Sulfur dioxide (SO2) is a chemical compound consisting of one sulfur atom and two oxygen atoms. It exists as a colorless, reactive gas with a pungent odor. This compound is recognized as a significant air pollutant, impacting environmental quality. SO2 belongs to a larger group of chemicals referred to as sulfur oxides (SOx). Its presence in the atmosphere can lead to various environmental concerns.
Primary Sources of Sulfur Dioxide
Sulfur dioxide enters the atmosphere from both natural occurrences and human activities. Natural sources include volcanic eruptions, geothermal activity, and the decay of organic matter in wetlands and oceans.
Human-made emissions account for a substantial portion of global sulfur dioxide releases. The largest human-made source is the burning of fossil fuels like coal and oil in power plants and industrial facilities, where sulfur in these fuels oxidizes to form SO2 during combustion.
Industrial processes such as metal extraction from ore, smelting, and manufacturing also contribute to SO2 emissions. Smaller contributions come from the burning of high-sulfur fuels in locomotives, large ships, and certain non-road equipment. Globally, fossil fuel combustion accounts for 75-85% of man-made sulfur dioxide emissions.
Atmospheric Transformation and Acid Deposition
Once released into the atmosphere, sulfur dioxide undergoes chemical transformations. It reacts with atmospheric compounds like oxygen and water vapor to form sulfuric acid, a step in acid deposition.
Acid deposition describes the process by which acidic compounds fall from the atmosphere to the Earth’s surface. It occurs in two main forms: wet deposition and dry deposition. Wet deposition involves acidic rain, snow, fog, or sleet, where sulfuric acid mixes with precipitation.
Dry deposition refers to acidic gases and particles settling directly onto surfaces without precipitation. These acidic particles and gases can be carried by wind over long distances. When washed off by rain, dry-deposited substances can further increase the acidity of runoff water.
Impact on Air Quality and Visibility
Sulfur dioxide affects air quality and visibility by forming fine particulate matter and aerosols. SO2 reacts to form tiny sulfate particles (PM2.5) that are small enough to be inhaled. These particles contribute to air pollution and can travel far.
The presence of sulfate aerosols and other particles reduces visibility, known as haze. Haze results from light scattering off these suspended particles, obscuring clear views. SO2 is a frequent cause of haze, particularly in areas with significant fossil fuel combustion.
SO2 also plays a role in the formation of smog, an air pollutant that impairs visibility and poses health risks. While often associated with photochemical reactions, SO2 contributes to smog by forming sulfuric acid droplets. These reactions, especially in the presence of atmospheric moisture, create a dense, opaque mixture of pollutants.
Effects on Natural Ecosystems
The deposition of sulfur dioxide and its acidic derivatives impacts natural ecosystems. Aquatic environments are particularly susceptible to acidification, as acid deposition lowers the pH of lakes, streams, and rivers. This increased acidity directly harms fish and other aquatic life.
Fish populations decline due to unhatched eggs, increased mortality, or deformities. Acidification mobilizes toxic substances like aluminum from soil into waterways, damaging fish gills and impairing respiration. This can simplify the aquatic food web, reducing species diversity.
Terrestrial ecosystems also experience adverse effects from acid deposition. Forests and other vegetation can suffer direct foliage damage and inhibited growth. Acid rain leaches essential nutrients like calcium and magnesium from the soil, making them less available for plants.
Acidified soils can release harmful aluminum, impeding trees’ ability to absorb water and nutrients. This combination of nutrient depletion and toxic substance exposure weakens trees, making them vulnerable to disease, insects, and harsh weather. These changes can lead to reduced biodiversity and altered ecosystem functions.