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ATMOS 397GATMOS 397GBiogeochemical Cycles and Global ChangeBiogeochemical Cycles and Global Change

Lecture 22: Sulfur Cycle 1Lecture 22: Sulfur Cycle 1

Don WuebblesDon Wuebbles

Department of Atmospheric SciencesDepartment of Atmospheric Sciences

University of Illinois, Urbana, ILUniversity of Illinois, Urbana, IL

April 16, 2003April 16, 2003

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OXIDATION STATES OF SULFUROXIDATION STATES OF SULFURS has 6 electrons in valence shell S has 6 electrons in valence shell oxidation states from –2 to +6oxidation states from –2 to +6

-2 +4 +6FeS2

Pyrite

H2S

Hydrogen sulfide

(CH3)2S

Dimethylsulfide (DMS)

CS2

Carbon disulfide

COS

Carbonyl sulfide

SO2

Sulfur dioxide

H2SO4

Sulfuric acid

SO42-

Sulfate

Decreasing oxidation number (reduction reactions)

Increasing oxidation number (oxidation reactions)

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THE GLOBAL SULFUR CYCLETHE GLOBAL SULFUR CYCLE

SO2

H2S

volcanoes industry

SO2

CS2

SO42-

OCEAN1.3x1021 g S107 years

deposition

runoff

SO42-

plankton

COS(CH3)2S

microbesvents

FeS2

uplift

ATMOSPHERE2.8x1012 g S1 week

SEDIMENTS7x1021 g S108 years

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GLOBAL SULFUR EMISSION TO THE GLOBAL SULFUR EMISSION TO THE ATMOSPHEREATMOSPHERE

1990 annual mean1990 annual mean

Chin et al. [2000]

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The modern global sulfur cycle.

The units in this figure are expressed as teragrams of sulfur

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Another example of the modern global sulfur cycle demonstrating the uncertainty in the sulfur fluxes

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The modern global sulfur cycle differs quite dramatically from the "pre-industrial" sulfur cycle.

Why?

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Sulfur Cycle

• S availability and plant nutrition

• Soil acidification, acid rain

• Ore extraction, acid mine drainage

• Trace gas production

. . . sulfur is a devilish substance . . . Discharged from the bowels of the earth,by volcanoes or evil-smelling hot springs . . . Surely the effluent of Hell itself.

– J.R. Postgate

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An important distinction between cycling of sulfur and cycling of nitrogen and carbon is that sulfur is "already fixed". That is, plenty of sulfate anions (SO4

2-) are available for living organisms to utilize.

By contrast, the major biological reservoirs of nitrogen atoms (N2) and carbon atoms (CO2) are

gases that must be pulled out of the atmosphere.

Sulfur fixation?

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Reservoirs of Sulfur AtomsReservoirs of Sulfur Atoms

The largest physical reservoir is the Earth's crust wherein sulfur is found in gypsum (CaSO4) and pyrite (FeS2).

The largest reservoir of biological useful sulfur is found in the ocean as sulfate anions (very concentrated at 2.6 g/L), dissolved hydrogen sulfide gas, and elemental sulfur.

Other reservoirs include:

Freshwater - contains sulfate, hydrogen sulfide and elemental sulfur; Land - contains sulfate; Atmosphere - contains sulfur oxide (SO2) and methane sulfonic acid (CH3SO3

-);

volcanic activity releases some hydrogen sulfide into the air.

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Amounts and Forms of S in Soils

• Total S ranges from 0.002 to 10%– Highest concentrations in tidal flat, saline, acid-

sulfate, and organic soils

• Organic S is 90% of total S– Ester sulfates 30 to 75% of organic S

• Microbial S is 2 to 3% of organic S

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Sulfur is one of the most abundant elements forming the Earth (along with Si, Fe, O). However, the majority of S resides in reduced form in Earth’s remote core, with Fe and Ni.

This S is effectively inaccessible, even on the time scale of plate tectonic processes. Sulfur near the surface of the Earth comprises ~11,000 x 1018 g; only 5 x 1018 g is in the organic reservoir.

The bulk of surficial sulfur dates back to the formation of the world’s ocean: released from Earth’s interior predominately through degassing associated with volcanism, becoming oxidized and accumulating with water as sulfate.

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•Some major steps in the sulfur cycle include: 1.Assimilative reduction of sulfate (SO4

=) into -SH groups in proteins.

2.Release of -SH to form H2S during excretion, decomposition, and desulfurylation.

3.Oxidation of H2S by chemolithotrophs to form sulfur (So) and sulfate (SO4=)

4.Dissimilative reduction of sulfate (SO4=) by anaerobic respiration of sulfate-reducing bacteria.

5.Anerobic oxidation of H2S and S by anoxygenic phototrophic bacteria (purple and green bacteria) •The sulfur cycle includes more steps than are shown here. Sulfur compounds undergo some interconversions due to chemical and geologic processes (not shown here). In addition, a number of organic sulfur compounds accumulate in significant amounts, especially in marine environments. •For example, about 45 tons of dimethyl sulfide are produced annually by degradation of dimethylsulfonium propionate, a chemical produced by marine algae for osmoregulation. This is gradually broken down by a variety of biotic and abiotic mechanisms.

