Outdoor air pollution consists of contaminants, either from natural sources or human activities, that cause health or environmental problems.
Of the anthropogenic sources of pollutants, there are stationary sources, such as industrial plants, and mobile sources, such as cars.
The diverse sources of pollution release a variety of chemicals and small particles directly into the air. These are primary pollutants. In addition, some primary air pollutants react with one another or with other chemicals in the air to form secondary air pollutants.
For example, ground-level ozone is a secondary pollutant that forms when some of the pollutants released during fossil fuel combustion react with atmospheric oxygen in the presence of sunlight.
Sulfuric acid and nitric acid are secondary pollutants that form when sulfur and nitrogen oxides react with water and oxygen in the atmosphere. The sulfuric and nitric acid can later fall to the ground as acid rain.
One of the most problematic characteristics of air pollution is that it moves, pushed by winds through the atmosphere. Air pollution produced in one city can end up harming humans and other species halfway around the globe.
In the atmosphere, these compounds react with oxygen and water to create acids.
With rain or snow, these compounds dissolve and fall to earth in wet deposition. Even dry particles can fall to Earth in dry deposition.
Acid deposition has serious consequences. For instance, acidified soils release aluminum that normally stays sequestered. Aluminum is taken up by plants and is toxic—too much will kill the plant.
In contrast, plants must have the ability to take up nutrients such as nitrogen and phosphorus. Acid decreases their ability to do so, thus also decreasing a plant's ability to grow and thrive.
Trees at higher elevations that stay shrouded in an acidic fog for much of the year are more affected than others—the acidic fog surrounds the tree for much longer than rainfall would.
Acid deposition can acidify lakes, harming aquatic life. "Dead lakes" with little or no life are often the first sign that acidification has reached harmful levels. Dying forests may follow.