Hg

Hg

Chapter 11: Atmospheric mercury

A brief human history of mercury poisoning

Qin Shi Huang,

1^st emperor of China

Mad hatters

Minimata disaster

Karen Wetterhahn,

Dartmouth professor

Iraq grain disaster

200 BC

19^th-20^th century

1950s

1971

1997

The perils�of eating too much fish

Jeremy Piven, actor

Richard Gelfond IMAX CEO

Both hospitalized for extended time due to mercury poisoning

from daily fish consumption over many years

Mercury from fish consumption: a global environmental issue

Children IQ deficits (fetal exposure)

Well-established

$8 billion per year cost in US

Adult cardiovascular, fertility effects

Suspected

EPA reference dose (RfD): 0.1 μg kg^-1 d^-1 (about 2 fish meals per week)

Mercury (Hg) is present in atmosphere as an elemental gas – �an amazing property shared only with noble gases

Mass number = 80: 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p⁶ 4d¹⁰ 4f¹⁴ 5s² 5p⁶ 5d¹⁰ 6s²

• Filling of subshells makes elemental Hg(0) stable, liquid, volatile
• Mercury can also shed its two outer electrons (6s²)

to produce Hg(II) (mercuric) compounds

6s²

oxidation

Biogeochemical cycle of mercury: critical role of atmosphere

Hg(0)

Hg(II)

particulate

Hg

burial

SEDIMENTS

uplift

volcanoes

erosion

​

oxidation

Hg(0)

Hg(II)

reduction

biological

uptake

ANTHROPOGENIC

PERTURBATION:

fuel combustion

mining

~20x natural

ATMOSPHERE

OCEAN/SOIL

VOLATILE

WATER-SOLUBLE

Atmosphere enables global transport of mercury

Observed variability of atmospheric Hg implies an atmospheric lifetime against deposition of 0.5 years

​

Implies gPresent-day cale transport of anthropogenic emissions

Present-day emission of mercury to atmosphere from coal and mining

UNEP [2013]; Shah et al. [2021]

Mercury wet deposition is controlled by global transport

EPA deposition data (circles), model (background)

Global Hg(II) pool

scavenging

Florida T-storm

Highest mercury deposition in US is along the Gulf Coast,

where thunderstorms scavenge globally transported mercury from high altitudes

Selin and Jacob [2008]

How to oxidize Hg(0) to Hg(II) in the atmosphere?

Radical oxidant X ≡ OH, Br

Shah et al. [2021]

Sources of bromine atoms

ocean plankton

CHBr₃

bromoform

OH

weeks

deposition

Br

HBr

BrO

O₃

CH₄

hv

plankton

sea-salt aerosol

Br^--

Br₂

hv

HOBr

hv

HO₂

Shah et al. [2021]

as simulated by GEOS-Chem model

UNEP Minimata Convention on Mercury (2017)�

• Requires best available technology

for coal-fired power plants

• Mercury mining to be banned in 15 years
• Regulation of mercury use in artisanal gold mining

Global biogeochemical cycling of mercury: major processes

Global biogeochemical model for mercury

thermocline

Grasshopper effect:�cycling between atmosphere/land/ocean keeps mercury in environment

​

​

Amos et al. [2014]

Land

Ocean

0

What can we hope from the Minimata Convention?

Effect of zeroing

all human emissions

by 2015

Zeroing human emissions right now would decrease ocean mercury by 50% by 2100, while keeping emissions constant would increase it by 40%

Amos et al. [2013, 2014]

The wild card of climate change:�potential mobilization of the large soil mercury pool

Global soils: 240,000 tons mercury

Oceans: 350,000 tons

Atmosphere: 5,000 tons

Increasing soil respiration

due to warmer temperature

Climate change may be as important as emission controls

for the future of environmental mercury in the century ahead.