Sulfur dioxide (\(\text{SO}_2\)) is a gaseous air pollutant composed of one sulfur atom and two oxygen atoms. It is a major component of sulfur oxides, released into the atmosphere primarily through human activity. The largest source of \(\text{SO}_2\) emissions is the combustion of sulfur-containing fossil fuels, such as coal and oil, used by power plants and industrial facilities. Other sources include metal smelting, refining, and the burning of high-sulfur fuel in locomotives and marine vessels. This gas initiates a cascade of chemical reactions that result in widespread environmental degradation.
Atmospheric Conversion and Acid Deposition
Once released, sulfur dioxide undergoes a complex transformation in the atmosphere to form sulfuric acid (\(\text{H}_2\text{SO}_4\)), the mechanism behind acid deposition. The initial step involves the oxidation of the \(\text{SO}_2\) molecule through different pathways. In the gas phase, \(\text{SO}_2\) reacts with the hydroxyl radical (\(\text{OH}\)) to form sulfur trioxide (\(\text{SO}_3\)). This \(\text{SO}_3\) combines rapidly with atmospheric water vapor to yield sulfuric acid.
Liquid-phase oxidation is also a significant pathway, where \(\text{SO}_2\) dissolves into water droplets found in clouds or fog. Within these droplets, the oxidation reaction is accelerated, often catalyzed by dissolved metal ions or oxidants like hydrogen peroxide (\(\text{H}_2\text{O}_2\)) and ozone (\(\text{O}_3\)).
The sulfuric acid returns to the Earth’s surface through two distinct processes. Wet deposition involves the acid dissolving into rain, snow, or fog, causing acidic precipitation. Dry deposition occurs when sulfur compounds, either as gaseous \(\text{SO}_2\) or fine sulfate particles, settle onto surfaces. Both wet and dry deposition deliver acids to ecosystems, driving the environmental impacts of \(\text{SO}_2\) pollution.
Effects on Terrestrial Ecosystems
Acid deposition severely impacts terrestrial ecosystems by injuring vegetation and altering soil chemistry. Direct injury occurs when acidic components in fog or rain contact foliage, leading to necrosis or browning of leaves and needles. This foliar damage impairs the plant’s ability to absorb sunlight, reducing photosynthesis and inhibiting growth.
The most profound long-term impact occurs in the soil, particularly in regions with limited natural buffering capacity. Acidic inputs accelerate the leaching of essential plant nutrients, including calcium and magnesium. The loss of these nutrients weakens trees and plants, making them more susceptible to disease, insect infestations, and climate stress.
Increased soil acidity also mobilizes toxic elements previously bound to soil particles. Aluminum is leached from clay minerals and becomes a dissolved inorganic form, which is highly toxic to fine root hairs. The combination of nutrient depletion and aluminum toxicity can lead to widespread forest decline, observed particularly in high-elevation forests.
Effects on Aquatic Ecosystems
Acid deposition into freshwater systems, such as lakes and streams, causes acidification by dropping the water’s pH. This change in water chemistry has severe biological consequences, particularly for organisms sensitive to acidity. Acid-sensitive aquatic life includes fish eggs and young fry, which often fail to hatch or survive at pH levels below 5.
Toxicity is compounded by aluminum leached from surrounding soil into the water. This dissolved inorganic aluminum is toxic to fish, as it interferes with gill function, causing physiological stress and mortality. Even species that tolerate moderate acidity may suffer indirect effects, as their food sources, such as aquatic insects and invertebrates, are more sensitive and disappear.
The resulting stress and mortality reduce species richness, leading to a loss of biodiversity and simplification of the aquatic food web. Episodic acidification, such as from the rapid melting of acidic snowpack, can cause sudden, lethal impacts.
Contribution to Atmospheric Haze and Aerosols
Sulfur dioxide plays a role in reducing visibility by acting as a precursor to secondary aerosols. In the atmosphere, \(\text{SO}_2\) transforms into fine sulfate particulate matter, which are microscopic particles suspended in the air. These sulfate particles are a major component of fine particle pollution (\(\text{PM}_{2.5}\)) and are highly efficient at scattering light.
The scattering of light by these fine particles leads to atmospheric haze, which reduces visual range and obscures landscapes. This impact is noticeable in scenic areas, including national parks and wilderness areas, often located far from the original emission sources.
Sulfate aerosols also exert an indirect influence on climate. These fine particles are highly reflective and scatter incoming solar radiation back into space. This reflection has a cooling effect on the planet, temporarily offsetting some warming caused by greenhouse gases. However, this reflective property is outweighed by the negative local and regional impacts of acid deposition and reduced air quality.