Photochemistry of oxidized Hg(I) and Hg(II) species suggests missing mercury oxidation in the troposphere

There is no author summary for this article yet. Authors can add summaries to their articles on ScienceOpen to make them more accessible to a non-specialist audience.

Significance

The atmospheric chemistry of mercury, a global priority pollutant, is key to its transport and deposition to the surface environment. Assessments of its risks to humans and ecosystems rely on an accurate understanding of global mercury cycling. This work shows that the chemical reactions and rates currently employed to interpret Hg chemistry in the atmosphere fails to explain observed atmospheric mercury concentrations and deposition. We report that model simulations incorporating recent developments in the photoreduction mechanisms of the oxidized forms of mercury (Hg ^I and Hg ^II) lead to a significant model underestimation of global observations of these oxidized species in the troposphere and their surface wet deposition. This implies that there must be currently unidentified mercury oxidation processes in the troposphere.

Abstract

Mercury (Hg), a global contaminant, is emitted mainly in its elemental form Hg ⁰ to the atmosphere where it is oxidized to reactive Hg ^II compounds, which efficiently deposit to surface ecosystems. Therefore, the chemical cycling between the elemental and oxidized Hg forms in the atmosphere determines the scale and geographical pattern of global Hg deposition. Recent advances in the photochemistry of gas-phase oxidized Hg ^I and Hg ^II species postulate their photodissociation back to Hg ⁰ as a crucial step in the atmospheric Hg redox cycle. However, the significance of these photodissociation mechanisms on atmospheric Hg chemistry, lifetime, and surface deposition remains uncertain. Here we implement a comprehensive and quantitative mechanism of the photochemical and thermal atmospheric reactions between Hg ⁰, Hg ^I, and Hg ^II species in a global model and evaluate the results against atmospheric Hg observations. We find that the photochemistry of Hg ^I and Hg ^II leads to insufficient Hg oxidation globally. The combined efficient photoreduction of Hg ^I and Hg ^II to Hg ⁰ competes with thermal oxidation of Hg ⁰, resulting in a large model overestimation of 99% of measured Hg ⁰ and underestimation of 51% of oxidized Hg and ∼66% of Hg ^II wet deposition. This in turn leads to a significant increase in the calculated global atmospheric Hg lifetime of 20 mo, which is unrealistically longer than the 3–6-mo range based on observed atmospheric Hg variability. These results show that the Hg ^I and Hg ^II photoreduction processes largely offset the efficiency of bromine-initiated Hg ⁰ oxidation and reveal missing Hg oxidation processes in the troposphere.

Related collections

Most cited references 61

Record: found
Abstract: not found
Article: not found

Description and evaluation of the Model for Ozone and Related chemical Tracers, version 4 (MOZART-4)

L. Emmons, S. Kloster, C. Wiedinmyer … (2010)

0 comments Cited 394 times – based on 0 reviews      Review now

Bookmark

Record: found
Abstract: found
Article: not found

Mercury as a Global Pollutant: Sources, Pathways, and Effects

Charles Driscoll, Robert P Mason, Laurie Chan … (2013)

Mercury (Hg) is a global pollutant that affects human and ecosystem health. We synthesize understanding of sources, atmosphere-land-ocean Hg dynamics and health effects, and consider the implications of Hg-control policies. Primary anthropogenic Hg emissions greatly exceed natural geogenic sources, resulting in increases in Hg reservoirs and subsequent secondary Hg emissions that facilitate its global distribution. The ultimate fate of emitted Hg is primarily recalcitrant soil pools and deep ocean waters and sediments. Transfers of Hg emissions to largely unavailable reservoirs occur over the time scale of centuries, and are primarily mediated through atmospheric exchanges of wet/dry deposition and evasion from vegetation, soil organic matter and ocean surfaces. A key link between inorganic Hg inputs and exposure of humans and wildlife is the net production of methylmercury, which occurs mainly in reducing zones in freshwater, terrestrial, and coastal environments, and the subsurface ocean. Elevated human exposure to methylmercury primarily results from consumption of estuarine and marine fish. Developing fetuses are most at risk from this neurotoxin but health effects of highly exposed populations and wildlife are also a concern. Integration of Hg science with national and international policy efforts is needed to target efforts and evaluate efficacy.

0 comments Cited 360 times – based on 0 reviews      Review now

Bookmark

Record: found
Abstract: found
Article: found

Is Open Access

A review of global environmental mercury processes in response to human and natural perturbations: Changes of emissions, climate, and land use

Daniel Obrist, Jane L. Kirk, Lei Zhang … (2018)

