Sulfate particles are sub-micron sized, sulfur-containing airborne particles. Most sulfate is a secondary pollutant, formed by the oxidation in the atmosphere of sulfur dioxide gas. Sulfur dioxide is emitted largely in fossil fuel combustion, particularly from power plants burning coal. A small fraction (generally well under 10%) of sulfur is emitted as primary sulfate at the combustion source. The use of coal-cleaning, scrubbers, and low sulfur coal have reduced sulfur dioxide emissions in the United States, and thus airborne sulfate concentrations have decreased.

In the atmosphere, sulfur dioxide (SO₂) emissions are slowly transformed to sulfate (SO₄) at a rate of 0.1–5% per hour, with the rate increased by higher temperatures, sunshine, and the presence of oxidants. Further reactions with water vapor may produce sulfuric acid (H₂SO₄), a corrosive acid which is injurious to ecosystems and humans, and also ammonium sulfate (NH₄)₂SO₄, which is particularly effective at...