United
Nations
Environment
Programme

Global Mercury
Assessment
2013
Sources, Emissions, Releases
and Environmental Transport

Copyright © United Nations Environment Programme, 2013
Citation: UNEP, 2013. Global Mercury Assessment 2013: Sources, Emissions, Releases
and Environmental Transport. UNEP Chemicals Branch, Geneva, Switzerland
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UNEP Global Mercury Assessment 2013

Table of Contents
Executive Summary. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . i
1. Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
2. Sources of mercury emissions to air and releases to water.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Anthropogenic emissions to the atmosphere. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
4. Trends in mercury emissions to the atmosphere. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
5. Atmospheric chemistry, monitoring, and trends. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
6. Atmospheric concentrations and deposition. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
7. Anthropogenic releases to the aquatic environment.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
8. Aquatic pathways, transport, and fate. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
9. Major gaps in knowledge. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
10. Key findings of the 2013 assessment.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

UNEP Global Mercury Assessment 2013

UNEP Global Mercury Assessment 2013

Executive Summary
Mercury is a global threat to human andenvironmental health. This report, focusing on
anthropogenic emissions of mercury and theirtransport and transformation in the environment,
is a contribution to international efforts to reduce
mercury pollution.
This summary report and the accompanying
Technical Background Report for the Global
Mercury Assessment 2013 are developed in response
to Decision 25/5, paragraph 36 of the Governing
Council of the United Nations Environment
Programme (UNEP), that:
Requests the Executive Director, in consultation with
Governments, to update the 2008 report entitled“Global Atmospheric Mercury Assessment: Sources,
Emissions and Transport,” for consideration by theGoverning Council/Global Ministerial Environment
Forum at its twenty-seventh session.
The report provides the most recent information
available on worldwide atmospheric mercury
emissions, releases to the aquatic environment, andthe transport and fate of mercury in the global
environment. The report emphasizes emissions to air
from human (anthropogenic) activities, but includes
releases to water because the aquatic environmentis the main route of exposure to humans andwildlife. It is in aquatic systems that the inorganic
mercury is transformed into the more toxic form,
methylmercury, which can accumulate in fish andmarine mammals consumed by humans.
This Executive Summary presents an overview of the
key findings of the Global Mercury Assessment 2013.
Total anthropogenic emissions of mercury to theatmosphere in 2010 are estimated at 1960 tonnes.2
The 2010 emissions inventory has several
improvements over the previous inventory for
2005, including:• A more detailed analysis of emissions from somemajor source sectors.

2

1 tonne = 1000 kilograms• A more detailed consideration of the mercury
content of fuels and raw materials used in different
countries/regions.
• New and updated information on artisanal andsmall-scale gold mining.
• The use of different pollution control technologies
in different countries and regions have beenfactored into the emissions estimates.
• Emission estimates for sectors not previously
included, such as aluminium production, oil
refining, and contaminated sites.
• More and better information on location of major
point sources such as individual power plants,
smelters and cement kilns.
• Better documentation and greater transparency
with respect to the data and information behindthe estimates
Using this approach, the global emissions to air fromanthropogenic sources is estimated as 1960 tonnes
in 2010. Despite recent progress in improving theavailable knowledge base, the emissions estimate
still has large associated uncertainties, giving a
range of 1010-4070 tonnes. The work also identifies
potentially important sectors that are not yet
quantified, including use of mercury in vinylchloride monomer production; secondary metals
production and ferro-alloys; oil and gas extraction
and transport; and industrial and some hazardous
waste incineration.
Present day anthropogenic emissions contribute toboth current and future emissions to the air
Current anthropogenic sources are responsible
for about 30% of annual emissions of mercury toair. Another 10% comes from natural geologicalsources, and the rest (60%) is from ‘re-emissions’of previously released mercury that has built up
over decades and centuries in surface soils andoceans. Although the original source of this reemitted mercury cannot be determined withcertainty, the fact that anthropogenic emissions
have been larger than natural emissions since
the start of the industrial age about 200 yearsiUNEP Global Mercury Assessment 2013

ago implies that most re-emitted mercury was
originally from anthropogenic sources. Reducing
current anthropogenic sources is therefore vital toreduce the amount of mercury that is cycling in theenvironment.
Artisanal and small-scale gold mining and coalburning are the major sources of anthropogenic
mercury emissions to air
The inventory confirms the role of artisanal andsmall-scale gold mining (ASGM) and coal burningas the largest components of anthropogenic
emissions, followed by the production of ferrous andnon-ferrous metals, and cement production.
Annual emissions from ASGM are estimatedat 727 tonnes, making this the largest sector
accounting for more than 35% of total
anthropogenic emissions. This is more than twice
the figure from this sector in 2005, however, mostof the increase is attributed to some new andbetter information. For example, West Africa wasthought in 2005 to have minimal ASGM activity
but is now recognized as an important source
region. It is thus difficult to determine whether
actual emissions from this sector have changed
because their estimation involves a great deal of
uncertainty. Much of the activity is unregulated
or even illegal, and thus reliable official data
are still hard to obtain. More work is needed toconfirm the emissions estimates from this sector,
including field measurements around ASGM sites
to better establish the amounts and fate of themercury used.
A large amount of coal is burned around theworld to generate electricity, to run industrial
plants, and for in-home heating and cooking.
Coal burning emitted some 475 tonnes of mercury
in 2010, the majority of which is from power
generation and industrial use. The estimate ofemissions from other coal burning (including
domestic and residential burning) is lower thanthat reported in the previous global assessment,
due to differences in estimates of the amounts
and mercury content of coal burned in these uses.
Use of coal for power generation and industry
is increasing, especially in Asia. However, wider
use of air pollution controls and more stringent
regulations in several countries, together withimproved combustion efficiency, have reduced
emissions from coal-burning power plants,
helping to offset most of the increase arising fromhigher coal consumption.
ii

Global anthropogenic mercury emissions fromindustrial sources may be rising.
Emissions to air are thought to have peaked in the
1970s, declined over the following two decades, andhave been relatively stable between 1990 and 2005.
There were some indications of slight increases inemissions between 2000 and 2005.
Any evaluation of trends needs to take into accountchanges in reporting and methods used to produce
inventory estimates, including the introduction of
additional sectors. Thus, a direct comparison of
the results of global inventories produced over the
past 25 years is not possible. A preliminary recalculation, using the improved methodology, of
global anthropogenic emissions in 2005 indicatesthat emissions from fossil fuel combustion, metal
and cement production increased between 2005 and2010, but continue to decline in other sectors suchas the chlor-alkali industry. Overall, indications arethat emissions from industrial sectors have increased
again since 2005.
Future emission trends have been examined using
scenarios and models. Without improved pollution
controls or other actions to reduce mercury
emissions, mercury emissions are likely to be
substantially higher in 2050 than they are today.
Comparing emissions estimates reported under
different reporting systems is not straightforward
The 2010 global inventory results were generally
consistent with nationally reported emissions
estimates for 2010, providing a degree of confidence
in the methods used. However, comparing estimatesfor individual countries and sectors is complicatedby differences in reporting methods, in particular the
specification and categorisation of sectors used indifferent national and international reporting systems.
National emissions estimates based on individual
facility reporting and site measurements should
be more accurate than those based on the global
inventory methodology. However, this is difficult
to evaluate as most nationally reported inventories
lack estimation of associated uncertainties. It is also
important to recognize that many measurementbased estimates are based on relatively few
measurements covering short periods that are thenextrapolated to produce annual emissions. It is
important that all reporting is subject to validationand that associated uncertainties are quantified. If
different reporting systems are to be compared, theyneed to be better aligned in terms of the emission
sources that are identified and used.

UNEP Global Mercury Assessment 2013

Asia contributes almost half of global anthropogenic
mercury emissions.

Sea suggest a similar pattern occurring there more
recently, likely as a result of Asian industrialization.

Increasing industrialization has made Asia the main
source region of mercury emissions to air, with
East and Southeast Asia accounting for about 40%of the global total, and South Asia for a further 8%.
The new data on ASGM and the related increase inemission estimates from this sector have increased
South America and sub-Saharan Africa’s shareof global emissions. However, modelling results
continue to indicate that East Asia is the dominantsource region for long-range airborne mercury
transport worldwide.

Monitoring capability continues to improve, butwhether this can be sustained is uncertain.

Anthropogenic releases of mercury to water total 1000
tonnes at a minimum.
Previous UNEP global mercury assessments
considered only atmospheric emissions. The 2013
report is thus the first attempt to compile a global
inventory of aquatic releases. Three types of sourceswere considered. Point sources are industrial sites
such as power plants or factories, and they release
an estimated 185 tonnes of mercury per year.
Contaminated sites, including old mines, landfills,
and waste disposal locations, release 8 - 33 tonnes
per year. Artisanal and small-scale gold mining wasevaluated separately, with total releases to waterand land totalling more than 800 tonnes per year.
Deforestation mobilizes another 260 tonnes of
mercury into rivers and lakes. Other sources remain
to be quantified, and so these estimates comprise
only a partial total. Thus, anthropogenic releases towaters are likely to be at least 1000 tonnes per year.
Mercury concentrations in the oceans and in marine
animals have risen due to anthropogenic emissions.
Anthropogenic emissions and releases havedoubled the amount of mercury in the top 100
meters of the world’s oceans in the last 100 years.
Concentrations in deeper waters have increased
by only 10-25%, because of the slow transfer of
mercury from surface waters into the deep oceans.
In some species of Arctic marine animals, mercury
content has increased by 12 times on average since
the pre-industrial period. This increase implies
that, on average, over 90% of the mercury in thesemarine animals today comes from anthropogenic
sources. The timing of the initial stage of theincrease, which started in the mid-19th century
and accelerated in the early 20th century before therise of Asian industrialization, indicates emissions
from Europe, Russia and North America were
probably responsible. Studies from the South China

Existing mercury monitoring networks such as theEuropean Monitoring and Evaluation Programme(EMEP), the Arctic Monitoring and Assessment
Programme (AMAP), the North American Mercury
Deposition Network (NAMDN), and others in thenorthern hemisphere have been complemented by
new monitoring sites in the southern hemisphere, inparticular, some sites established under the Global
Mercury Observing System (GMOS) initiative. The
longer-term status of many of the newly establishedsites however depends on availability of sustained
funding to continue operations.
Anthropogenic emissions and releases over time haveincreased mercury loads in the environment, so theeffects of reductions in emissions will often take time
to become apparent.
Large amounts of mainly inorganic mercury haveaccumulated in the environment, in particularin surface soils and in the oceans, as a result of
past emissions and releases. Owing to their larger
volumes, intermediate and deep ocean waters below
100 metres actually store much larger tonnages ofanthropogenic mercury than surface waters. There
are also relatively large tonnages of natural mercury
circulating in the intermediate and deep waters. A
significant fraction of the mercury in intermediatewaters is recycled back to the surface each year
by upwellings. Today’s anthropogenic emissions
continue to load the oceans, and the catchments
and sediments of lakes and rivers, with inorganic
mercury. This mercury, which is the “feed-stock” fortoxic methylmercury production, is stored and recycled in the bioavailable part of the environment for
decades or centuries before it eventually is removed
by natural processes. One consequence is that
there will likely be a time-lag of years or decades,
depending on the part of the water column, before
emissions reductions begin to have a demonstrable
effect on mercury levels throughout the environment
and in the fish and marine mammals which arepart of the human food-chain. At the same time,
mercury levels in parts of the Atlantic Ocean are
decreasing, likely due to reduced emissions in past
decades in North America and Europe, indicatingthat emissions reductions can eventually lead todecreases in mercury levels in surface oceans. This
reinforces the need to continue and strengtheniii

