Atmospheric Composition

The ancient Greeks thought that air was one of the four fundamental elements from which all other substances were made. In fact, air is a combination of gases, such as nitrogen and oxygen, and particles, such as dust, water droplets, and ice crystals. These gases and particles form Earth’s atmosphere, which surrounds Earth and extends from Earth’s surface to outer space.

Permanent atmospheric gases About 99 percent of the atmosphere is composed of nitrogen (N2) and oxygen (O2). The remaining 1 percent consists of argon (Ar), carbon dioxide (CO2), water vapor (H2O), and other trace gases, The amounts of nitrogen and oxygen in the atmosphere are fairly constant over recent time. However, over Earth’s history, the composition of the atmosphere has changed greatly. For example, Earth’s early atmosphere probably contained mostly helium (He), hydrogen (H2), methane (CH4), and ammonia (NH3). Today, oxygen and nitrogen are continually being recycled between the atmosphere, living organisms, the oceans, and Earth’s crust.

Variable atmospheric gases The concentrations of some atmospheric gases are not as constant over time as the concentrations of nitrogen and oxygen. Gases such as water vapor and ozone (O3) can vary significantly from place to place. The concentrations of some of these gases, such as water vapor and carbon dioxide, play an important role in regulating the amount of energy the atmosphere absorbs and emits back to Earth’s surface.

Water vapor Water vapor is the invisible, gaseous form of water. The amount of water vapor in the atmosphere can vary greatly over time and from one place to another. At a given place and time, the concentration of water vapor can be as much as 4 percent or as little as nearly zero. The concentration varies with the seasons, with the altitude of a particular mass of air, and with the properties of the surface beneath the air. Air over deserts, for instance, contains much less water vapor than the air over oceans.

Carbon dioxide Carbon dioxide, another variable gas, currently makes up about 0.039 percent of the atmosphere. During the past 150 years, measurements have shown that the concentration of atmospheric carbon dioxide has increased from about 0.028 percent to its present value. Carbon dioxide is also cycled between the atmosphere, the oceans, living organisms, and Earth’s rocks.

The recent increase in atmospheric carbon dioxide is due primarily to the burning of fossil fuels, such as oil, coal, and natural gas. These fuels are burned to heat buildings, produce electricity, and power vehicles. Burning fossil fuels can also produce other gases, such as sulfur dioxide and nitrous oxides, that can cause various respiratory illnesses, as well as other environmental problems.

Ozone Molecules of ozone are formed by the addition of an oxygen atom to an oxygen molecule. Most atmospheric ozone is found in the ozone layer, 20 km to 50 km above Earth’s surface,The maximum concentration of ozone in this layer—9.8 × 1012 molecules/cm3—is only about 0.0012 percent of the atmosphere. 

The ozone concentration in the ozone layer varies seasonally at higher latitudes, reaching a minimum in the spring. The greatest seasonal changes occur over Antarctica. During the past several decades, measured ozone levels over Antarctica in the spring have dropped significantly. This decrease is due to the presence of chemicals called chlorofluorocarbons (CFCs) that react with ozone and break it down in the atmosphere.

Atmospheric particles Earth’s atmosphere also contains variable amounts of solids in the form of tiny particles, such as dust, salt, and ice. Fine particles of dust and soil are carried into the atmosphere by wind. Winds also pick up salt particles from ocean spray. Airborne microorganisms, such as fungi and bacteria, can also be found attached to microscopic dust particles in the atmosphere.