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Sulfur Cycling

Sulfur Cycling. Like N, S can form gaseous and dissolved aqueous chemical species S : elemental sulfur (solid) S 2- : sulfide (dissolved), can bond with metals to form minerals, i.e. FeS 2 (pyrite), ZnS (sphalerite), PbS (galena)

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Sulfur Cycling

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  1. Sulfur Cycling • Like N, S can form gaseous and dissolved aqueous chemical species • S : elemental sulfur (solid) • S2-: sulfide (dissolved), can bond with metals to form minerals, i.e. FeS2 (pyrite), ZnS (sphalerite), PbS (galena) • SO42- : sulfate (dissolved) or as minerals like gypsum (CaSO4·2H2O) • SO2 : sulfur dioxide (gas) • H2S : hydrogen sulfide (gas) SO2 oxidation reduction SO42- H2S, S2-

  2. Sources/Distribution of Atmospheric Sulfur • Biogenic reduced S gases: especially DMS (CH3SCH3) over the open ocean; released by natural organic decay, bacterial metabolism, transpiration, plantonic algae ~65Tg S/yr (DMS alone = 43 Tg S/yr) • Sea salt aerosols: evaporation of seaspray, sulfate anion; relatively inert, short residence time ~144 Tg S/yr • Terrestrial dust: sulfate from gypsum (CaSO4·2H2O); relatively inert, short residence time ~20 Tg/yr • Natural biomass burning, Volcanic emissions (H2S, SO2) ~28 Tg/yr • Anthropogenic: especially coal-burning, ore smelting ~93 Tg/yr

  3. Major Sulfur Compounds • SO2(g) • Produced by volcanoes • Also formed from oxidation of other sulfur gases (such as H2S) by free radicals in the atmosphere (•OH) • Also produced by burning fossil fuels (mainly COAL) • Greater than 10x the amount from volcanoes • Down to 58 Tg/yr in 1999 from 69 Tg/yr in 1990 • Change mainly due to US amendments to the Clean Air Act in 1990 • Fall of USSR also reduced amount of S output from industrial coal burning • European Union (EU) has tightened limits on S emissions from coal burning • Trend may not continue… China’s use of coal is increasing, but with no S emission controls

  4. US EPA

  5. Major Sulfur Compounds • SO2(g) • Short residence time (2-7 days), then oxidizes in the atmosphere forming H2SO4 • H2SO4 = Sulfuric acid • Results in acid deposition (acid rain) -- more on this later • Residence time 5-12 days, so may travel far from point of origin before being deposited • Also forms dry sulfate aerosols • Combination of sulfuric acid in cloud droplets with dissolved Ca2+ or NH3 • Forms particulate sulfate aerosols • Can scatter sunlight, cooling the Earth’s surface • Can contribute to cloud formation (droplet nucleation)

  6. Biogeochemical Cycles: Sulfur • Recently scientists have been pointing out that SO2 counterbalances CO2 in global climate change • BUT, SO2 has a much lower residence time in the atmosphere than CO2 • 1816: “Year without a Summer” huge weather related disruptions in New England & Western Europe. Why? Eruption of Tambora volcano in 1815 puts huge amounts of SO2(g) into the stratosphere • Why does this matter? Because, it means that reduced fossil fuel emissions (particularly of S-rich fossil fuels) should lead to a sudden T rise! (due to lack of counterbalancing short-lived SO2)

  7. Pre-Industrial S Cycle (from Chameides and Perdue, 1997) Atmospheric Sulfur 32 Tg

  8. Pre-Industrial S Cycle (from Chameides and Perdue, 1997) Atmospheric Sulfur 32 Tg Wet & Dry Deposition 61 Tg/yr DMS, H2S, seaspray 77 Tg/yr Oceanic Sulfur 1,600,000,000 Tg

  9. Pre-Industrial S Cycle (from Chameides and Perdue, 1997) Atmospheric Sulfur 32 Tg Wet & Dry Deposition 61 Tg/yr DMS, H2S, seaspray 77 Tg/yr Oceanic Sulfur 1,600,000,000 Tg Volcanoes 2.1 Tg/yr Pyrite deposition 26 Tg/yr ReducedSediments (Pyrite) 4,800,000,000 Tg

  10. Pre-Industrial S Cycle (from Chameides and Perdue, 1997) Atmospheric Sulfur 32 Tg Wet & Dry Deposition 61 Tg/yr DMS, H2S, seaspray 77 Tg/yr Oceanic Sulfur 1,600,000,000 Tg Volcanoes 2.1 Tg/yr Volcanoes 1.1 Tg/yr Pyrite deposition 26 Tg/yr Oxidized Sediments (Gypsum) CaSO4•2H2O 6,400,000,000 Tg Gypsum Deposition 17.1 Tg/yr ReducedSediments (Pyrite) 4,800,000,000 Tg

  11. Pre-Industrial S Cycle (from Chameides and Perdue, 1997) Atmospheric Sulfur 32 Tg Reduced S Emissions 19.2 Tg/yr Wet & Dry Deposition 61 Tg/yr Wet & Dry Deposition 38.4 Tg/yr DMS, H2S, seaspray 77 Tg/yr Soils 96,000 Tg Oceanic Sulfur 1,600,000,000 Tg Volcanoes 2.1 Tg/yr Weathering & Volcanoes 24 Tg/yr Volcanoes 1.1 Tg/yr Weathering & Volcanoes 16 Tg/yr Pyrite deposition 26 Tg/yr Oxidized Sediments (Gypsum) CaSO4•2H2O 6,400,000,000 Tg Gypsum Deposition 17.1 Tg/yr ReducedSediments (Pyrite) 4,800,000,000 Tg River runoff 59 Tg/yr

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