UNEP Global Mercury Assessment 2013

international efforts to reduce current mercury
emissions and releases, as delays in action now will
inevitably lead to slower recovery of the world’s
ecosystems in future from mercury contamination.
Global climate change may also complicate
the response of global ecosystems to mercury
emission reductions, through its profound effects
on many aspects of the movement and chemicaltransformations of mercury in the environment. For
example, warmer temperatures may increase ratesof organic productivity in freshwater and marine
ecosystems, and rates of bacterial activity, possibly
leading to faster conversion of inorganic mercury
to methylmercury. Thawing of the enormous areasof northern frozen peatlands may release globallysignificant amounts of long-stored mercury andorganic matter into Arctic lakes, rivers and ocean.

iv

UNEP Global Mercury Assessment 2013

Introduction

1

Background and mandate
Global inventories for mercury emissions to air fromhuman sources have been produced at approximately
5-year intervals since 1990 by scientific groups.
UNEP produced its first Global Mercury Assessment
in 2002. In 2007, the Governing Council of UNEP
through its decision 24/3 requested the Executive
Director of UNEP:
to prepare a report, drawing on, among other things,
ongoing work in other forums, addressing:(a) Best available data on mercury atmosphericemissions and trends including where possible ananalysis by country, region and sector, including
a consideration of factors driving such trends andapplicable regulatory mechanisms;(b) Current results from modelling on a global
scale and from other information sources on thecontribution of regional emissions to deposition which
may result in adverse effects and the potential benefits
from reducing such emissions, taking into accountthe efforts of the Fate and Transport partnershipestablished under the United Nations Environment
Programme mercury programme.
The results of this request were presented as asummary report entitled “The Global AtmosphericMercury Assessment: Sources, Emissions andTransport”, released in December 2008, and anaccompanying “Technical Background Report to the
Global Atmospheric Mercury Assessment”.
The technical background report, which formed
the basis for statements made in the summary
report and was prepared in cooperation with the
Arctic Monitoring and Assessment Programme(AMAP) Secretariat, included an updated inventoryof anthropogenic emissions of mercury to theatmosphere. The inventory was based on nationalemissions data for the year 2005 submitted bygovernments, as well as estimates prepared forcountries that did not provide data, and wascoordinated with work related to mercury under the
UN Economic Commission for Europe Convention

UNEP’s 2008
Global AtmosphericMercury Assessment
Report.

on Long-range Transboundary Air Pollution(LRTAP). The report also drew on the work of the
UNEP Global Mercury Partnership, in particular
the Mercury Air Transport and Fate Research
partnership area.
In 2009, the Governing Council of UNEP, throughits Decision 25/5, paragraph 36, requested the
Executive Director of UNEP:… in consultation with Governments, to update the2008 report entitled Global Atmospheric Mercury
Assessment: Sources, Emissions and Transport forconsideration by the Governing Council/Global
Ministerial Environment Forum at its twentyseventh session.
This updated Global Mercury Assessment 2013
and its accompanying and updated TechnicalBackground Report are the response to that
Governing Council request.

Developing the 2013 Report
As in 2008, the Technical Background Report for
the Global Mercury Assessment 2013 forms thebasis for the statements made in the Summary
1

UNEP Global Mercury Assessment 2013

Report and is fully referenced according tostandard scientific practice. As such, it is the single
reference for the Summary Report. It has again
been prepared in co-operation with the Arctic
Monitoring and Assessment Programme (AMAP)
and uses national data and information submitted
by several governments. Contributions havealso been incorporated from the UNEP Global
Mercury Partnership, in particular its partnershipareas on mercury control from coal combustion,
reducing mercury in artisanal and small-scale gold
mining (ASGM), and mercury air transport andfate; AMAP mercury expert group; UN Economic
Commission for Europe (UN ECE) Long-range
Transboundary Air Pollution (LRTAP) Convention
groups; industry; and non-governmentalorganizations. Each section was prepared by a teamof experts and then reviewed to ensure its scientificaccuracy. The evaluation of information of mercury
released into the aquatic environment benefits
from contributions from the Group of Experts
on Scientific Aspects of Marine Environmental
Protection (GESAMP).

Scope and coverage
This update to the Global Mercury Assessment
provides the most recent information available forthe worldwide emissions, releases, and transport ofmercury in atmospheric and aquatic environments.
To the extent possible, the information comes fromthe published scientific literature, supplemented
where necessary by other sources. Since the Global
Mercury Assessment is intended as a basis for
decision making, emphasis is given to anthropogenic
emissions (mercury going into the atmosphere) andreleases (mercury going into water and land), that is,
those associated with human activities.
The Technical Background Report includes a detailed
inventory of global mercury emissions to air basedon data for 2010. “Inventory” in this context means acompilation of the estimated emissions and releases
from various sectors and sources. While it attempts
to catalogue all major sources of mercury emissions
worldwide, it should not be regarded as complete
and exhaustive.
In addition, for the first time, this Global Mercury
Assessment includes an evaluation of information of
mercury released into the aquatic environment andits associated pathways and fate. The information
available for this evaluation is less complete than2that for emissions to air and is based on data fromrecent years. The inclusion of this new element of
the Global Mercury Assessment is as a responseto the requests from many governments in theIntergovernmental Negotiating Committee (INC) for
more information on releases to land and water. The
aquatic environment is the main route of exposure
to humans and wildlife, because it is in water that
inorganic mercury is transformed into highly
toxic methylmercury.
The Technical Background Report includes
chapters on:• Global Emissions of Mercury to the Atmosphere,
describing sources, anthropogenic emissions, andtrends in emissions;• Atmospheric Pathways, Transport and Fate,
examining pathways, levels and trends in air
and deposition, and modelling of pathways anddeposition;• Global Releases of Mercury to Aquatic Environments,
containing global estimates of releases towater; and• Aquatic Pathways, Transport and Fate, examining
mercury pathways in aquatic systems that result in
important routes of human exposure.
Technical Background Report chapters wereprepared by teams of experts and then reviewed toensure their scientific validity. This Summary Reportis based on the content of the Technical Background
Report and has been reviewed by the authors of theTechnical Background Report. It was also circulated
for national review.
This Summary Report provides, in Chapter 2, an
overview of natural and anthropogenic sources of
mercury, outlining the main sectors involved. The
global inventory of anthropogenic emissions to air
is presented in Chapter 3, and Chapter 4 describes
trends in mercury emissions to the atmosphere.
What happens after mercury is released to theatmosphere is covered in Chapter 5 on atmosphericchemistry, monitoring, and deposition, and Chapter
6 on atmospheric concentrations and deposition.
Chapter 7 provides a first attempt to estimate
global releases to water, followed in Chapter 8 bya discussion of aquatic pathways, transformations,and fate. A review of gaps in knowledge is given in
Chapter 9. Chapter 10 summarizes key findings.

UNEP Global Mercury Assessment 2013

What’s new in the Global
Mercury Assessment 2013
Global mercury emissions inventories continue toimprove as new data and better data become available
concerning some sources. The method for compilingthe inventory of anthropogenic emissions to theatmosphere has also been revised and improved.
Differences in air pollution control technologies anddifferences in the mercury content of raw materials
and fuels in different countries and regions have beenfactored into emissions calculations to better reflect
actual conditions in each country.
In the Global Mercury Assessment 2013, a new
updated inventory, based on data from 2010, ispresented in which some new sources (including
emissions from combustion of natural gas andprimary aluminium production and emissions
associated with oil refining) have been quantified forthe first time. A more detailed analysis has also been
made of some of the major mercury emission sectors,
including the break-down of emissions from coalburning in power plants, industrial and other uses.
In addition, new information acquired through the
UNEP Global Mercury Partnership area on Reducing
Mercury in Artisanal and Small-scale Gold Mining,in particular from the Artisanal Gold Council, hasresulted in a significant re-evaluation of emissions
from the ASGM sector.
New observational data and new modelling results
provide fresh insight into atmospheric mercury
transport and fate.

One of the objectives of the updated assessment has
been to provide transparent documentation andcomprehensive compilations of the data that form
the basis for all of the estimates presented. Thisapproach will allow a more consistent and replicable
method for compiling the global mercury emission
inventory, so that valid comparisons can be made in
the future. In addition, a wider range of experts fromaround the world have been involved in preparing
the Technical Background Report on which thissummary is based.
The inclusion of an assessment of releases of
mercury to the aquatic environment and its
subsequent pathways and fate is a further significant
development as these topics were not addressedin the 2008 UNEP Global Atmospheric Mercury
Assessment. The aquatic environment is critical forthree reasons:• Hundreds of tonnes of mercury is released directly
into water, so an inventory limited to mercury
emissions to air provides an incomplete assessmentof anthropogenic impacts on the mercury cycle.
• Mercury in aquatic environments can betransformed into methylmercury, which is far
more toxic to humans and animals and can enterand biomagnify in food webs more readily thanother forms of mercury.
• Much human exposure to mercury is throughthe consumption of fish and other marine foods,
making aquatic pathways the critical link tohuman health.

Kevin Telmer

A large open artisanal andsmall-scale gold mining pit.

3

UNEP Global Mercury Assessment 20132

Sources of mercury emissions
to air and releases to water

Mercury is contained in many minerals, including
cinnabar, an ore mined to produce mercury. Muchof the present day demand for mercury is met by
supply from mercury recovered from industrial
sources and stockpiles rather than from mercury
mining. Mercury is also present as an impurity
in many other economically valuable minerals, inparticular the non-ferrous metals, and in fossil fuels,
coal in particular.
Human activity, especially mining and the burningof coal, has increased the mobilization of mercury
into the environment, raising the amounts in theatmosphere, soils, fresh waters, and oceans. The
majority of these human emissions and releases of
mercury have occurred since 1800, associated with
the industrial revolution based on coal burning,base-metal ore smelting, and gold rushes in various
parts of the world. To some extent the same driversof mercury emissions and releases are continuingwith fossil-fuel-based energy generation powering
industrial and economic growth in Asia and SouthAmerica, which in turn helps drive high demand formetals including gold, spurring artisanal and smallscale gold mining (ASGM) around the world.
In preparing inventories of mercury emissions
and releases, it is important to distinguish various
categories of sources. Three main types of emissions
and releases can be distinguished, each of which isbriefly introduced here in qualitative terms, withparticular emphasis on anthropogenic sources.

Natural sources of mercury
emissions and releases
Mercury in the earth’s crust can be emitted andreleased in several ways to air, water, and land.
Natural weathering of mercury-containing rocks
4

farbled / Shutterstock.com

Mercury is a naturally occurring element and isfound throughout the world. There are thus many
natural sources of mercury, creating backgroundenvironmental levels that have been present sincelong before humans appeared.

Cinnabar: the principal ore of mercury.
Year
2000
Mt St Helens (1980)

1950
WWII manufacturing

1900

Industrial
periodKrakalau (1883)

Pre-industrial
periodGold rush (circa 1850-84)

1850

Tambora (1815)

1800
1998 core
1991 core
Natural background
Natural events (volcanic eruptions)
Local/regional anthropogenic sources
Global anthropogenic (industrialization)

1750

1700

0

5

10

25
15
20
Total mercury, ng/L

30

35

40

Ice core record of deposition from Wyoming, USA. The elevatedlevels associated with the 1850-84 US gold rush probably
reflect local/regional sources rather than a global signature.
Increasing environmental levels of mercury associated withindustrialization, however, are found in environmentalarchives like this ice core around the globe.

UNEP Global Mercury Assessment 2013

Global mercury cycling
through entrapment in stable mineral compounds.
Methylmercury, the most toxic and bioaccumulative
form of mercury, which presents the greatest
health risk to humans and wildlife, is mainly
formed in aquatic environments through natural
microbial processes.

