Sulfur dioxide is a chemical compound with the simple chemical formula SO2. This compound consists of one sulfur atom covalently bonded to two oxygen atoms. It is a common atmospheric pollutant that is regulated globally. It is a colorless gas that has a characteristic pungent odor, often described as the smell of a burnt match. The presence of this gas in the atmosphere is closely linked to both human industrial activity and certain natural processes.
Molecular Structure and Key Physical Properties
The molecular geometry of sulfur dioxide is bent or V-shaped. This shape is determined by the electron arrangement around the central sulfur atom, a concept described by Valence Shell Electron Pair Repulsion (VSEPR) theory. The sulfur atom is bonded to the two oxygen atoms and also possesses one lone pair of non-bonding electrons. This lone pair exerts a stronger repulsive force than the bonding pairs, resulting in an O-S-O bond angle of approximately \(119^{\circ}\).
This asymmetrical, bent structure makes the SO2 molecule polar. Although the sulfur-oxygen bonds are polar, the asymmetrical shape means the individual bond dipoles do not cancel each other out. The molecule possesses a net dipole moment, which influences its physical behavior, such as its high solubility in water. Under standard conditions, SO2 is a gas, but it can be liquefied under pressure or by cooling it to its boiling point of approximately \(-10^{\circ}\text{C}\).
Primary Sources of Sulfur Dioxide
The release of SO2 into the atmosphere originates from both human-caused (anthropogenic) and natural sources. Anthropogenic emissions are the primary source globally. The largest single source is the combustion of sulfur-containing fossil fuels, such as coal and heavy fuel oil, predominantly utilized in power plants for electricity generation.
Industrial processes also contribute significantly, particularly facilities involved in metal extraction and smelting of sulfur-containing ores. The transportation sector, including ships and locomotives that burn high-sulfur fuels, is another element. In contrast, the most significant natural source of sulfur dioxide is volcanic activity, where erupting volcanoes and constant degassing release substantial amounts of the gas into the atmosphere.
Environmental and Respiratory Health Impacts
Once released, sulfur dioxide contributes to negative environmental and health outcomes, beginning with its transformation in the atmosphere. SO2 reacts with water vapor, oxygen, and other chemicals in the air to form acidic compounds. This process leads to the formation of sulfuric acid (H2SO4), which is the main component of acid rain.
Acid rain causes damage to ecosystems when it falls to the ground. It can acidify lakes and streams, harming aquatic life and impacting freshwater ecosystems. The acid deposition also damages vegetation, contributing to the deterioration of trees and other plant life, especially at high elevations. Beyond biological systems, acid rain accelerates the corrosion of human-made structures, including building materials, paints, and culturally significant objects like statues and monuments.
Sulfur dioxide gas and its atmospheric derivatives also significantly affect human health, acting as a respiratory irritant. Short-term exposure can cause immediate symptoms like wheezing, coughing, and chest tightness. The gas irritates the mucous membranes of the eyes, nose, throat, and lower respiratory tract, often leading to increased mucus secretion.
People with existing respiratory illnesses are particularly vulnerable to SO2 exposure. Exposure can exacerbate conditions such as asthma and chronic bronchitis, making breathing difficult, especially during physical activity. Furthermore, SO2 reacts in the air to form fine sulfate particles, which contribute to particulate matter pollution and haze that reduces visibility. These sulfate aerosols can penetrate deep into the lungs, increasing the risk of hospital admissions and emergency room visits for children, the elderly, and asthmatics.