- Types of air pollutants - Declining emissions - Acid deposition
Primary air pollutants include particulates, volatile organic compounds, carbon monoxide, nitrogen oxides, sulfur dioxide, lead and air toxics. They are the direct result of combustion and evaporation. This happens from human industrial activity and the combustion of fossil fuels primarily.
Secondary air pollutants include ozone, peroxyacetyl nitrates, sulfuric acid and nitric acid. These occur after primary pollutants undergo further chemical changes. These air pollutants are created through photochemical reactions between air pollutants and oxidation of primary air pollutants by OH radicals.
The EPA tracks pollutants and measures ambient conditions in air quality throughout the country.
In 2011, emissions were at 99 million tons of primary pollutants. In 1970, before the Clean Air Act was passed, emissions were at 301 million tons. Despite increasing domestic activity and more vehicles on the road, air quality continues to improve over the years.
Acid deposition is precipitation, whether it is rain, fog, mist or snow, that is more acidic than usual.
Acid precipitation in North America and Europe has been between 10 and 1,000 times more acidic than usual. Acid precipitation is widespread in North America. It increases primary air pollutants in the air, kills aquatic life and makes lands toxic with aluminum.
The Clean Air Act has helped reduce acid precipitation as many of the industrial plants that generate it have had to improve their facilities and reduce emissions.