Mercury is released to the environment from natural
sources and processes and as a result of humanactivities. Once it has entered the environment,
mercury cycles between air, land, and water until
it is eventually removed from the system throughburial in deep ocean sediments or lake sediments andDeposition
to land /freshwater
80600

30060017002800

2000
3200

Geogenic

Deposition
to oceans

20002950

3700

Biomass
Soil and Anthropogenic
burning vegetation

Anthropogenic

Oceans380

RiversUnits: tonnes
Anthropogenic
NaturalRe-emissions / Re-mobilization<600

1 tonne = 1000 kilograms

Geogenic

Global mercury budgets, based on models, illustrate the main environmental compartments and pathways that are ofimportance in the global mercury cycle, and the ways in which natural and anthropogenic releases to air land and water
move between these compartments. Emissions to air arise from natural sources and anthropogenic sources, as well as
re-emissions of mercury previously deposited from air onto soils, surface waters, and vegetation.

onime / Shutterstock.com

is continuous and ubiquitous, allowing mercury toescape to air and to be washed into lakes and rivers.
Volcanoes emit and release mercury when they erupt.
Geothermal activity can also take mercury fromunderground and emit it to the atmosphere andrelease it to the deep oceans. Some recent modelsof the flow of mercury through the environment
suggest that natural sources account for about 10%of the estimated 5500-8900 tonnes of mercury
currently being emitted and re-emitted to theatmosphere from all sources.

5

UNEP Global Mercury Assessment 2013

Anthropogenic sourcesof mercury emissions
and releases
Anthropogenic sources of mercury emissions
account for about 30% of the total amount of
mercury entering the atmosphere each year.
As found in previous global mercury assessments, themain industrial sources of atmospheric mercury arecoal burning, mining, industrial activities that processores to produce various metals or process otherraw materials to produce cement. In these activities,
mercury is emitted because it is present as an impurity
in fuels and raw materials. In these cases, mercury
emissions and releases are sometimes referred to as‘by-product’ or ‘unintentional’ emissions or releases.
A second category of sources includes sectors where
mercury is used intentionally. Artisanal and smallscale gold mining (ASGM) is the largest of these, inwhich mercury emissions and releases result fromthe intentional use of mercury to extract gold fromrocks, soils, and sediments. Other intentional-use
release sectors include waste from consumer products(including metal recycling), the chlor-alkali industry,
and the production of vinyl-chloride monomer.
Coal burning, and to a lesser extent the use of otherfossil fuels, is one of the most significant anthropogenic
source of mercury emissions to the atmosphere. Coaldoes not contain high concentrations of mercury, but
the combination of the large volume of coal burned
and the fact that a significant portion of the mercury
present in coal is emitted to the atmosphere yieldlarge overall emissions from this sector. The mercury
content of coal varies widely, introducing a high degreeof uncertainty in estimating mercury emissions fromcoal burning. Data on mercury content is now available
from many countries, including major emittersof mercury.
Mining, smelting, and production of iron andnon-ferrous metals are also a large source of
global mercury emissions to air, and also a very
important sector with regard to releases to water. In
the mining and processing of metals, most of themercury is captured and either stockpiled or soldfor use in various products, creating anthropogenic
sources associated with intentional use, discussed
below. Nonetheless, the volume of ores and metals
involved result in large total emissions and releases.
The relatively small volume of primary mercury
production makes mercury mining a far smaller
source today than it has been in the past.
6

Cement production, which typically involves theburning of fossil fuels to heat the materials required tomake cement, is another major anthropogenic sourceof mercury emissions. Both the raw materials and thefuel may contain mercury and lead to emissions. The
amount of mercury involved varies greatly with themercury content of these fuels and especially withthe raw materials. In some countries, cement kilns
are burning increasing amounts of alternative fuels,
including wastes that may contain mercury. This may
add to the emissions from cement kilns.
Oil refining emits and releases mercury, as oil
deposits are known to contain mercury, generally
at low concentrations. Mercury is removed frommost petroleum products and natural gas prior to
combustion, and therefore combustion-related
emissions are low. Most of the mercury in crude oil
is associated with solid waste that is disposed of in
landfills. However, emissions and releases duringrefining of crude oil do occur and these have been
quantified for the first time in the 2010 inventory.
The inventory does not quantify other emissions andreleases during oil and gas extraction and transport
or from flaring.
Unintentional mercury emissions from these sectors
can be reduced by the application of pollution
control measures at power plants and industrial
plants. Some of the mercury captured is refined andenters the commercial supply chain; however, large
amounts of mercury captured in materials such as
fly ash and oil refinery waste need to be disposed of.
Some of the resulting wastes are themselves used as
raw materials, for example in construction materials,
but large amounts are disposed of in landfills, which
can thus become a potential source of mercury
emissions and releases.
Among intentional-use sectors, Artisanal and smallscale gold mining is a major source for emissions
and releases of mercury worldwide. In ASGM, miners
use mercury to create an amalgam separating gold
from other materials. They then have to separate themercury from the gold. Calculating emissions fromthis sector presents a particular challenge because it
is typically widely dispersed and often unregulated
and may be illegal. Uncertainties regarding release
estimates from the ASGM sector are therefore high.
Furthermore, the miners are typically poor andperhaps have little awareness of the hazards of mercury,and pollution control devices may be hard to obtain.
Wastes from consumer products containing
mercury can end up in landfills or incinerators.

UNEP Global Mercury Assessment 2013

Mercury is also used as a catalyst in the productionof vinyl chloride monomer (VCM) from acetylene,
mainly in China.
Mercury releases to aquatic systems as a result of
current human activities arise from many of the sameuses or the presence of mercury in various products
and processes that emit mercury to the air. As withemissions to the atmosphere, aquatic releases comefrom two main sources. First, mercury is released
with water effluent from the sites where mercury isused. Second, mercury can leach into water from
disposal sites that have mercury in the waste.

bikeriderlondon / Shutterstock.com

Mercury is also used in a number of industrial
processes. A major industrial use is in the chlor-alkaliindustry where mercury-cell technology may be usedin the production of chlorine and caustic soda.

Tomas Pavelka / Shutterstock.com

The amount of mercury released in the recycling ofscrap metals, for example in secondary steel and nonferrous metal production, is generally assumed to bemuch lower than that released during primary metal
production, which is why the present global inventory
only addresses primary metal production. However,lower emissions from recycling may not be the case in
all countries. Much of the scrap steel in some countries
comes from automobiles that may still have devices
that contain mercury, and which may not be removedprior to recycling of the steel. These devices are largely
being phased out so mercury emissions from scrap
steel may be expected to decrease. However, large
differences exist between countries in the way theytreat their mercury-containing wastes, including scrap
metals that are recycled in secondary metal production.

zhu difeng / Shutterstock.com

Another use of mercury is in dental amalgam forfilling teeth. When bodies are cremated, mercury infillings can be emitted. Mercury can also be emitted
and released during production and preparation offillings and from the disposal or removed fillings.
In addition, mercury from removed fillings can berecycled or go into solid waste and wastewater.

Gunnar Futsæter

Mercury is still used in a wide range of products,
including batteries, paints, switches, electrical andelectronic devices, thermometers, blood-pressuregauges, fluorescent and energy-saving lamps,
pesticides, fungicides, medicines, and cosmetics.
Once used, many of the products and the mercury
they contain enter waste streams. While mercury
in landfills may slowly become re-mobilized tothe environment, waste that is incinerated can bea major source of atmospheric mercury, especiallyfrom uncontrolled incineration. Incinerators withstate-of-the-art controls have low emissions.

Anthropogenic sources of mercury from industry andintentional use.
7

UNEP Global Mercury Assessment 2013

Re-emission and remobilization of mercury

of aquatic sediments due to wave action or storm
events is an additional way for mercury to re-enter
the aquatic ecosystems.

Re-emissions constitute the third category ofsources, presently comprising about 60% of mercury
emissions to air. Mercury previously deposited fromair onto soils, surface waters, and vegetation from pastemissions can be emitted back to the air. Re-emission
is a result of natural processes that convert inorganicand organic forms of mercury to elemental mercury,which is volatile and therefore readily returns to
the air. Mercury deposited to plant surfaces can bere-emitted during forest fires or biomass burning.
Mercury may be deposited and re-emitted manytimes as it cycles through the environment.

Estimating re-emission and re-mobilization rates isdifficult. It is often done using modelling approaches.
These models are based on data on atmosphericlevels and other observations as well as current
understanding of chemical transformations andother processes that determine how mercury moves
between air, land, and water. The models aim tobalance the amount of mercury in circulation at
any given time while remaining consistent withobservational data. Temperature is a key factor.
With lower temperatures, re-emission rates are
generally lower.

It is important that re-emitted mercury should not
be considered a natural source. It may originallyhave come from natural or anthropogenic sources,
but by the time it is re-emitted, it is difficult or
impossible to identify its specific origin. Nonetheless,human activity has increased the environmental
burden of mercury, resulting in higher levels of reemission. This is compounded by changes in land
use practices as well as increasing temperatures due
to climate change.

Re-emission is also a major factor in determining
the length of time needed for anthropogenic
emission reductions to be reflected in decreasing
environmental levels of mercury. Mercury emitted
in one year may be deposited to and retained insoils and waters for some time before being reemitted or re-mobilized in subsequent years. This
cycle can be repeated, keeping levels in air andwater elevated even after anthropogenic sources
have been lowered. Conversely, continuing to add
to the global pool will leave an ever-longer legacy ofanthropogenic mercury contamination worldwide. It
is thus imperative that international efforts to reduce
mercury emissions continue and are strengthened assoon as possible.

In the aquatic environment, re-mobilization of
mercury occurs when mercury deposited on andaccumulated in soils or sediments is re-mobilized by,for example, rain or floods that cause the mercury toenter or re-enter the aquatic system. Resuspension
Arnold John Labrentz / Shutterstock.com

Forest fires re-emit mercury deposited to vegetation.
8

UNEP Global Mercury Assessment 2013

3

Anthropogenic emissions
to the atmosphere
Global emissions inventory
The global emissions inventory for 2010 estimates that1960 tonnes of mercury was emitted to the atmosphereas a direct result of human activity. The leading sectors
remain the same as those identified in the 2005
inventory. Improved data, however, have changed therelative contributions of some of these sectors, as havesome actual changes in emissions. All coal burningemissions taken together, for example, represent alower percentage of the total emissions than in the 2005
inventory. This is due in part to the increased estimatesfrom artisanal and small-scale gold mining (ASGM),
and in part to much lower estimates for domestic use ofcoal. When the actual amount of mercury emitted fromcoal combustions in power generation and industrial
uses is considered, and the estimates are based on thesame 2010 methodology, the emissions in 2010 are thesame and perhaps slightly higher than in 2005. Even
though new coal-fired power plants are being built,
combustion efficiency and emissions controls are also
improving in most parts of the world.

Results by selected sector
The updated inventory of emissions to air confirms
coal burning as a continuing major source ofemissions, responsible for some 475 tonnes of
mercury emissions to air annually, compared witharound 10 tonnes from combustion of other fossil
fuels. According to the new inventory, more than
85% of these emissions are from coal burningin power generation and industrial uses. In theprevious assessment, emissions from domestic
and residential coal burning were highlighted asa possible larger contribution. Better informationon coal consumption for domestic and residential
uses indicates that these activities are a smaller
contribution to total emissions from coal burning
than previously thought.
Emissions from the cement production industry
are largely dependent on the raw materials andthe fuels used. The new inventory avoids double

Emissions from various sectors, in tonnes per year with the range ofthe estimate, and as a percentage of total anthropogenic emissions.
Note: These numbers cannot be compared directly with thosepresented in the 2008 assessment (see Chapter 4, Trends inmercury emissions to the atmosphere).
SectorEmission (range),
tonnes*%**Coal burning (all uses)

474 (304 - 678)

24

Oil and natural gas burning

9.9 (4.5 – 16.3)

1

45.5 (20.5 – 241)2

Primary production of nonferrous metals (Al, Cu, Pb, Zn)

193 (82 – 660)

10Large-scale gold production

97.3 (0.7 – 247)

5

Mine production of mercury

11.7 (6.9 – 17.8)<1173 (65.5 - 646)

9

Oil refining

16 (7.3 - 26.4)

1

Contaminated sites

82.5 (70 - 95)

4Artisanal and small-scale
gold mining

727 (410 – 1040)

37

Chlor-alkali industry

28.4 (10.2 – 54.7)

1

Consumer product waste

95.6 (23.7 – 330)5

3.6 (0.9 - 11.9)<1

1960 (1010 – 4070)

100

By-product or unintentional emissions
Fossil fuel burning

Mining, smelting, & productionof metals
Primary production offerrous metals

Cement production

Intentional uses

Cremation (dental amalgam)
Grand Total* Values rounded to 3 significant figures.
** To nearest percent
Sectors for which emissions are not currently quantified
biofuel production and combustion
vinyl-chloride monomer production, emissions duringsecondary metals production and ferro-alloys
oil and gas extraction, transport and processing other than refinery
emissionsindustrial / some hazardous waste incineration and disposal
sewage sludge incineration
preparation of dental amalgam fillings and disposal of removed
fillings containing mercury

9UNEP Global Mercury Assessment 2013

Mercury-cellchlor-alkaliindustry

Disposal of waste from
mercury-containing
products

Cremation

Coal combustion
Primary ferrous
metal production
Oil and natural
gas combustion

Primary
non-ferrous
metal(Al, Cu, Pb, Zn)
Artisanal andsmall-scale
gold production

Large-scale
gold production

Hg production
Contaminated
sites

Oil
refining

Cementproduction

Relative contributions to estimated emissions to air fromanthropogenic sources in 2010.

