Sulfur's Many Faces
Sulfur is a redox chameleon, existing in oxidation states from −2 (hydrogen sulfide) to +6 (sulfate). This chemical versatility drives its biogeochemical cycle through volcanic eruptions, rock weathering, biological metabolism, atmospheric chemistry, and ocean processes. Sulfur compounds range from the rotten-egg smell of H₂S to the life-essential amino acids cysteine and methionine to the industrial acid H₂SO₄.
The Acid Rain Era
The burning of sulfur-rich coal and oil during the 20th century released up to 65 TgS/yr as SO₂, dwarfing natural volcanic and biogenic sources combined. Atmospheric oxidation of SO₂ to sulfuric acid produced devastating acid rain across industrialized regions — killing fish in Scandinavian lakes, stripping nutrients from Appalachian forest soils, and corroding limestone buildings. The US Clean Air Act (1990) and similar regulations have cut SO₂ emissions dramatically.
Aerosol Climate Effects
Sulfate aerosols formed from SO₂ oxidation scatter incoming sunlight and serve as cloud condensation nuclei, increasing cloud brightness and lifetime. This produces a significant cooling effect (−0.5 to −1.0 W/m²) that has partially masked greenhouse warming. As nations clean up SO₂ pollution, this cooling mask is removed — a dilemma where improving air quality may temporarily accelerate warming.
Ocean Sulfur and DMS
The oceans are a major sulfur reservoir, containing ~1.3 billion TgS as dissolved sulfate. Marine phytoplankton produce dimethylsulfide (DMS), the source of the 'smell of the sea.' DMS emissions (~20 TgS/yr) oxidize to form sulfate aerosols over remote oceans, potentially creating a climate feedback loop where warmer seas produce more phytoplankton, more DMS, more clouds, and thus cooling. This CLAW hypothesis remains one of the most intriguing questions in Earth system science.