Sulfur and Nitrogen Compounds

For our consideration of acidic deposition, we will limit discussion to sulfur and nitrogen compounds. The major sulfur-based pollutant is sulfur dioxide (SO₂). Sulfur is a constituent of most coal, but the sulfur content varies with the location of the mine. Coal from the western United States is very low in sulfur, whereas eastern-US coal has higher sulfur content. At one time it was a state law that Iowa State University was required to burn coal mined in Iowa. This coal had a quite high sulfur content and produced large amounts of sulfur dioxide as a byproduct of combustion.

SO₂ gas oxidizes to a sulfate (SO₄) at a rate of 4% per hour. Airborne sulfate particles are therefore deposited back to the earth's surface before they travel very far from their sources. Later in the semester we will discuss the rapidly growing economies of China and other Asian countries. For many of these developing countries, including India, their growth likely will be based on expanding power production that depends on coal of quite high sulfur. The consequences of this are more serious than simply increased acid deposition, as we will learn when we talk about global warming and the effects of sulfates.

Other sulfur compounds are given in Figure 3.

Hydrogen sulfide, H₂S, sometimes known as rotten-egg gas, is produced naturally in the soils and decaying vegetation, but also is produced by combustion. Dimethyl sulfide also has natural sources and over the ocean is believed to be an agent in enhancing natural precipitation.

Nitrogen compounds in the lower atmosphere include nitric oxide, nitrogen dioxide, nitrous oxide, nitrogen trioxide, and ammonia.

Of these, nitrous oxide does not really belong with acid-producing chemicals because it is a very stable compound and does not decompose rapidly enough to participate in chemical reactions in the troposphere.

PREVIOUS: Atmospheric Transport

NEXT: pH