Mercury-cell technology is becoming less common
in the chlor-alkali industry as other, more costeffective processes are adopted. No new plants arebeing constructed, though many older plants remain
to be converted. Old chlor-alkali plants and otherdecommissioned industrial sites may constitutecontaminated sites that continue to release mercury to
the environment for many years and emissions fromcontaminated sites are now part of the inventory.
Global emissions from use of mercury in dental
amalgam resulting from cremation of humanremains are estimated at 3.6 (0.9 – 11.9) tonnes in2010. Some 340 tonnes of mercury is used per year
in dentistry, of which about 70-100 tonnes (i.e. 2030%) likely enters the solid waste stream.
In the production of vinyl chloride monomer,
information is still lacking on the lifecycle andeventual fate of the mercury catalyst. Most of thisproduction occurs in China, and about 800 tonnes of
mercury is thought to have been used by this industry
in China in 2012. Used mercury catalyst is recycled
and reused by enterprises that hold permits forhazardous waste management. The amounts that may
be emitted or released are unknown.
Ronald de Hommel

counting of emissions from conventional fuels(such as coal and oil) that are included under theindustrial fossil fuel burning emissions. However,it does attempt to account for emissions from otherfuels, including alternative fuels (such as old tyres
and other wastes) and from raw materials. Increasing
amounts of waste are being co-incinerated in the
cement industry both as fuel but also, in some plants,
as a means of disposing of hazardous wastes, someof which may contain mercury. In some regions,additional measures are being introduced to makesure that mercury emissions associated with waste
co-incineration do not increase overall emissions
from cement plants.

recent information on this sector in China is lacking.
China prohibited ASGM in 1996 and therefore
records no emissions from this sector.

Increased application of air pollution control devices,
including some mercury-specific technologies,
together with more stringent regulations in several
countries have the effect of reducing mercury
emissions from coal burning sectors and thus
offset some part of the emissions arising fromincreased activity.
Artisanal and small-scale gold mining emissions
are, in the 2010 inventory, the major source ofemissions to air, at 727 tonnes per year. The reasonsfor the large increase in the estimate compared
to 2005 are discussed below (see page 16). The
global estimate for emissions from ASGM includes
a significant contribution from China, although
10

Artisanal and small-scale gold mining. Unlike most small-scale
gold miners, the green gold miners of Oro Verde, shown here,
employ an environmental way of mining gold that does not use
mercury or other chemicals.

UNEP Global Mercury Assessment 2013

Results by region
New data on emissions from ASGM have increased
the proportion of global emissions attributed toSouth America and Sub-Saharan Africa, largely due
to increased estimates associated with improved
information about ASGM activities in these areas.

The greatest proportion of anthropogenic mercury
emissions to the atmosphere comes from Asia,
which contributes about 50% of the global total. The
majority of Asian emissions come from East andSoutheast Asia. China accounts for three-quartersof East and Southeast Asian emissions, or about one
third of the global total.

Emissions to air from other major source sectors arehigher in Europe, North America, and Oceania.

Mercury emission 2010, g/km2
02

5

10 100 1000

Global distribution of anthropogenic mercury emissions to air in 2010.

Emissions from various regions, in tonnes per year with the range of the estimate, and as a percentage of total global
anthropogenic emissions. Note: These numbers cannot be compared directly with those presented in the 2008 assessment(see Chapter 4, Trends in mercury emissions to the atmosphere).
Region*Emission (range), tonnes**%

Australia, New Zealand & Oceania

22.3 (5.4 - 52.7)

1.1

Central America and the Caribbean

47.2 (19.7 - 97.4)

2.4

CIS & other European countries

115 (42.6 - 289)

5.9

East and Southeast Asia

777 (395 - 1690)

39.7

European Union (EU27)

87.5 (44.5 - 226)

4.5

Middle Eastern States

37.0 (16.1 - 106)

1.9

North Africa

13.6 (4.8 - 41.2)

0.7

North America

60.7 (34.3 - 139)

3.1

South America

245 (128 - 465)

12.5

South Asia

154 (78.2 - 358)

7.9

Sub-Saharan Africa

316 (168 - 514)

16.1

Undefined (global total for emissions fromcontaminated sites)

82.5 (70.0 - 95.0)

4.2

1960 (1010 – 4070)

100

Grand Total* S ee figure on the following
page for map withspecification of regions.
**Values rounded to 3
significant figures.

11

UNEP Global Mercury Assessment 2013

North America

EU27

CIS & other
European countries

500

100

50
10

tonnesCentral America andthe Caribbean

North Africa

Middle Eastern States

East and Southeast Asia

South America

Sub-Saharan Africa

South Asia

Australia, New
Zealand & Oceania

Non-ferrous metals(Cu, Pb, Zn, Al)
Mercury

North America

Unintentional emissions
Stationary combustion (coal)
Stationary combustion(oil and gas)
Oil refining
Pig iron and steel

Large-scale gold
CementCIS & other
European countries

Middle Eastern
East andNorth Africa States
Southeast Asia
South Asia
Sub-Saharan Africa

South America
Australia, New Zealand& Oceania

Intentional uses
Artisanal and small-scale
gold mining
Chlor-alkali

Central America
and the Caribbean

EU

Product waste
Cremation

Estimates of 2010 anthropogenic mercury emissions to air from different main sectors in different regions. ASGM is shown
separately to highlight its geographic distribution and better allow regional comparisons to be made for other sectors.
12

UNEP Global Mercury Assessment 2013

Compiling the 2010 inventoryof anthropogenic mercury
emissions to air
The inventories used in successive Global Mercury
Assessments continue to improve as better data
become available. The 2013 update presents aninventory of emissions for 2010 that has a numberof improvements over the 2008 assessment and its
inventory for 2005:• A more detailed analysis of emissions from somemajor source sectors. For example, fossil fuelconsumption is now broken down into categories
for combustion in power plants, industry, andother uses. The type of coal or oil used is also
considered, providing a more accurate estimate of
mercury emissions.
• A more detailed consideration of the mercury
content of fuels and raw materials used in different
countries and regions.
• New and updated information on ASGM.
• The use of different pollution control technologies
in different countries and regions have beenfactored into the emissions estimate.
• Emission estimates for sectors not previously
included, such as aluminium production, oil
refining, and contaminated sites.
• More and better information on location of major
point sources such as individual power plants,
smelters and cement kilns.
• Better documentation and greater transparency
with respect to the data and information behindthe estimates.
The methods for estimating emissions fromindustrial sectors is complemented by othermethods used for more dispersed emissions sources.
For example, estimating emissions from someintentional-use sectors requires intensive work withthose sectors. Relevant and accurate information
about ASGM is generally not available throughofficial channels. Estimating mercury emissions
from its intentional use in lighting, batteries, andother products requires analysing the entire
production, use, and waste stream to determine
where mercury is likely to be released and inwhat quantities. The approach used in the 2013assessment addresses the emissions from breakages
and wastes resulting from use of these mercurycontaining products in society, the majority ofwhich are incinerated or end up in landfills. It does
not, however, address industrial wastes or sewage
sludge incineration.

Uncertainties inemission estimates
Estimates of mercury emissions are just that:
estimates. To compile a global assessment
requires making a number of assumptions andgeneralizations. Uncertainty associated with the2010 inventory arises from each of the factors used
to estimate the emissions: the correctness of theactivity data, the validity of the emission factors
applied, and the validity of assumptions regarding
the effectiveness and use of emission-control
technologies.
Based on an evaluation of these sources of
uncertainty and the relative contributions of sectors
with reliable information and those with less reliable
data, uncertainty in the 2010 inventory assessmentof total anthropogenic mercury emissions to
air gives a range of emissions from 1010-4070
tonnes. This range is greater than that reported
in some previous assessments, reflecting a greaterappreciation of the sources of uncertainty due tothe improved estimation methods and perhaps amore realistic appraisal of the state of knowledge
concerning some aspects of emissions.
The best estimate, taking a conservative approach,is 1960 tonnes. If nationally produced estimates for2010 available from some countries (Canada, Japan,
Korea, Mexico, the United States and European
countries reporting to LRTAP) are introduced
into the global inventory in place of the inventory
estimates, the corresponding estimated total global
anthropogenic emission to air is 1940 tonnes. As
most national inventories do not include uncertainty
ranges, it is not possible to assign a range to thisnumber. The recognition of uncertainties is an
important consideration and presenting single
national estimates can convey a misleading pictureof what is known and, more importantly, not known
about emissions.
The numbers derived using the methods employed
to produce the global inventory for 2010 werecompared with a number of national inventories andemissions reported under other systems covering thesame period. In general, the level of agreement wasgood, in particular when the significant uncertainties
13

UNEP Global Mercury Assessment 2013

Methods for estimating emissions
All global mercury emissions inventories to date haveused the same basic approach for the major sectors
emitting mercury to the atmosphere. For specificemission sectors, national emissions estimates arecalculated by multiplying the amount of activity (i.e.
amounts of fuels burned, raw materials consumed, ormaterials produced) by an emission factor that is an
estimate of the mercury emitted per unit of activity.
For example, emissions from coal-fired power
generation are estimated by multiplying the tonnesof coal used by the amount of mercury estimatedto be released per tonne of coal. In most suchinventories, “abated” emission factors have been
employed to quantify both the emissions of mercury
that occur during the processes and the effects ofmercury emission controls. In the new methodology
used to develop the 2013 report these components
are split. “Unabated” emission factors are employed
to quantify the emissions, and “technology profiles”
have been developed to represent the effects ofmercury emission controls.
Various methods are employed to estimate emissionsof mercury at individual sources and at national,
regional and global levels. In general, the methods fallunder one of two main categories:• Mass-balance/substance-flow based estimates,
which are based on the principle that what goes inmust come out. Amounts of mercury in fuels andraw materials constitute the inputs; and the outputs
are the amounts of mercury emitted to air, released
to water or land, retained in products or in wastes,or otherwise recovered and stored or disposed of.

involved in both global and national estimates aretaken into account. It is important to note that there
are good reasons why estimates of mercury emissions
produced in the 2010 inventory may not fully agreewith national inventories made by countries using
other methodologies. These reasons include:• Reporting schemes may define and distinguishemissions source sectors in very different ways,
and aligning these sectors may not be possible.
• Industry reporting to national government may belimited to sources with emissions above a certain
threshold level so that emissions from smaller
sources, below the threshold, are not reported.
Where smaller sources make up a significant
part of the source category, reported inventories

14• Measurement-based estimates, which rely on
measurements made at appropriate points in theindustrial process or in the product/waste output
streams. These measurements are used to estimate
where the mercury goes, as in the method above.
In principle the two approaches should produce
the same results. In practice, mass-balance based
approaches tend to result in higher emission
estimates than most measurement-based estimates.
Since future emissions reporting is likely to involve a
combination of these two approaches, further work is
required to understand why results may differ and toreconcile the two approaches.
Since the 2008 UNEP assessment was produced, thenumber of direct measurements of emissions fromcertain point sources (in particular power plants andsome metal and cement production plants and waste
incineration facilities) has increased considerably,
resulting in a much improved knowledge base.
A number of countries require regular reporting ofemissions. Increasing use is being made in thesereporting systems of measurement-based estimatesand facility-level reporting, in particular for major point
sources. In other countries, national mercury emissions
are only being quantified for the first time. Since
2005 and the start of the UNEP negotiating process in2010, many countries have initiated work on nationalemission inventories which, in several cases, haveyielded much improved information on activity data,
sector characteristics and mercury emissions. Suchinventories often make use of the UNEP Toolkit for
identification and quantification of mercury emissions.

may therefore significantly underestimate total
mercury emissions.
• National inventories in some countries are making
increasing use of actual measurements of mercury
emissions at individual facilities. At the global
scale it is not yet feasible to base an inventory on
individual site emissions. Furthermore, actual
measurements of mercury emissions at a source
may be taken only a few times during a year
and may not be fully representative of normal
operations. In the 2013 assessment, a mass-balance
based approach was employed to be consistent,transparent, and replicable.
• National reporting and monitoring schemes
may provide information that is not available toexternally produced inventories.

