Sulfur dioxide (\(\text{SO}_{2}\)) is a colorless, reactive gas composed of one sulfur atom and two oxygen atoms. It is a well-known atmospheric pollutant released from both natural and human-made activities. \(\text{SO}_{2}\) is classified as a primary pollutant because it is emitted directly into the atmosphere in its harmful form. Understanding its sources and fate requires looking at how pollutants are categorized and how \(\text{SO}_{2}\) transforms after release.
Defining Primary and Secondary Pollutants
Air pollutants are categorized based on how they enter the atmosphere. A primary pollutant is any substance introduced directly into the air from a source. These substances cause harm immediately upon emission, without needing chemical change. Carbon monoxide (CO) from car exhaust is a common example.
A secondary pollutant is not emitted directly from a source. Instead, it forms in the atmosphere through chemical reactions involving primary pollutants, water vapor, or other atmospheric components. Ground-level ozone, a major component of smog, is a classic example, created when sunlight reacts with nitrogen oxides and volatile organic compounds. \(\text{SO}_{2}\) is often described as a precursor because, while primary, it initiates the formation of several harmful secondary pollutants.
Sulfur Dioxide’s Status and Key Emission Sources
The largest source of \(\text{SO}_{2}\) emissions globally is the combustion of sulfur-containing materials. These emissions are predominantly anthropogenic, meaning they are caused by human activity.
The burning of fossil fuels, particularly coal and oil, at power plants and industrial facilities accounts for the vast majority of human-caused \(\text{SO}_{2}\). In the United States, power plants are the single largest source, historically accounting for about 73% of total \(\text{SO}_{2}\) emissions. Other significant industrial sources include processing metals from ore and petroleum refining operations.
Smaller sources include large mobile sources, such as locomotives, ships, and heavy equipment, that burn high-sulfur fuels. Natural sources also contribute \(\text{SO}_{2}\), with volcanic eruptions being the most substantial non-anthropogenic source. However, human-related emissions remain the dominant factor contributing to widespread pollution.
Atmospheric Transformation and Environmental Impact
Once released, sulfur dioxide rarely stays in its original form for long. \(\text{SO}_{2}\) undergoes chemical transformations, transitioning to a precursor for secondary pollutants. This process involves oxidation by atmospheric agents, such as the hydroxyl radical (\(\text{OH}\)) in the gas phase or hydrogen peroxide and ozone within cloud droplets.
Oxidation converts \(\text{SO}_{2}\) into sulfur trioxide (\(\text{SO}_{3}\)), which rapidly reacts with atmospheric water vapor (\(\text{H}_{2}\text{O}\)). The final result is the formation of sulfuric acid (\(\text{H}_{2}\text{SO}_{4}\)), a highly corrosive secondary pollutant. Sulfuric acid is a major constituent of acid deposition, which can be transported by wind over hundreds of miles.
Acid deposition damages sensitive ecosystems, acidifies soils, and accelerates the corrosion of buildings and monuments. The sulfuric acid also forms fine sulfate particles, a major component of particulate matter pollution. These particles penetrate deep into the lungs, causing or aggravating respiratory illnesses like asthma. The atmospheric lifetime of \(\text{SO}_{2}\) is typically only a few days before removal through these transformations and deposition.