Sulfur, an abundant element, exists throughout Earth’s systems, including its atmosphere. It cycles through various forms and compartments, impacting the planet’s chemistry and climate. Sulfur enters the air through a range of processes, continually introducing different sulfur compounds. These processes involve both natural phenomena and human activities, each contributing distinct forms and quantities of sulfur to the air.
Natural Pathways for Sulfur Entry
Volcanic activity is a natural source of atmospheric sulfur. Eruptions release sulfur dioxide (SO2) and hydrogen sulfide (H2S) directly into the atmosphere. Explosive events can inject SO2 into the stratosphere, where it can persist for weeks and form sulfate aerosols.
Oceans also play a role, primarily through the emission of dimethyl sulfide (DMS). Marine organisms, particularly phytoplankton, produce DMS as a byproduct of their metabolic processes. Once released, DMS oxidizes in the atmosphere to form other sulfur compounds, including sulfur dioxide and sulfate aerosols.
Biological decomposition contributes to natural sulfur emissions. The anaerobic decay of organic matter in wetlands, swamps, and soils releases hydrogen sulfide (H2S). This colorless gas, known for its rotten egg odor, is produced by bacteria. H2S has a relatively short atmospheric lifetime, but undergoes further chemical transformation.
Wildfires also introduce sulfur compounds into the air, though typically in smaller quantities. Smoke plumes from burning biomass, including forest and agricultural fires, contain sulfur dioxide. The amount of sulfur released varies based on fuel type and geographic location.
Human Activities Releasing Sulfur
The combustion of fossil fuels is a human-induced source of atmospheric sulfur. Burning coal, oil, and natural gas for energy production, particularly in power plants and industrial facilities, releases sulfur dioxide (SO2). Sulfur is present as an impurity in these fuels. The burning process converts this sulfur into SO2.
Beyond energy generation, industrial processes contribute to sulfur emissions. Metal smelting, which involves extracting metals from sulfur-containing ores, releases SO2. Other chemical manufacturing activities are also sources of sulfur compounds.
Agricultural practices also release sulfur compounds. Livestock manure emits hydrogen sulfide, which can convert to sulfur dioxide. Fertilizer and pesticide applications to croplands are a source of sulfur in the environment. This sulfur can react and enter the atmosphere through various pathways.
Forms of Sulfur in the Atmosphere
Once sulfur enters the atmosphere, it exists in several chemical forms, undergoing transformations influenced by atmospheric conditions.
Sulfur dioxide (SO2) is one of the most common forms directly released. It is a colorless gas with a distinct odor, originating from both natural sources like volcanoes and human activities such as fossil fuel combustion and industrial processes. SO2 is a precursor to sulfuric acid in the atmosphere.
Hydrogen sulfide (H2S) is another gaseous sulfur compound found in the atmosphere. It is primarily emitted from natural sources like anaerobic decomposition and some volcanic activity, but also from industrial processes and livestock. H2S has a characteristic rotten egg smell at low concentrations. In the atmosphere, H2S reacts with other chemicals and typically breaks down within a few days.
Sulfate aerosols (SO4^2-) are small airborne particles that form when SO2 and H2S undergo oxidation in the atmosphere. These particles are created through reactions with water vapor and other gases. Sulfate aerosols contribute to the overall particulate matter in the air and influence atmospheric processes.
Dimethyl sulfide (DMS), primarily of oceanic origin, is a precursor molecule that transforms into other atmospheric sulfur compounds. It is oxidized in the marine atmosphere to compounds like sulfur dioxide, dimethyl sulfoxide, and sulfuric acid. The production of sulfuric acid from DMS can contribute to the formation and growth of new particles in the atmosphere.