UNEP Global Mercury Assessment 2013

Despite these considerations, most of thediscrepancies noted between the 2010 inventorymass-balance-based estimates and national estimates
based on measurement- approaches are modest
and within the margins of error associated with thedifferent approaches. In cases where the differences
are larger, it is important to recognize that there
are sources of error in all methods for estimating
mercury emissions.

Gunnar Futsæter

Sampling of mercury
emissions at the Kendal coalfired power plant in South
Africa, conducted under a
UNEP project. A probe withmercury traps is inserted intoa sampling port in the stack
to collect mercury present
in the flue gas. The mercury
traps are subsequently
analysed according to the US
EPA Mercury Monitoring
Toolkit sampling protocol.

15

UNEP Global Mercury Assessment 2013

Trends in mercury emissions
to the atmosphere
Assessing global andsectoral trends
A key question in assessing anthropogenic
mercury emissions worldwide is whether they
are increasing or decreasing. While it is tempting
simply to compare the 2010 inventory with its
2005 counterpart, the results would mean little.
As inventory methods develop and as additionalsources are considered, the estimate of total
anthropogenic mercury emissions worldwide alsochanges. While some changes reflect real trends,
others reflect changes in methods or scope. Thus,
comparing the results of previous assessments withthe present update requires considerable care andinvolves an understanding of the way in whichthese inventories have been produced and theuncertainties associated with them.
It is possible, however, to compare certain sectors
over time by attempting to compensate for any
methodological changes between the various
inventories since 1990.
Emissions to air are thought to have peaked in the
1970s. From 1990 to 2005, total anthropogenic
emissions of mercury to the atmosphere appear to
Emissions to air, t
1400

1000
800

1995
1990

600
400

AfricaAsia

EuropeRussiaNorth Oceania SouthAmerica
America

Estimates of annual anthropogenic mercury emissions fromdifferent continents/regions, 1990-2005.
16

The changes introduced in the 2010 inventory
methodology prevent a straight-forward continuationof this analysis. However, an evaluation of trends forsome sectors between 2005 and 2010 has been madeby applying the new methodology to activity datafor 2005. This comparison suggests a further slightincrease in the combined amount of mercury emitted
by industrial sectors (coal combustion, production of
cement, pig iron and steel, and non-ferrous metals).
The following paragraphs describe the trend results forthree sectors, to illustrate some of the developments
between 2005 and 2010, and some of the thedifficulties in comparing emissions estimates between
different years, made with different methods.
Coal burning for power generation and forindustrial purposes continues to increase, especiallyin Asia. However increases in the application of
air pollution controls, including some mercury
specific technologies, together with more stringent
regulations in a number of countries have reduced
mercury emissions from coal burning in power
plants in particular, and thus offset some part of theemissions arising from increased coal consumption.

In China, many of the new coal-fired power plants
have state-of-the-art pollution controls installed.

200
0have been relatively stable, with decreases in Europe
and North America being offset by increases in Asia.
A reanalysis of the available inventories since 1990
however, indicates that emissions from industrial
sectors at least may be starting to increase again.

In the United States, for example, emissions from coalburning at power plants have reportedly decreased
from about 53 tonnes in 2005 to 27 tonnes in 2010.
This decrease is largely due to new regulations thathave resulted in changes in the sources of the coal thatis burned in large power plants and the installationof mercury controls as well as controls on sulphur
dioxide and particulates that have the co-benefit offurther reducing mercury emissions.

2005
2000

1200

4

Emissions of mercury from artisanal and smallscale gold mining (ASGM) reported for 2010 are
more than twice those reported for 2005. While theUNEP Global Mercury Assessment 2013

Artisanal and small-scale gold mining

Intentional-use sectors

1000

1000

1000

800

800

600

600

Unintentional emission sectors
Emission, t

Step change from 2005 to2010 due to new information(under-estimation in 2005?)

800

600

?
400

400

400

200

200

200

0

0
2005

2010

0
2005

2010

2005

Australia, New Zealand & Oceania

European Union (EU27)

South America

Central America and the Caribbean

Middle Eastern States

South Asia

CIS & other European countries

North Africa

Sub-Saharan Africa

East and Southeast Asia

North America

2010

Unintentional emission sectors: Coal burning, ferrous- and non-ferrous (Au, Cu, Hg, Pb, Zn) metal production, cement production. Intentional-use
sectors: Disposal and incineration of product waste, cremation emissions, chlor-alkali industry.

Comparison of emissions in 2005 and 2010, by selected sector and region.

Waste from consumer products is affected by theamount of mercury used. For most products inwhich mercury is used, mercury-free alternatives
exist. Consequently, many of these uses of mercury
are declining, at least in some regions, as alternative
products or processes are adopted. Compact
fluorescent light bulbs are an exception. Even
though the mercury content of individual lightbulbs has decreased, use of this type of light bulb isincreasing rapidly.

In order to make valid assessments of trends inemissions from global inventories, comparable
data on activity levels are required, together withinformation on changes in fuel and raw material
characteristics and applied air pollution control
technology. One aim of the 2010 inventory methods
is to create a firmer foundation for such future
trend analysis.
Ralf Siemieniec / Shutterstock.com

rise in the price of gold (from USD 400 per ounce
in 2005 to USD 1100 per ounce in 2010), along withincreased rural poverty, may indeed have caused
more activity in this sector, the increased estimate
for mercury emissions is considered to be due
primarily to some more and better data from many
countries and regions. West Africa, for example, wasregarded as having minimal ASGM in 2005, but is
now recognized as a region with considerable activity.
Thus, the baseline has improved, without necessarily
any change in actual activity or emission levels.

Use of mercury-containing energy-saving lamps is increasing.
17

UNEP Global Mercury Assessment 2013

Emission scenarios andfuture trends
The 2008 Global Atmospheric Mercury Assessment
included a first attempt at projecting future
emissions inventories. These were based on three
scenarios: continuing with the status quo, applying
current emissions controls worldwide, and achieving
maximum feasible technological reductions. At thetime, this effort was regarded as highly provisional.
Since then, additional studies have extended
and improved this work, but the results are notmarkedly different from earlier projections of future
mercury emissions.
Global mercury models have been used to evaluate
future scenarios. Four global and hemisphericmodels projected mercury levels around the worldin 2020, based on the three emission scenarios
described earlier. In industrial regions, the statusquo will cause an increase in mercury levels of
2-25%, and in remote areas of 1.5-5%. The twoemission control scenarios, on the other hand, yielda decrease in mercury in industrial areas of 25-35%,
and in remote areas of 15-20%.
Another global model was coupled with estimatesof surface reservoirs of mercury in order to quantify
source-receptor relationships for the present andfor 2050. Under the best-case scenario of maximum
feasible reductions, projected estimates fordeposition in 2050 is similar to estimates for today.
In the other scenarios, increasing emissions leadto increasing deposition. A greater proportion of
mercury emissions is expected to be in the oxidized
form, so that a greater proportion will be depositednear the source instead of being transported far away.
The model results support the conclusion that
reducing anthropogenic emissions will slowly reduce
the amount of mercury in biologically available
reservoirs. Over time, mercury in the environment
will be taken out of circulation by natural processes,
for example from ocean waters down into sediments,
and biologically available mercury will decrease.
Increased emissions, on the other hand, willcontinue to build up the amount of mercury incirculation.

18

With new databases and methods, it may soon bepossible to create scenarios that incorporate activity
levels as well as technology use for each country. If
this were done, it would allow countries to assess
the effects of different mercury reduction strategies
on their national emissions. For example, a country
could determine the relative contributions ofemission reduction technology as opposed to, forexample, changes in raw materials.

UNEP Global Mercury Assessment 2013

Atmospheric chemistry,monitoring, and trends
Forms of mercury in air
Mercury in the atmosphere is in three primaryforms. Gaseous elemental mercury is the mostcommon in anthropogenic and natural emissions
to the atmosphere. Gaseous oxidized mercury andmercury bound to particulates are less common.
The transport and deposition of atmosphericmercury depend greatly on whether the mercury
is elemental or oxidized. Elemental mercury
stays in the atmosphere long enough for it to betransported around the world, whereas oxidized
and particulate mercury are more readily captured
in existing pollution control systems or deposited
relatively rapidly after their formation. As aresult, most mercury in the air is in the gaseous
elemental phase. Relatively little elemental
mercury is deposited directly, but instead mustfirst be oxidized.
Although gaseous oxidized mercury is very
important in mercury cycling between air andother environmental compartments, the processof oxidation in the air is poorly understood, withreactions and resulting compounds yet to be verified
in observations.
When mercury moves from air to water and land,
it is generally in an oxidized gaseous or particulate
form, whereas when it is re-emitted to air it
has been converted back to gaseous elemental

Global transport

Gaseous elemental
mercury

Equator

Pole

Particulate associatedmercury

Gaseous elemental mercury can be transported globally.
Mercury emitted in particulate form tends to deposit closer
to sources.

5

mercury. Sunlight appears to play a large role inboth oxidation and reduction of mercury, buttemperature and biological interactions are also
likely to be involved to some degree. Here, too, much
uncertainty remains. Nonetheless, the reactions areimportant in determining net deposition and fateof mercury.

Monitoring of mercury in air
Monitoring of mercury in air focuses on the three
primary forms of mercury. The measurementof gaseous elemental mercury is routine androbust. Measuring gaseous oxidized mercury andparticulate-bound mercury, however, is challenging.
Concentrations are typically very low, and these
forms are chemically unstable, leading to highuncertainty in the measurements. Nonetheless, these
forms are critical for defining and modelling the fate
and transport of airborne mercury.
In the past two decades, coordinated mercury
monitoring networks and long-term monitoringsites have been established in a number of regions,
measuring mercury concentrations in the air aswell as deposition of mercury in precipitation.
In Europe and North America, high-quality,
continuous monitoring has been going on for
more than 15 years, especially in the Arctic. Highquality monitoring has started more recently in
East Asia and South Africa, as part of a global effort
to expand the coverage provided by long-termmonitoring sites.