We review recent progress in our understanding of the global cycling of mercury (Hg), including best estimates of Hg concentrations and pool sizes in major environmental compartments and exchange processes within and between these reservoirs. Recent advances include the availability of new global datasets covering areas of the world where environmental Hg data were previously lacking; integration of these data into global and regional models is continually improving estimates of global Hg cycling. New analytical techniques, such as Hg stable isotope characterization, provide novel constraints of sources and transformation processes. The major global Hg reservoirs that are, and continue to be, affected by anthropogenic activities include the atmosphere (4.4–5.3 Gt), terrestrial environments (particularly soils: 250–1000 Gg), and aquatic ecosystems (e.g., oceans: 270–450 Gg). Declines in anthropogenic Hg emissions between 1990 and 2010 have led to declines in atmospheric Hg0 concentrations and HgII wet deposition in Europe and the US (− 1.5 to − 2.2% per year). Smaller atmospheric Hg0 declines (− 0.2% per year) have been reported in high northern latitudes, but not in the southern hemisphere, while increasing atmospheric Hg loads are still reported in East Asia. New observations and updated models now suggest high concentrations of oxidized HgII in the tropical and subtropical free troposphere where deep convection can scavenge these HgII reservoirs. As a result, up to 50% of total global wet HgII deposition has been predicted to occur to tropical oceans. Ocean Hg0 evasion is a large source of present-day atmospheric Hg (approximately 2900 Mg/year; range 1900–4200 Mg/year). Enhanced seawater Hg0 levels suggest enhanced Hg0 ocean evasion in the intertropical convergence zone, which may be linked to high HgII deposition. Estimates of gaseous Hg0 emissions to the atmosphere over land, long considered a critical Hg source, have been revised downward, and most terrestrial environments now are considered net sinks of atmospheric Hg due to substantial Hg uptake by plants. Litterfall deposition by plants is now estimated at 1020–1230 Mg/year globally. Stable isotope analysis and direct flux measurements provide evidence that in many ecosystems Hg0 deposition via plant inputs dominates, accounting for 57–94% of Hg in soils. Of global aquatic Hg releases, around 50% are estimated to occur in China and India, where Hg drains into the West Pacific and North Indian Oceans. A first inventory of global freshwater Hg suggests that inland freshwater Hg releases may be dominated by artisanal and small-scale gold mining (ASGM; approximately 880 Mg/year), industrial and wastewater releases (220 Mg/year), and terrestrial mobilization (170–300 Mg/year). For pelagic ocean regions, the dominant source of Hg is atmospheric deposition; an exception is the Arctic Ocean, where riverine and coastal erosion is likely the dominant source. Ocean water Hg concentrations in the North Atlantic appear to have declined during the last several decades but have increased since the mid-1980s in the Pacific due to enhanced atmospheric deposition from the Asian continent. Finally, we provide examples of ongoing and anticipated changes in Hg cycling due to emission, climate, and land use changes. It is anticipated that future emissions changes will be strongly dependent on ASGM, as well as energy use scenarios and technology requirements implemented under the Minamata Convention. We predict that land use and climate change impacts on Hg cycling will be large and inherently linked to changes in ecosystem function and global atmospheric and ocean circulations. Our ability to predict multiple and simultaneous changes in future Hg global cycling and human exposure is rapidly developing but requires further enhancement.

0 comments Cited 119 times – based on 0 reviews      Review now

Bookmark

All references

Author and article information

Journal

Journal ID (nlm-ta): Proc Natl Acad Sci U S A

Journal ID (iso-abbrev): Proc Natl Acad Sci U S A

Journal ID (hwp): pnas

Journal ID (pmc): pnas

Journal ID (publisher-id): PNAS

Title: Proceedings of the National Academy of Sciences of the United States of America

Publisher: National Academy of Sciences

ISSN (Print): 0027-8424

ISSN (Electronic): 1091-6490

Publication date (Print): 8 December 2020

Publication date (Electronic): 23 November 2020

Publication date PMC-release: 23 November 2020

Volume: 117

Issue: 49

Pages: 30949-30956

Affiliations

[1] ^aDepartment of Atmospheric Chemistry and Climate, Institute of Physical Chemistry Rocasolano , Spanish National Research Council (CSIC), 28006 Madrid, Spain;

[2] ^bMeteorological Synthesizing Centre–East of EMEP , 115419 Moscow, Russia;

[3] ^cGéosciences Environnement Toulouse, CNRS/Observatoire Midi-Pyrénées (OMP)/Université de Toulouse , 31400 Toulouse, France;

[4] ^dHarvard John A. Paulson School of Engineering and Applied Sciences, Harvard University , Cambridge, MA 02138;

[5] ^eInstitut de Ciència Molecular, Universitat de València , 46071 València, Spain;

[6] ^fDepartamento de Química Física, Universitat de València , 46100 València, Spain;

[7] ^gUniversité de Lorraine , CNRS, Laboratoire de Physique et Chimie Théoriques (LPCT), F-54000 Nancy, France;

[8] ^hDepartment of Environmental Sciences, University of Basel , 4056 Basel, Switzerland;

[9] ⁱDepartment of Environment and Geography, Centre for Earth Observation Science, University of Manitoba , Winnipeg, MB R3T 2N2, Canada;

[10] ^jHelmholtz-Zentrum Geethacht, Institute of Coastal Research , 21502 Geesthacht, Germany;

[11] ^kSchool of Chemistry, University of Leeds , LS2 9TJ Leeds, United Kingdom;

[12] ^lDepartment of Earth and Environmental Science,University of Pennsylvania , Philadelphia, PA 19104;

[13] ^mDepartment of Chemistry, University of Pennsylvania , Philadelphia, PA 19104

Author notes

¹To whom correspondence may be addressed. Email: a.saiz@ 123456csic.es , Oleg.Travnikov@ 123456msceast.org , or frjoseph@ 123456sas.upenn.edu .

Contributed by Joseph S. Francisco, October 26, 2020 (sent for review December 27, 2019; reviewed by Michael E. Goodsite and Anthony J. Hynes)

Author contributions: A.S.-L. and J.S.F. designed research; A.S.-L., O.T., J.E.S., and A.U.A. performed research; A.S.-L., O.T., J.E.S., C.P.T., D.J.J., J.C.-G., A.F.-M., D.R.-S., A.U.A., J.Z.D., C.A.C., M.J., F.W., J.B., J.M.C.P., and J.S.F. analyzed data; and A.S.-L. and J.S.F. wrote the paper.

Reviewers: M.E.G., University of Adelaide; and A.J.H., University of Miami.