Sulfur compounds undergo frequent metabolic transformations in bacteria

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Volatile Biogenic S Compounds

16.5 – 70.6

15.0

3.8 – 4.7

2.7 – 3.5

39.6 – 45.4

1.3 – 3.4

0.2 – 1 ppb

0.2 – 5 ppb

0.1 – 0.4 ppb

0.2 – 0.6 ppb

58 ppt

H2S

SO2

CS2

COS

CH3SH

CH3CH2SH

CH3SCH3

CH3SSCH3

Hydrogen sulfide

Sulfur dioxide

Carbon disulfide

Carbonyl sulfide

Methyl mercaptan

Ethyl mercaptan

Dimethyl sulfide

Dimethyl disulfide

Production

(Tg y-1)

Atmospheric

concentration

FormulaCompound

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Global Sulfur Cycle

BIOSPHERE 7.9x103

SOILS 2.6x105

SOM 1x104

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Simplified

biogeochemical

cycle for sulfur in

organic-matter-rich

sediments of South

Florida.

CH3Hg+ = methyl

mercury ion.

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T h e S u l f u r C y c le

A n im a lm a n u r e s

a n d b io s o lid s

M in e r a lfe r t i liz e r s

C r o p h a r v e s t

R u n o f f a n de r o s io n

L e a c h in g

A b s o r b e d o rm in e r a l s u lfu r

P la n t r e s id u e s

P la n tu p ta k e

S u lfa teS u lfu r( S O 4 )

A tm o s p h e r ics u lfu r

E le m e n ta ls u lfu r

O r g a n ics u lfu r

R e d u c e d s u lf u r

In p u t t o s o i lC o m p o n e n t L o s s f ro m s o i l

V o la t i liz a t io nA tm o s p h e r ic

d e p o s i t io n

-

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Aerosol’s Effect on Cloud Formation

• Polluted cloud contains eight times as many droplets of half the size, twice the surface area, twice the optical depth, and higher reflectivity than the natural cloud

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Aerosol Modification of Marine Clouds

• A false color image of ship tracks (white streaks) in a boundary layer cloud deck (mottled white) offshore from the northwestern United States (green)

• Cloud-free ocean is dark blue, high-altitude clouds are light blue

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Consequences of Sulfur Oxidation

• Acid mine drainage

• Acid sulfate soils

• Corrosion of concrete

• Corrosion of metals

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Acid Rain

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Fossil fuels with high sulfur content produce sulfur dioxide when they burn. In the atmosphere, the gas reacts with hydroxyl radicals (OH), ozone, and peroxide (H2O2), creating sulfuric acid (H2SO4). In the cold air of the High Arctic, sulfuric acid takes the form of sub-micrometer particles, which are the main components of Arctic haze. Sulfate particles can adhere directly to surfaces as dry deposition. Sulfuric acid can also react with water in rain, snow, and fog, dissociating into hydrogen and sulfate ions, which get washed out as wet deposition. Biogenic sulfur compounds, such as dimethyl sulfide (DMS) from plankton and hydrogen sulfide (H2S) from volcanoes, enter the same chemical cycle in the atmosphere via a reaction with hydroxyl radicals (OH). The rates of different chemical reactions in the sulfur cycle depend on energy from the sun. In the Arctic, lack of sunlight during the polar winter limits production of the hydroxyl radical, which in turn slows production of sulfuric acid from sulfur dioxide. When the sun returns in the early spring, there is a load of sulfur dioxide in the air, ready to be converted into sulfate aerosols. This photochemical mechanism explains why Arctic haze is most pronounced in March and April, after the Arctic sunrise.

Sulfur dioxide turns into haze and acid precipitation

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Sulfur dioxide concentration in air, monthly averages, Viksjöfjäll,

Norway, 1992, and extent of vegetation damage on the Kola

Peninsula.

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Effects of Acid Rain

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Emission of sulfur in the

Northern Hemisphere, gigagrams

(1000 tonnes) in 1985.

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Summer and winter visibility

in North America

showing the effects of Arctic haze in winter.

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Acidification depletes nutrients Most Arctic mineral soils are naturally acidic, because slow weathering limits the rate at which they can replace the base ions that trees use for nutrients. Acid deposition amplifies this natural acidification process when hydrogen ions replace base ions, causing the base ions to leach further down into the soil or to be washed away in runoff. As a result, the pool of nutrients in the soil decreases. Moreover, once the easily available base ions such as calcium and magnesium are used up, another buffering process starts freeing previously bound aluminum ions, which are toxic to plants. Tree damage from acidification has many causes, but the lack of nutrients and the excess of aluminum ions are two important culprits. The figure shows the pH at which different base ions become

mobile.

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Sensitivity of different organisms to decreasing pH.

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Sensitivity of

terrestrial ecosystems

to acid deposition

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Carbonyl Sulfide

COS

NOAA CMDL Measurements