Measurements and trends inatmospheric mercury
Monitoring stations around the world have provided
information about trends in atmospheric mercury,though the time periods vary depending on how
long the site has been active. Overall, a declining
trend in background mercury levels over the past
decade has been recorded from monitoring stations
19

UNEP Global Mercury Assessment 2013

in many regions. Other regions, however, show anincrease in mercury levels.

mercury and local and regional influences. Mercury
concentrations at remote sites in Asia are higher
than in other regions of the Northern Hemisphere.
Coastal cities in China have lower levels than inlandsites, likely due to the influence of relatively clean

The sites also provide information about geographicalpatterns, reflecting both background levels ofTrends in atmospheric measurements of mercury
Site(s)PeriodMeasurement

Trend

Mace Head, Ireland

1996-2011

Gaseous elemental mercury

Decrease of 1.4-1.8% per year

North America, rural sites

1995-2005

Total gaseous mercury

Decrease of 2.2% to 17.4% in total

High Arctic, sub-Arctic,
mid-latitudes

Up to 20 years of records

Total gaseous mercury

Decreasing trend at some stations,increasing at others

Monitoring networks
Mercury monitoring in Europe is carried out under
the European Monitoring and Evaluation Programme(EMEP), one of the first international environmental
measurement networks. Heavy metals such as
mercury have been included in the EMEP program
since 1999. Mercury measurements are available only
from northern and northwestern Europe.
Three monitoring networks currently operate in North
America, providing good coverage of Canada and theUnited States: the Mercury Deposition Network, theCanadian Air and Precipitation Monitoring Network,and the Atmospheric Mercury Network. More recently,
new sites have been established in Mexico, extending
coverage on the continent. The stations monitor
atmospheric mercury and mercury in precipitation,
which has measured since the mid-1990s. Monitoringof mercury in the air and in precipitation has been
underway in Asia for nearly a decade.
Alert

The monitoring network of the Arctic Monitoringand Assessment Programme (AMAP) includes air anddeposition monitoring sites located in Arctic regionsof Canada, Greenland, Iceland, Norway, Russia,
and Sweden.
Building on existing national and regional monitoring
networks, the European Union-financed project“Global Mercury Observation System” (GMOS)
started in November 2010. Its goal is to develop acoordinated global system for monitoring mercury,including a large network of ground-based monitoring
stations. New sites are being installed in regions
where few monitoring stations exist, especially in
the Southern Hemisphere. Two sites have been
established in Antarctica, one on the Antarctic
Plateau and one on the coast.

Nord

Ny-Ålesund

Point Barrow
NuukRåö Bredkälen

Mace Head

Mauna Loa

Celestún

Listvyanka

Diabla Gora

NiembroMt Bachelor

Amderma

PallasStórhöfði

IskrbaCap Monte Cimone
Ferrat Longobucco

Mt Changbai
Mt Waliguan
Ev-K2

Mt Ailao

CalhauGanghwa Island
Minamata
Cape Hedo

Lulin

Nieuw
Nickerie

Kodiakanal
Mahé Island

ManausCape Point

Amsterdam Island

Bariloche

Cape Grim

Dumont d’Urville

AMAP

CAMnetEMEP

GMOS

Air + precipitation

20

GMOS associatedAir

NADP/AIRMoN

Precipitation

NADP/MDN

UNEP Global Mercury Assessment 2013

John Munthe

The mercury background air monitoring station at Zeppelin mountain, Svalbard.

air from over the ocean. Sites off the Asian coast,
however, show higher levels of gaseous elemental
mercury than the Northern Hemisphere background,suggesting an outflow from the Asian mainland.
Weather patterns have also been found to have astrong influence on seasonal patterns in mercury
levels in the air at monitoring sites. At the Mount
Waliguan Observatory on the Tibetan Plateau, forexample, northeasterly and easterly winds produced
the highest levels of mercury.

Measurements athigh altitudes
Measurements from ground-based stations above 2700
meters suggest that there is an inverse relationshipbetween gaseous elemental mercury and gaseousoxidized mercury at these elevations. In other words,
when there is more elemental mercury, there is lessoxidized mercury, and vice versa. These measurements
are in good agreement with modelling results, which
gives additional confidence in current understanding of
the behaviour of mercury at high altitude.
An inverse relationship between total gaseous
mercury and particulate concentration has also
been observed in the high stratosphere. From thetransformation rate of total gaseous mercury toparticulate-bound mercury, it appears that gaseous
mercury lasts about two years in the stratosphere.
21

UNEP Global Mercury Assessment 2013

6

Atmospheric concentrations
and deposition
Measurements and trends ofmercury in precipitation
Mercury deposited by precipitation decreased fromthe 1990s into the 2000s at many, but not all, sites
in North America and Western Europe where suchmeasurements were taken.
Regional patterns are also evident, including slightly
higher levels in southern Europe than northern Europe,and higher levels in Asia than in North America. Total
mercury concentrations in precipitation in China weremuch higher than those in Japan and South Korea.
This was mostly attributed to higher levels of gaseousoxidized mercury and particulate-bound mercury,which are readily deposited by precipitation and thus
not transported far.
A comparison of data from Canadian and American
monitoring sites found that deposition of mercury in
precipitation was highest in summer.
Dry deposition of mercury, which is deposition
not associated with precipitation, is difficult tomeasure, and consequently there is little information
on trends in mercury dry deposition. This lack of
information means that it is also difficult to validate
model results for dry deposition.

Modelled mercury air
concentrations anddeposition patterns
Atmospheric mercury concentrations are highest
in major industrial regions. Models examining
global patterns show that East and South Asia,
Europe, North America, and South Africa havethe highest levels of elemental mercury in the air.
Concentrations are generally lower in the Southern
Hemisphere than the Northern Hemisphere because
most industrial activity is in northern regions. Thesemodel results are based on simulations using the2005 global mercury emission inventory because the2010 inventory has only recently become available.
Intercontinental transport of mercury in the air has
been examined to determine its impact on regional
mercury levels. Four models using different parameters
produced consistent results. Depending on the region,
mercury emitted by distant sources contributes
between 10% and 30% of annual mercury deposition.
Where local sources are low, mercury transported
from distant emission sources can account for morethan half of the deposition. East Asia is the dominantsource region, adding 10-14% to the deposition inContribution from sources
outside receptor region, %
40

10.6%

Model
GEOS-Chem
GRAHM

4.4%

7.2%

47.4%

9.1%

GLEMOS
CMAQ-Hg

30

20
5%

7%

7.1%

10

0.5

1

5

10

100

500

1000

Total mercury emission (2005), kg/cell

0

ArcticEuropeNorth
America

East Asia

South Asia

Receptor region(Left) Global distribution of anthropogenic mercury emissions in 2005, with source regions that were considered in the analysis(North America, South America, Europe, Africa, Central Asia, South Asia, East Asia, Australia and Oceania) and (right) thecontribution of anthropogenic sources outside a particular receptor region to deposition within the receptor region.
22

UNEP Global Mercury Assessment 2013

Global models reproduce the movement of mercury
in the atmosphere, during which the mercury
may move between continents. They have beeneffective in examining sources of mercury to
the Arctic, and also for examining the outflow ofpollution from South and Southeast Asia and its
impact on western North America, as well as theflow from eastern North America to Europe.

benefits of proposed emission control measures
found that mercury transport from outside thecountry accounts for 68% of mercury depositionin the Northeast, and up to 91% in the west-central
United States. Large point sources contribute up
to three-quarters of nearby deposition and power
plants contributed half of the deposition in theeastern part of the country. This suggests that new
emissions standards would primarily benefit easternregions of the U.S.

Regional models, on the scale of a continent ora basin such as the Mediterranean, can provide
more detail about specific source areas, even to thelevel of individual industrial areas. Reliable results,
however, require a detailed understanding of
mercury oxidation in the atmosphere, about which
there is considerably uncertainty.

In Asia, studies show high seasonal variation
in mercury concentration and deposition.
Anthropogenic emissions are responsible for about
75% of deposition in East Asia. In this region,
there is a net removal of gaseous oxidized mercury
from the atmosphere, and a net export of gaseous
elemental mercury.

Most atmospheric transport models consider thefull chain of mercury processes in the atmosphere.
Some models also consider how mercury is cycled
in other atmospheric compartments such as soil,
vegetation, snow, freshwater, and seawater.

In the Arctic, gaseous elemental mercury levelsin the atmosphere are depleted rapidly in spring
when, after the long Arctic winters, sunlight returns.
Ozone is depleted at the same time, and it is believedthat bromine-containing compounds are thecause of the photo-chemical reactions that depleteozone and mercury. During these events, gaseousoxidized mercury and particulate-bound mercury
levels increase sharply, suggesting that the gaseous
mercury is oxidized and deposited, enhancing
mercury deposition in Arctic areas in springtime.
Recent research, however, has found that 50-80%
of the deposited mercury is re-emitted within a few
days. Similar springtime mercury depletion events
have been seen in Antarctica.

Modelling atmospheric transportand deposition

One of the largest sources of uncertainty inmercury models is the chemical mechanism used
to determine how mercury changes forms in the
air. Improved experimental data can help improvemodel performance by making sure that thecorrect reactions are simulated. The processes thatlead from deposition to re-emission also need to
be understood better. Advances in this area showpromise, with model results becoming closer toestimates based on experimental data.

other regions of the world. Natural emissions andre-emissions account for 35-70% of total depositionin most regions. Deposition is generally greater in low
and mid-latitudes, due to stronger sunlight, higher
concentrations of oxidants, and higher precipitation.
Regional mercury models show patterns at the scalesof continents or individual seas. The MediterraneanSea, for example, is a net source of mercury, as itemits more than is deposited to it. In that region, dry
deposition generally exceeds deposition of mercury
in precipitation. In Central and Southern Europe,
total gaseous mercury concentrations and deposition
in precipitation were both consistently higher thanthe European average.
Across the United States, dry deposition accounts
for two-thirds of total annual deposition, mainly
from gaseous oxidized mercury. A study examining
sources and deposition of mercury to assess the23

UNEP Global Mercury Assessment 2013

Anthropogenic releases tothe aquatic environment
Global assessment
Previous UNEP global mercury assessments only
considered emissions to the atmosphere. The 2013
Report presents the first attempt at compiling a global
inventory of mercury releases to aquatic environments.
Releases directly into the aquatic environment present
a completely different chemistry, set of pathways,
and fate to those released to air. They are also more
directly linked to the risks posed by mercury to global
environmental and human health. Unlike mercury
releases to air, which are predominantly in the formof gaseous elemental mercury, releases to water are
predominantly inorganic mercury and to a lesser
extent liquid elemental mercury.
At present, only some sources of aquatic releases
can be quantified with any confidence. Thus, manysources that may be important are not included inthis first assessment of global aquatic releases. For
example, land management practices that expose
new geological sources of mercury in rocks andmineral soils, or that re-mobilize previously deposited
mercury, cannot yet be assessed. Two broad categories
of current anthropogenic sources are considered
here: point sources of mercury release to water,and diffuse releases of mercury as a result of its
remobilisation from areas where it was previously
deposited or accumulated due to human activity (i.e.,
contaminated sites). Artisanal and small-scale gold
mining (ASGM), mercury-containing pesticides andfungicides used in agriculture, and deforestation are
considered separately.
Point source releases to water from various sectors, in tonnes
per year with the range of the estimate, and as a percentage of
total anthropogenic emissions.
Sector24

Releases (range), tonnes

Non-ferrous metal production

92.5 (19.3 - 268)

Consumer product waste

89.4 (22.2 - 308)

Chlor-alkali production

2.8 (1.0 – 5.5)

Oil refining

0.6 (0.3 – 1)

Grand Total

185 (42.6 – 582)

7

The global estimate of mercury release to water frompoint sources has been derived from the atmosphericemissions assessment and the approach employedin the UNEP Toolkit to partition total mercury
releases between air, land, and water. This approach
has considerable uncertainty and may omit sectors
where releases to water are high but air emissions
are unimportant and therefore not addressed in theinventory. Global releases from point sources were
estimated to be 185 tonnes per year.
For diffuse sources, evaluating the relative
contribution of anthropogenic and natural sources
to the aquatic environment requires determining
considerations of the various mercury inputs. For
some sectors, data exist that allow a preliminary
assessment of the amounts of mercury re-mobilized
into aquatic systems.
For mercury-contaminated sites, the range of
average soil concentrations of mercury reported forsites where mercury was mined, used, or otherwise
released by human activity was considered. The
area of contamination is then used to determine
how much mercury is available in soils and, whencombined with a factor for how much mercury isreleased from those soils in a year, the total releasecan be estimated. Among identified contaminatedsites, mercury mining is identified as the largest
sector, followed by mining of precious metals.
Total releases to aquatic environments fromcontaminated sites are estimated to be 8.3-33.5
tonnes per year.

