Evaluation Programme monitoring network. Speciated measurement data of atmospheric elemental Hg, gaseous oxidised Hg and Hg associated with particulate matter, from a Mediterranean oceanographic campaign (June 2009), has permitted the model’s ability to simulate the atmospheric redox chemistry of Hg to be assessed. The model results highlight the importance of both the boundary conditions employed and the accuracy of the mercury speciation in the emission database. The model has permitted the reevaluation of the deposition to, and the emission from, the Mediterranean Sea. In light of the well-known high concentrations of methylmercury in a number of Mediterranean fish species, this information is important in establishing the mass balance of Hg for the Mediterranean Sea. The model results support the idea that the Mediterranean Sea is a net source of Hg to the atmosphere and suggest that the net flux is ≈30 Mg year−1 of elemental Hg.

Model study of global mercury deposition from biomass burning

Mercury emissions from biomass burning are not well characterized and can differ significantly from year to year. This study utilizes three recent biomass burning inventories (FINNv1.0, GFEDv3.1, and GFASv1.0) and the global Hg chemistry model, ECHMERIT, to investigate the annual variation of Hg emissions, and the geographical distribution and magnitude of the resulting Hg deposition fluxes. The roles of the Hg/CO enhancement ratio, the emission plume injection height, the Hg(g)0 oxidation mechanism and lifetime, the inventory chosen, and the uncertainties with each were considered. The greatest uncertainties in the total Hg deposition were found to be associated with the Hg/CO enhancement ratio and the emission inventory employed. Deposition flux distributions proved to be more sensitive to the emission inventory and the oxidation mechanism chosen, than all the other model parametrizations. Over 75% of Hg emitted from biomass burning is deposited to the world’s oceans, with the highest fluxes predicted i...

Chasing quicksilver northward: mercury chemistry in the Arctic troposphere

Environmental Context. ‘Mercurial storms rage over the Arctic’ wrote Fred Pearce in New Scientist in June of 1997: he was referring to the recent discovery by Bill Schroeder and his colleagues (Nature, Vol. 394, 1998) of periods soon after Arctic dawn when the concentration of mercury in the atmosphere literally plummets to levels so low that they can be undetectable, even by the most sensitive of modern instruments. A decade and many measurement campaigns later, we think we understand how these so-called depletion events occur, if not all the mechanisms that go towards providing the conditions for them to happen. Nor do we really know what happens to the mercury removed from the atmosphere; the fear is that it is deposited and enters the Arctic ecosystem, where it is potentially extremely harmful. The present study questions whether that fear is grounded. Abstract. The tropospheric boundary layer chemistry of Hg has been simulated using a two-phase photochemical box model to see if our current (experimental and theoretical) understanding of Hg(g)0 reaction rates can account for the depletion events seen during Arctic spring, when the so-called ‘bromine explosion’ in the model is constrained by the measured ozone depletion rate. The simulations reveal that the observed rate of Hg(g)0 depletion can be accounted for; however, the measured concentrations of gas-phase oxidised Hg and HgP (Hg associated with particulate matter) cannot. Simulating the emission of Hg(g)0 from the snow pack to mimic the observed concentration recovery after a depletion event suggests the net Hg deposition from a depletion event is all but irrelevant.