What Are Some Gases That Make Up Photochemical Smog?

Photochemical smog is a secondary pollutant made up of several gases. The primary gases found in photochemical smog are nitrogen oxides, Volatile Organic Compounds (VOCs), ground-level ozone (O3) and Peroxyacyl Nitrate (PAN). In addition to primary pollutants being present, for photochemical smog to form there must be an abundance of strong sunlight, the air must be still, and the temperature must be greater than 64° Fahrenheit or 18° Celsius.

Nitrogen Oxides

• Primary gases in photochemical smog are nitrogen oxides. Nitrogen oxides are a mixture of gases, two of which are significant in the production of photochemical smog. These two are nitric oxide and nitrogen dioxide. Nitrogen oxides come from combustion processes such as the burning of gas in motor vehicles, burning of coal or even forest fires.

VOCs

• VOCs are organic compounds that easily turn to gases and are the result of gasoline exhaust (from cars, trucks, buses and gasoline stations), industrial emissions or paint fumes. They are composed of hydrogen, carbon and other elements such as sulfur and nitrogen. VOCs are sometimes referred to as hydrocarbons.

Ozone

• The type of ozone found in photochemical smog is located in the troposphere, the layer of the atmosphere that is closest to the Earth's surface. This type of ozone is referred to as ground-level ozone and is harmful. Ground-level ozone forms from a chemical reaction involving nitrogen oxides, VOCs and sunlight. When sunlight and hot weather are present, ground-level ozone can reach harmful concentrations if there is no wind.

PANs

• PAN forms as the result of complex chemical reactions between nitric oxide (NO) and VOCs. Scientists began studying PAN as early as the 1940s when plants on farms near Los Angeles began showing ill effects, found to result from the city's onset of smog. Scientists found PAN is 10 to 50 times more toxic than oxone to plants and is a major eye irritant.

Forming Photochemical Smog

• For photochemical smog to form, several chemical reactions need to take place between nitrogen oxides, VOCs, and sunlight. Although photochemical smog is the result of a series of complex chemical reactions, it can be illustrated by a simple example that starts with heavy traffic during the morning commute. The by-products of the fuel burning vehicles, nitrogen oxides and VOCs, are emitted in the exhaust fumes coming from the vehicles. The nitrogen oxides and VOCs quickly react to form nitrogen dioxide (NO2). Then, as the day progresses, the sunlight becomes stronger and more intense. The sunlight reacts with the nitrogen dioxide (NO2) to form ground-level ozone. The nitrogen dioxide (NO2) also reacts with VOCs to form PAN. This combination of toxic gases makes up photochemical smog. These chemical reactions are ongoing throughout the day and the photochemical smog, under the right circumstances, increases as the day goes on and can get so bad it can reduce visibility.