Releases to water from contaminated sites, in tonnes per yearwith the range of the estimate.
SectorReleases (range), tonnes

Primary mercury mining sites

6.7 – 26.6

Precious metal production sites

1.4 – 5.5

Non-ferrous metal production sites

0.1 – 0.5

Chlor-alkali production sites

0.1 – 0.5

Other industrial sites

0.1 – 0.3

Grand Total

8.3 – 33.5

UNEP Global Mercury Assessment 2013

Deforestation, especially in the Amazon Basin, canlead to extensive soil erosion and thus the releaseof mercury previously accumulated in soils. Using
2010 figures for deforestation rates around theworld, and an estimate of soil concentrations of
mercury, as much as 260 tonnes of mercury may
have been released into rivers in 2010 as a result of
deforestation worldwide.

Uncertainties inrelease estimates
Estimating global releases of mercury to aquatic
environments is being done here for the first time.
Many data are missing, and others are imprecise.
A number of assumptions have been made, basedon what measurements are available or on othergrounds, to extrapolate from known quantities or to
calculate aquatic releases in relation to atmosphericBureau of Land Management

Because this is the first attempt to quantify global
mercury releases to the aquatic environment, there
is no previous measurement against which theseresults can be compared. Thus, it is not possible atthis time to evaluate trends in releases.

Phil Augustavo / istockphoto.com

Mercury continues to be used in pesticides andfungicides. 2100 tonnes of mercury were released in
the decade of the 1960s through such agrochemical
uses. Current data are unavailable, although the use of
mercury in these products has been greatly reduced.

or other emissions. Thus, the results should betreated with great caution. It is nonetheless clearthat anthropogenic sources and human activity
contribute hundreds of tonnes of mercury to aquatic
environments each year, a substantial amountrelative to estimated natural releases 150-960 tonnes
per year from terrestrial environments.

Lapshin / Shutterstock.com

Much of the mercury released from artisanal andsmall-scale gold mining goes into rivers, lakes,
soils and sediments, and tailings. From soils andtailings, it may be re-mobilized by leaching anderosion. In addition, mining may disturb mercurycontaining soils and sediments that may then erode
more quickly, releasing more mercury than would
otherwise have become available from natural
erosion. A key factor in these processes is the local
hydrological cycle of precipitation, evaporation,
run-off, and river flow. To account for this in theassessment, countries were designated as dry, wet, orintermediate, to determine the relative importanceof erosion. The tropical and subtropical countries
which have the greatest activity in this sector also
tend to experience high precipitation and run-off,exacerbating this source of mercury release and remobilization. Total worldwide releases of mercury toland and water from ASGM were estimated at over
800 tonnes per year. How much of this is released towater cannot yet be determined.

Contaminated sites (top), erosion following deforestation(middle) and a flooded store of mercury-contaminated waste(lower) illustrate potential sources of mercury releases to water.
25

UNEP Global Mercury Assessment 20138

Aquatic pathways,
transport, and fateMercury in aquatic
environments
The pathways and fate of mercury in aquatic
environments are important because it is in waters,
sediments, and wetland soils that inorganic mercury
is converted into methylmercury, which is toxic andconcentrates in animals. The majority of human
exposure to mercury, and the health risk that comeswith mercury exposure, is from consumption ofmarine foods. Thus, this section focuses on theworld’s oceans. Some freshwater systems, however, areimportant sources of fish for human consumption,
especially for subsistence and recreational fisheries,
often among indigenous peoples. Artisanal and smallscale gold mining (ASGM) communities may also
be affected through fish consumption and drinking
water, if those are contaminated by local releases.

The global mercury cycle is chemically and physicallycomplex. Thus, it is necessary to use models to study,
describe, and predict what happens and will happen
to mercury in the aquatic environment. Until
recently, only one model existed that incorporated
26

Pathways and fate of mercury
in the oceans
Model simulations suggest that anthropogenic impacts
on mercury levels in the ocean are greatest in thesurface waters, the top 100 meters or so of the watercolumn. While model results vary, one recent estimate
suggests that anthropogenic emissions over the last
100 years have doubled the concentration of mercury
in the surface layer of the ocean, and increased it by
25% in intermediate waters, and by 10% in deep waters.
The difference is due to the length of time it takes forsurface waters to circulate to the depths.
Models and measurements agree that direct
deposition from the atmosphere is the dominant
pathway by which mercury reaches the oceans. The
exceptions are smaller, semi-enclosed basins such as
the Mediterranean Sea or the Arctic Ocean, whereriver runoff, coastal erosion, and ocean currents
Ugo Montaldo / Shutterstock.com

The major processes of mercury cycling are generally
similar for all aquatic systems. Inorganic mercury indissolved or particulate form is the dominant mercury
type in most marine and fresh waters. Dissolved
gaseous elemental mercury accounts for less than
30% of total mercury in water. Methylmercury isoften present at trace levels, but may in some settings
reach 30% of total mercury. In freshwater and coastal
environments, inorganic mercury is transformed
into methylmercury primarily in sediments. In the
open ocean, this conversion takes place largely atintermediate depths, between 200 and 1000 metersin the water column. Mercury is lost from aquatic
systems in two ways. When inorganic mercury is
reduced to elemental mercury, it can be re-emitted tothe atmosphere. When inorganic mercury binds toparticulates in water, it can settle out rapidly and beburied in sediments. Deep burial in ocean sediments
is one of the major pathways by which mercury isremoved from the biologically active environment.

air, land, and water into a unified whole. The model
has been validated against observations of mercury,and represents the current scientific consensus. The
largest potential errors in this model in terms of
the aquatic cycle concern details of the exchange of
mercury between air and water.

UNEP Global Mercury Assessment 2013

account for about half of mercury inputs. The
most recent modelling effort suggests that total
deposition input of mercury to the oceans in 2008
was 3700 tonnes.
Other mercury pathways appear less important on a
global basis. Rivers are estimated to carry more than
2800 tonnes of mercury each year, but only about
380 tonnes of this is transported offshore. The rest
is trapped by particles in estuaries. Groundwaterand re-mobilization from sediments provide 100800 tonnes of mercury to the oceans each year.
Undersea hydrothermal vents add less than 600
tonnes of mercury to the oceans. There are highconcentrations of mercury in vent fluids, but a great
deal of the mercury is precipitated as solids once thevent fluids enter seawater.
Models suggest that, globally, about 70% of themercury deposited into the ocean is re-emitted
to the atmosphere. Oxidized mercury in surface
waters is reduced by biological and photochemical
processes to the elemental form that is volatile andreadily re-emitted. The removal of this mercury
ultimately reduces the pool of mercury that could
be converted to methylmercury and be accumulated
by marine organisms. The re-emission process thus
simultaneously prolongs the lifetime of mercury
cycling through the atmosphere and upper ocean,
and acts to reduce mercury availability for marine
food webs.
Any changes in the efficiency of mercury reduction
in surface waters or the rate of re-emission is likely
to impact mercury concentrations in surface waters
and the air. One example is the effect of sea ice in
the Arctic Ocean, which blocks the re-emission
Tony Magdaraog / Shutterstock.com

Mercury accumulated in marine food-webs can enter thehuman diet.

of elemental mercury, leaving elevated levels inwaters under ice. Changes in primary productivity
could change the rate at which mercury is boundto particles, thus affecting re-emission to theatmosphere as well as rates of downward transport
in the ocean. Changes in oxidant levels in theatmosphere, such as ozone and bromine, may also
influence the rate of oxidation of gaseous elemental
mercury and thus deposition rates to the ocean.

Methylmercury in the ocean
Natural bacterial processes in seawater, and in
sediments in coastal environments, convert inorganic
mercury to methylmercury. Methylmercury levels
are highest in the subsurface waters of many oceans
because it is formed at these intermediate depths,
likely as a result of the decomposition of organic
material falling from surface waters. Current
understanding indicates that about 300 tonnes ofmethylmercury is produced in the upper ocean by
this and related processes. By contrast, only about
80 tonnes of methylmercury reaches the ocean fromother sources such as atmospheric deposition, rivers,and diffusion from sediments.
Current modelling indicates that methylmercury
stays in the upper ocean for about 11 years. Demethylation, by photochemical reaction or by
microbial activity, is the major removal process formethylmercury in the ocean. About 240 tonnes ofmethylmercury is removed by this process from thesurface waters of the ocean per year.
The other important pathway for methylmercury inseawater is uptake into marine food webs. Although
only about 40 tonnes per year is taken up in this way,
it is this fraction that poses risks to marine animals
and human consumers of seafood. Methylmercury
is a problem for several reasons. First, it is taken up
by plankton much more efficiently than is inorganic
mercury, resulting in concentrations in plankton
that are as high as 10,000 times the concentration inseawater. Second, methylmercury is absorbed throughthe intestine of animals much more easily than isinorganic mercury. Third, methylmercury biomagnifies
as it moves up the food web. Thus, methylmercury
becomes an increasingly greater proportion of themercury in organisms higher in the food web. This
explains why some indigenous populations that
consume top marine predators such as fish, seals, andwhales have some of the world’s highest concentrationsof methylmercury, giving rise to health concerns.
27

UNEP Global Mercury Assessment 2013

Pathways and fate of mercury Anthropogenic impacts onin freshwateraquatic mercury levels
Observations and modelling both show that
there are in general many similarities between
mercury chemistry, pathways, and fate in lakes
and in the oceans, but with obvious differences inscale and in the relative importance of different
processes. Modelling of mercury dynamics wasdone for four lakes with a history of direct mercury
inputs, from the largest freshwater lake in theworld (Lake Superior) to small lakes in Ontario,
Canada. Atmospheric deposition and river inflow
were the main sources of inorganic mercury.
Much of the mercury input from lake and rivercatchments was associated with dissolved andparticulate organic matter. The dominant mercury
removal mechanisms, as in saltwater, were burial
in sediments and photochemical reduction ofoxidized mercury to elemental mercury followedby re-emission to the air. In Lake Superior andLake Michigan, there is evidence for methylationof mercury in the water column, as is seen in the
open ocean.
Methylmercury levels in freshwater fish varywith the level of deposition of mercury from theair, though other factors such as inputs from thewatershed and the number of trophic levels within
the lake’s food web are also important. Methylation
predominantly occurred in lake sediments andwetlands and floodplain soils, where sulphatereducing bacteria are believed to be primarily
responsible. Methylmercury is de-methylated in
sediments and also taken away in the outlet riverfrom each lake.
Some 400-1400 tonnes of mercury are estimatedglobally to be trapped behind dams and otherman-made impoundments each year. This has
in many cases led to significant increases in total
mercury and methylmercury levels in water, andof methylmercury in fish and other aquatic species.
When artificial lakes are created, plant material issubmerged, leading to decay and anoxia. In these
conditions, microbially driven methylation of
inorganic mercury takes place. In some cases, theelevated mercury levels have also been observed
downstream from the artificial lakes and reservoirs.

