Presentation on theme: "1 UIUC ATMOS 397G Biogeochemical Cycles and Global Change Lecture 22: Sulfur Cycle 1 Don Wuebbles Department of Atmospheric Sciences University of Illinois,"— Presentation transcript:
1 UIUC ATMOS 397G Biogeochemical Cycles and Global Change Lecture 22: Sulfur Cycle 1 Don Wuebbles Department of Atmospheric Sciences University of Illinois, Urbana, IL April 16, 2003
2 UIUC OXIDATION STATES OF SULFUR S has 6 electrons in valence shell oxidation states from –2 to +6 -2+4+6 FeS 2 Pyrite H 2 S Hydrogen sulfide (CH 3 ) 2 S Dimethylsulfide (DMS) CS 2 Carbon disulfide COS Carbonyl sulfide SO 2 Sulfur dioxide H 2 SO 4 Sulfuric acid SO 4 2- Sulfate Decreasing oxidation number (reduction reactions) Increasing oxidation number (oxidation reactions)
3 UIUC THE GLOBAL SULFUR CYCLE SO 2 H 2 S volcanoes industry SO 2 CS 2 SO 4 2- OCEAN 1.3x10 21 g S 10 7 years deposition runoff SO 4 2- plankton COS (CH 3 ) 2 S microbes vents FeS 2 uplift ATMOSPHERE 2.8x10 12 g S 1 week SEDIMENTS 7x10 21 g S 10 8 years
4 UIUC GLOBAL SULFUR EMISSION TO THE ATMOSPHERE 1990 annual mean Chin et al. 
5 UIUC The modern global sulfur cycle. The units in this figure are expressed as teragrams of sulfur
6 UIUC Another example of the modern global sulfur cycle demonstrating the uncertainty in the sulfur fluxes
7 UIUC The modern global sulfur cycle differs quite dramatically from the "pre-industrial" sulfur cycle. Why?
9 UIUC 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 (SO 4 2- ) are available for living organisms to utilize. By contrast, the major biological reservoirs of nitrogen atoms (N 2 ) and carbon atoms (CO 2 ) are gases that must be pulled out of the atmosphere. Sulfur fixation?
10 UIUC Reservoirs of Sulfur Atoms The largest physical reservoir is the Earth's crust wherein sulfur is found in gypsum (CaSO 4 ) and pyrite (FeS 2 ). 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 (SO 2 ) and methane sulfonic acid (CH 3 SO 3 - ); volcanic activity releases some hydrogen sulfide into the air.
12 UIUC 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 10 18 g; only 5 x 10 18 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.
13 UIUC Some major steps in the sulfur cycle include: 1.Assimilative reduction of sulfate (SO 4 = ) into -SH groups in proteins. 2.Release of -SH to form H 2 S during excretion, decomposition, and desulfurylation. 3.Oxidation of H 2 S by chemolithotrophs to form sulfur (S o ) and sulfate (SO 4 = ) 4.Dissimilative reduction of sulfate (SO 4 = ) by anaerobic respiration of sulfate-reducing bacteria. 5.Anerobic oxidation of H 2 S 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
30 UIUC 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
31 UIUC Sulfur dioxide concentration in air, monthly averages, Viksjöfjäll, Norway, 1992, and extent of vegetation damage on the Kola Peninsula.
44 UIUC Summer and winter visibility in North America showing the effects of Arctic haze in winter.
45 UIUC 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.
46 UIUC Sensitivity of different organisms to decreasing pH.
47 UIUC Sensitivity of terrestrial ecosystems to acid deposition
48 UIUC Carbonyl Sulfide COS NOAA CMDL Measurements