Acid deposition occurs when acidic components fall to the Earth from the atmosphere, encompassing both wet and dry forms. Wet deposition is commonly recognized as acid rain, snow, or fog, where the acidic compounds are dissolved in precipitation. Dry deposition occurs when acidic gases and fine particles settle directly onto surfaces like soil, vegetation, and buildings during dry weather periods. This phenomenon results from specific atmospheric gases released primarily through human activity that transform into strong acids.
The Chemical Precursors
The formation of acid deposition begins with the emission of two primary gaseous pollutants: sulfur dioxide (\(\text{SO}_2\)) and nitrogen oxides (\(\text{NO}_x\)). Once released into the atmosphere, these gases react with water vapor, oxygen, and other chemicals. Sulfur dioxide is oxidized to form sulfur trioxide (\(\text{SO}_3\)), which quickly combines with atmospheric water to produce sulfuric acid (\(\text{H}_2\text{SO}_4\)). Nitrogen oxides, typically emitted as nitric oxide (\(\text{NO}\)), are oxidized to nitrogen dioxide (\(\text{NO}_2\)). The nitrogen dioxide then reacts with hydroxyl radicals (\(\text{OH}\)) to form nitric acid (\(\text{HNO}_3\)). Both sulfuric and nitric acids dissolve easily into water droplets, lowering the \(\text{pH}\) of rain, or they can remain as fine acidic particles that contribute to dry deposition.
Primary Anthropogenic Sources: Energy Production and Smelting
The combustion of fossil fuels for electricity generation constitutes the largest source of the gases that lead to acid deposition globally. Power plants, particularly those burning coal, are the major fixed sources, releasing significant quantities of sulfur dioxide. Sulfur is a natural impurity within coal and oil, and when these fuels are burned at high temperatures, the sulfur content is oxidized and released as sulfur dioxide gas. Industrial processing, specifically the smelting of metal ores, is another large-scale source of sulfur dioxide emissions. Smelting involves high-temperature processes used to extract metals like copper, lead, and zinc from their ores. Many metal ores contain high concentrations of sulfur, and the smelting process drives off this sulfur in the form of sulfur dioxide gas. This makes smelters a localized yet substantial source of the precursor gases.
Secondary Anthropogenic Sources: Transportation and Agriculture
The transportation sector is a primary source of nitrogen oxides. Mobile sources, including cars, trucks, buses, aircraft, and ships, generate \(\text{NO}_x\) when the high heat of their internal combustion engines causes atmospheric nitrogen and oxygen to chemically combine. This is known as thermal \(\text{NO}_x\) formation and is particularly pronounced in densely populated urban corridors with heavy traffic. Agricultural practices contribute to nitrogen oxide emissions through the use of synthetic nitrogen fertilizers. When these fertilizers are applied to soil, microbial processes convert the nitrogen into various gaseous forms, including nitrogen oxides, which are then released into the atmosphere.
Natural Contributions
Natural events also contribute a background level of the precursor gases. Volcanic eruptions release substantial amounts of sulfur dioxide and other sulfur compounds directly into the atmosphere. These eruptions can be intermittent but powerful sources of acid precursors, sometimes affecting regional air quality for extended periods. Lightning strikes are a natural source of nitrogen oxides. The intense heat generated by lightning causes the nitrogen and oxygen gases naturally present in the air to react, forming small amounts of \(\text{NO}_x\). Additionally, biological decay processes, such as the decomposition of organic matter in wetlands and forests, release reduced sulfur gases that are eventually converted to sulfur dioxide in the atmosphere. Although these natural sources exist, their contribution is typically dwarfed by the emissions from industrial and mobile sources in industrialized regions.