28

Two studies have reported that the Atlantic andPacific Oceans have shown opposing trends overrecent decades. While the influence of changing ocean
currents and other oceanographic factors may be afactor, the trends do make sense in light of patterns ofanthropogenic emissions. In the North Pacific Ocean,mercury concentrations appear to have increased at
depths between 200 and 1000 meters over the lastfew decades. This increase coincides with the rapid
industrialization of East Asia.
In the North Atlantic, however, results from near
Bermuda suggest a substantial decrease in mercury
concentration and a change in the mercury profile
through the water column. The Mediterranean Sea, too,showed a decrease in mercury concentration between
1990 and 2004. These findings are consistent withatmospheric data from around the North Atlantic. The
conclusion is that mercury levels in seawater in the
North Atlantic were elevated by human activity but
may now be declining, likely as a result of decreased
emissions from North America and Europe.
A complementary approach to measuring mercury
in seawater is to measure mercury in aquatic
animals, which accumulate and integrate mercury
over time. Teeth, hair, feathers, and eggshells areuseful long-term archives of mercury concentrations,and can provide data from the pre-industrial period(i.e., before 1800) as well as more recent periods.
Among marine animals, samples are available fromseveral regions, particularly from the Arctic, wherecold, dry conditions help preserve animal tissues.
Mercury concentrations between the 13th and 16th
centuries were relatively stable. From the mid- to late
19th century to the end of the 20th century, mercury
levels increased by an average of 12 times. On average,
therefore, about 92% of the mercury in Arctic marine
wildlife today is likely to be of anthropogenic origin.
Similar findings were found for a 700-year sequenceof seabird eggshells from the South China Sea.
Mercury levels increased steadily between 1800
and 2000, with a particularly rapid increase after
1970. Concentrations since 2000 are about 10
times higher than pre-industrial levels, with 91%
of the mercury now likely to be of anthropogenic
origin. In the Antarctic, studies of seal hairs showthat pre-industrial mercury levels were only 60%
of recent levels. The relatively small difference

UNEP Global Mercury Assessment 2013

80

Rune Dietz

Historical Hg concentration as a proportion of present-day, %100
Human teeth
Beluga teeth
Ringed seal teeth
Polar bear hairGyrfalcon feathers
Perigrine falcon feathers

60
40
20
0
1200

1300

1400

1500

1600

1700

1800

1900

2000

between pre-industrial and modern mercury levels
in Antarctic is consistent with lower mercury
emissions and atmospheric concentrations in
the Southern Hemisphere as compared with theNorthern Hemisphere.
The timing of long-term increases of mercury inaquatic animals can shed light on the likely sourcesof mercury emissions that were responsible. The
industrialization of Europe and North America appear
responsible for the large increases in marine mercury
levels that began in the 19th century, as these increases
occurred prior to recent Asian industrialization. As
with water concentrations in the Pacific Ocean, recent
rapid increases in mercury in seabird eggs in the South
China Sea coincide with industrialization in Asia.

The time lag in aquatic
ecosystem response
One important conclusion from air-ocean modelingis that the oceans have not yet reached equilibriumwith current atmospheric levels of mercury. Thislag is due to several factors. First, mercury stays
in the upper ocean, above 200 meters depth, forabout 30 years, and for centuries in intermediate
and deep waters. This is much longer than the one
year residence time in the lower atmosphere. Thus,
removal from the ocean takes much longer thandoes removal from the air, and so concentrations
will change more slowly.
Second, because of the long lifetime of mercury indeeper ocean waters, a great deal of natural mercury
is present in the ocean already, about 100,000 tonnes
out of a total of 135,000 tonnes in intermediate waters,and 200,000 out of 220,000 tonnes in deep waters.

Analysis of museum samples reflects the increase in mercury
emissions and releases since the mid-19th century associatedwith anthropogenic activities.

Third, vertical transport of mercury fromintermediate depths back to the surface returns asubstantial amount of mercury to the biologically
active zone each year. As a result, average mercury
concentrations in seawater and marine animals arelikely to increase slowly for decades if not centuries,
even if atmospheric levels stabilize at present levels.
In short, the effects of historical anthropogenic
emissions from Europe and North America are still
being observed in the oceans, at the same time thatthe effects of the recent rise in emissions from Asia
are being seen.
Regionally, different trends can be expected
depending on distance from sources and ocean
circulation patterns. In the North Atlantic, northof 55° N, it will take 50-600 years to achieve
equilibrium between atmospheric and seawater
mercury levels. In the North Pacific, this process isestimated to take 500-700 years, and in the Antarctic,
700-1000 years. The Arctic Ocean, by contrast, issmaller and will only take about 35 years to reflectchanges in atmospheric mercury levels. Surface
waters of the Mediterranean Sea should show alsochanges within 10-50 years.
Time lags in the responses of many freshwaters
and their food-webs to changes in atmosphericmercury levels are also expected. Although some
immediate recovery following reductions inatmospheric mercury deposition rates is likely, fullrecovery may take decades, centuries, or longer
depending on the characteristics of the area. Highly
polluted freshwaters with catchments and sediments
containing large amounts of anthropogenic mercury
will recover most slowly, as is already observed
around point sources such as smelters that haveclosed down.
29

UNEP Global Mercury Assessment 2013

Major gaps in knowledge
Large uncertainties remain in global estimatesof mercury emissions to the air. These stem fromvarious sources, including the availability of
information on activity levels, but mainly fromthe lack of information concerning the mercury
content of some raw materials and the validity ofassumptions regarding processes and technologies
employed to reduce mercury emissions, including
their rates of application and effectiveness. The
accuracy and precision of measurement-based
estimates can depend on the validity of extrapolating
measurements made at infrequent intervals to longer
periods, or measurements made at one plant to otherfacilities with similar operations.
Potentially important emissions sectors that are still
not quantified in the emissions inventory include
the use of mercury in vinyl-chloride monomer
production; secondary metals production; oil andgas extraction, transport, and processing; industrial
and hazardous waste incineration and disposal;
sewage sludge incineration; and dental fillings
preparation and removal.
Current measurements and their evaluation
are inadequate for determining spatial andtemporal trends. Improved, more extensive, andbetter coordinated measurements are needed. A
permanent global integrated monitoring network
for atmospheric mercury and mercury depositionis needed to achieve this goal. The atmospheric
network sites should be closely integrated with water,soil, and biological monitoring networks. The results
can be used for model testing and evaluation as
well as for more accurately detailing the geographic
impact of anthropogenic emissions. Monitoring of
mercury concentrations in the important marine
food species is essential to understanding possible
human health impacts and the effects of future
emissions changes.
Better data on mercury distribution in the
troposphere is needed to improve understandingof long-range transport and source-receptor
relationships. Coordinated studies at high altitudes
and by aircraft are needed. This information will30

9also help validate regional and global scale models,
improving their prediction capabilities with regard
to different policy scenarios.
Key processes related to global transport and cyclingof mercury remain inadequately understood. The
chemical form of gaseous oxidized mercury is
unknown. Reduction and oxidation rates for
mercury in the presence of atmospheric oxidants
need further study, including determining which
oxidants are important. New measurements andmodelling studies are needed to examine key
chemical and physical processes that affect global
transport and cycling.
Few data are available for reporting mercury releases
to aquatic systems. Systematic and consistentmonitoring of mercury releases to the air is required,especially for contaminated systems. Releases of
mercury from soils into waters depend greatly on
climate and topography, and these parameters need
to be better accounted for. Consistent approaches
for measuring and reporting releases from point
sources are needed to ensure comparability of datafrom around the world. In particular, the actual roleof artisanal and small-scale gold mining (ASGM)
in emissions to air and releases to water needs to be
more accurately estimated.
The link between mercury deposition, methylation,and uptake by living organisms needs further study.
The parameters that determine the rates of exchangeof mercury compounds between air and sea, air andsoil, and air and vegetation are not fully understood.
Whole-ecosystem studies of mercury are needed
to better understand biogeochemical cycling.
Methylation/demethylation rates, and their spatial
and temporal variations and relationship to climatic
factors, need to be determined in most of world’s
major ocean basins, as well as in representative
freshwaters.

UNEP Global Mercury Assessment 2013

Key findings of the2013 assessment

10

Atmosphere

Aquatic environments

Total anthropogenic emissions of mercury to theatmosphere in 2010 are estimated at 1960 (10104070) tonnes which is about 30% of the total
mercury that was emitted and re-emitted fromanthropogenic and natural sources in that year. The
largest anthropogenic sources are associated withartisanal and small-scale gold mining (ASGM) andcoal burning, which together account for about 62%of the annual total anthropogenic emissions to air.
Other major sectors include ferrous and non-ferrous
metal production and cement production.

Natural processes in aquatic systems convert
less toxic elemental and inorganic mercury intomuch more toxic methylmercury. Methylmercury
concentrates and accumulates in the food web,leading to high concentrations in some species of
seafood and fish that many people eat.

East and Southeast Asia are responsible for about
40% of global anthropogenic emissions. About
75% of the mercury from this region comes from
China, which is about one-third of the global
total. Increased estimates for emissions from
ASGM, largely a result of newly obtained data andimproved information, mean that South America
and Sub-Saharan Africa are also responsible for agreater proportion of global emissions than waspreviously assumed.
Anthropogenic emissions over time have increased
mercury loads present in the environment. This
leads to higher rates of re-emission, and also meansthat there will be a time lag of years or decades
between emissions reductions and lower mercury
levels in the food web, including pathways of human
exposure.
Mercury emissions to the atmosphere likely peakedin the 1950s to 1970s and subsequently declined
because of reductions in Europe, Russia, and North
America. Emission trends are unclear due to changes
in methods employed to produce inventories anddifferences in the sectors that have been accounted at
different times. There are, however, some indications
that emissions may be rising again, with increases
from East Asia offsetting continuing reductions in
Europe and North America.

Anthropogenic releases of mercury to aquatic
environments contribute hundreds of tonnes tothe amount of mercury cycling in the environment.
Diffuse releases and point sources appear equallyimportant. Atmospheric deposition, however,remains the most important input of mercury toland and oceans, and has increased the amount of
mercury in many environmental compartments by afactor of two or three since the start of the industrial
age. On land, mercury that is deposited is largely
retained in soils and vegetation, increasing theavailable pool for re-mobilisation to adjacent aquatic
systems. Re-emission from soils, however, is a major
addition to atmospheric mercury.
Hydrology is the most important factor in
the transport of mercury from catchments todownstream environments. Relatively little of thismercury reaches the open ocean, as most is captured
in sediments behind dams, in estuaries, and near thecoast. Changes in land cover and land use can havea large effect on mercury mobilisation, as exposedsoils erode and mercury leaches into the water table.
Concentrations of mercury in Arctic marine animals
today are about 10-12 times higher than in preindustrial times (i.e., prior to about 1800). Thismeans that on average about 92% of the mercury
in marine predators such as seabirds, seals, andwhales is anthropogenic in origin. The timing of
the increase suggest that 19th and early 20th century
emissions from Europe, North America, and Russia
were responsible. Increases mercury concentrations
in seabirds in the South China Sea appear to be
more recent than the Arctic increases, coinciding
with increasing industrialisation in East andSoutheast Asia.
31

UNEP Global Mercury Assessment 2013

K+S Entsorgung GmbH

Recovered mercury is being stockpiled at long-term storage sites above- and, as shown here, below ground.

The upper 100 meters of the oceans have twice themercury that they did a century ago. Intermediate
and deeper waters have 10-25% more mercury on
average, reflecting the slow transport of mercury
downwards in the oceans. Seawater concentrations
will thus be especially slow in responding to changes
in mercury inputs from atmospheric deposition
and river flow. As a result, mercury concentrations
in marine biota are likely to increase slowly for
decades to centuries, even without an increase inatmospheric emissions.
In freshwater ecosystems, atmospheric deposition
and re-mobilisation from soils slow the reductionof mercury levels, even in regions where
atmospheric concentrations have decreased due toemission controls.
Further increases in atmospheric emissions will havelong-term consequences for commercial fisheries
and all consumers of marine and freshwater foods.

32

It is likely to be years or decades before reductionsin anthropogenic emissions and releases of mercury
have a demonstrable effect on mercury levels
throughout the environment and in the fish andmarine mammals which are part of the humanfood-chain. This only reinforces the need to act now
to continue and strengthen international efforts
to reduce current mercury emissions and releases.
Delays in action now will inevitably lead to slower
recovery of the world’s ecosystems in future from
mercury contamination, leaving an even greater
legacy of pollution for future generations.

Division of Technology,
Industry and Economics (DTIE)
Chemicals Branch
GenevaSwitzerland
January, 2013

Job number: DTI/1636/GE