THE FOG SQUAD IN CALTECH YEARS (Jan 1982)

“No problem too obscured”

Jed WaldmanBill MungerDaniel Jacob

THE ROTATING ARM COLLECTOR

A lethal contraption for collecting fog droplets

Jed and the RACat Pt. Reyes,Aug 1982

WHAT TO DO WHEN YOU’RE INEPT IN THE LAB AND IN THE FIELD?

Try modeling

Group photo (2013)

And now …using global models of atmospheric composition and climateto interpret observations and gain knowledge of processes

GEOS-Chem global 3-D model of atmospheric compositionSatellites

Surface networks Aircraft

GEOS-Chemchemical transport model

The GEOS-Chem user community: 70 institutions, 18 countries

7th GEOS-Chem meeting next week at Harvard – 200 participants

Good reasons not to choose a career in research:

• Independence• Joy of discovery• Shielding from human suffering and stupidity• Job security• See the world

• “My experience as grad student/postdoc isn’t fun” (it doesn’t get better) • “Research is too narrow, too slow-paced”• “I want to become filthy rich”

Bad reasons:

• “I’m not creative enough” • “I don’t want to publish or perish” • “It’s too hard to get a good job”• “I want to have a personal life”

Is a research career for you?

…but it may not be for you.

A lot has to be said for a career in research…

If not you,then who?

Hg is present in atmosphere as an elemental gas

…a property that it shares only with the noble gases!

XeKrNe

ArHe

Hg Rn

Electronic structure of mercury

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

• Complete filling of subshells gives Hg(0) a low melting point, volatility• Two stable oxidation states: Hg(0) and Hg(II)

Biogeochemical cycle of mercury

Hg(0) Hg(II)

particulate

Hg

burial

SEDIMENTS

uplift

volcanoeserosion

oxidation

Hg(0) Hg(II)reduction biological

uptake

ANTHROPOGENIC PERTURBATION:fuel combustion

mining

ATMOSPHERE

OCEAN/SOIL

VOLATILE WATER-SOLUBLE

Rising mercury in the environment

Global mercury deposition has roughly tripled since preindustrial times

Dietz et al. [2009]

Human exposure to Hg is mainly through ocean fish consumption

Tuna is the #1 contributor

Mercury biomagnification factor

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

Mercury is a global pollutant

Anthropogenic Hg emission

Streets et al. [2009]; Soerensen et al. [2010]

Transport around northern mid-latitudes:

1 month

Transport to southern hemisphere: 1 year

Implies global-scale transport of anthropogenic emissions

Hg(0) lifetime = 0.5-1 year

Hg emitted anywherecan deposit to oceans worldwide

Atmospheric redox chemistry of mercury:driver of mercury deposition

Hg(0) Hg(II)OH, O3,

• Oxidation of Hg(0) by OH or O3 is endothermic

HO2(aq)

Older models

• Our current hypothesis is that oxidation by Br atoms is dominant:

2 2 , , , ,

Hg Br M HgBr M

HgBr X M HgBrX M X OH Br Cl NO HO

?

X X Cl, Br

• No viable mechanism identified so far for atmospheric reduction of Hg(II)

X

Horowitz et al. , in prep

UNEP Minimata Convention on Mercury

• Requires best available control technology

for coal-fired power plants• Mercury mining to be banned in 15 years• Many mercury-containing commercial

products to be banned by 2020

Opened for signatures in October 2013; already signed by 128 countries

• Convention requires ratification by 50 countries to go into effect; 9 have ratified so far

Historical inventory of global anthropogenic Hg emissions

Large past (legacy) contribution from N. American and European emissions; Asian dominance is a recent phenomenon

Streets et al. , 2011

Global biogeochemical model for mercury (Amos et al., 2013)• 7-box model with 7 coupled ODEs dm/dt = s(t) – km where s is primary emission• Transfer rate constants k are specified from best knowledge

Model is initialized at natural steady state, and then forced with anthropogenic emissions for 2000 BC – present; % present-day enrichments are indicated

Primaryemissions

thermocline

Observational constraints:

1. present-day atmosphere 4600-5600 Mg2. present-day ocean 0.5-2.5 pM3. 2-5 x atmospheric increase since 1850

Characteristic time scales for Hg global biogeochemical cyclefrom eigenanalysis of 7-box model

Amos et al. [2013]

~1-year time scale for exchangebetween atmosphere and surface/subsurface ocean;

~100-year time scale for transfer from surface reservoirs to deep ocean;

~10,000-year time scale for dissipation of perturbation to deep mineral reservoir

Time scale for dissipation of an atmospheric emission pulseR

eser

voir

fra

ctio

n

• Pulse gets transferred to subsurface ocean within a few years and stays there ~100 years, maintaining a legacy in the surface ocean

• Pulses injected in surface ocean or terrestrial reservoirs have similar fates

Amos et al. [2013]

Global source contributions to Hg in present-day surface ocean

• Human activity has increased 7x the Hg content of the surface ocean

• Half of this human influence is from

pre-1950 emissions

• N America, Europe and Asia share similar responsibilities for anthropogenic Hg in present-day surface ocean

Amos et al. [2013]

EuropeAsia

N America

S America

former USSR

ROW

pre-1850natural

emissions

What can we hope from the Minimata Convention?

Effect of zeroing global anthropogenicemissions by 2015

• Zeroing anthropogenic emissions would decrease ocean Hg by 30% by 2100, while keeping emissions constant would increase it by 40%

• Elevated Hg in surface ocean will take centuries to fix; the only thing we can do in short term is prevent it from getting worse.

Amos et al. [2013]

Conundrum: decrease of atmospheric Hg in past two decadesCircles = observations Background = GEOS-Chem model (after improvements)

The decreasing trend is inconsistentwith standard emission inventories

Zhang et al., in prep

Disposal of Hg in commercial products:a missing component of the Hg biogeochemical cycle?

Global production of commercial Hg peaked in 1970

Horowitz et al., 2014

• Commercial Hg enters environment upon use or disposal; much larger source than inadvertent emission

• Could this explain the observed environmental Hg decreases over the past two decades?

Global Hg production Global inadvertent Hg emission[Streets et al., 2011]

Hg is found in many commercial products

Wiring Devices &Industrial Measuring Devices

Medical Devices

Pharmaceuticals & Personal Care Products

Pesticides and Fertilizer Explosives/Weapons

Dyes/Vermilion

Hg is found in many commercial products (cont.)

Horowitz et al., 2014

Global historical Hg consumption

Horowitz et al. [2014]

Tracking the ultimate environmental fate of commercial Hg

Total Global Mined Hg

Developed Countries Use Developing Countries Use

% GDP % GDP

Disposal Disposal

Air Land Water Air Land Water

Landfill Landfill

Horowitz et al., 2014

Historical releases of commercial Hg to environmental reservoirs

Additional releases from commercial Hg in the context of atmospheric emissions

Historical contribution of commercial Hg to environmental release

Horowitz et al., 2014

Much larger source than coal combustion – how can we make it compatible with constraint on atmospheric inventory?

Could it explain observed atmospheric trend?

Sink from sequestration of riverine Hg in coastal sediment

• 55 Mg a-1 of Hg is discharged to oceans from rivers, comparable to atmospheric deposition

• About 70% of this riverine Hg settles in estuaries and coastal sediments; long-term sink?

Amos et al., 2014

Inclusion of shallow sediment sequestration in Hg budget

Ocean margin sediments: dominant natural reservoir,speeds up removal of environmental Hg

without sequestration

with sequestration

Amos et al., 2014

Amos et al., 2014

Sequestration in ocean margin sediments speeds up effect of Minimata Convention