Earth's atmosphere is a dynamic system of natural gases that are necessary to sustain life, and while the planet has defense mechanisms to absorb small quantities of air pollutants, high levels of gases can cause ozone depletion and other problems for living organisms. The main sources of gaseous air pollutants are fuel combustion in stationary sources, such as coal-burning power plants, as well as emissions from automobiles. While these gases are not the only ones contributing to air pollution, they are regarded as dominant sources of this world-wide problem.
Carbon oxides are among the most well-known greenhouse gases that contribute to the air pollution plaguing the Earth's atmosphere. Carbon monoxide (CO) is a poisonous gas, dangerous due to its lack of odor and color, that is released into the atmosphere with the incomplete combustion of fuels, such as coal, wood or other natural sources, as well as exhaust from automobiles. Carbon dioxide (CO2) is the greenhouse gas widely considered the main air pollutant in the Earth's atmosphere. Despite the fact that carbon dioxide is essential to support living organisms, it is considered a dangerous air pollutant caused by human activities such as deforestation and the burning of fossil fuels. Responsible for more than half of the global warming trend, carbon dioxide restricts infrared radiation leaving the Earth's surface, causing the "greenhouse effect."
Nitrogen oxides (NOx) are air pollutants that contribute the most contaminants to the Earth's atmospheric. Like carbon oxides, vehicle emissions are a major source of nitrogen oxides, and these air pollutants are easily recognizable by the brown plume or haze that forms over areas with high concentrations of the gases. Nitrogen dioxide (NO2) is one of the most prominent and dangerous air pollutants, and this toxic gas is easily identifiable by its reddish-brown color and distinctive, sharp odor.
Sulfur oxides (SOx) are another group of gases polluting the Earth's atmosphere. Of particular concern is sulfur dioxide (SO2), one of the major components of smog and a primary cause of acid rain. While sulfur dioxide is naturally produced by erupting volcanoes, the combustion of sulfur-containing fuels such as petroleum oils and coal have caused this gas to become a dangerous air pollutant that is eating away at the Earth's fragile atmosphere. Dangerous to both plants and animals, sulfur oxides can injure organic matter when deposited in high concentrations and cause respiratory problems by irritating air passages and lungs.