Earth's atmosphere is divided into layers based on temperature variations. These layers include the troposphere, stratosphere, mesosphere, thermosphere, and exosphere. Atmospheric pressure and density decreases with altitude.

There are several major categories of air pollutants. Particulate matter, sulfur dioxides, nitrogen oxides, greenhouse gases, and tropospheric ozone are several common types of pollutants over urban areas. Tropospheric ozone is a component of photochemical smog, the brown haze common over cities. Nitrogen oxides enter the atmosphere primarily from automobile exhaust, and sulfur dioxides mainly from the combustion of fossil fuels. Greenhouse gases are found naturally in trace amounts in Earth's atmosphere; however, human activities have increased atmospheric concentrations of carbon dioxide in the relatively recent past. All these air pollutants have harmful effects on humans, other organisms, and entire ecosystems.