PAN (Peroxy Acetyl Nitrate) is a secondary pollutant.

Definition: A primary pollutant is an air pollutant emitted directly from a source. A secondary pollutant is not directly emitted as such, but forms when other pollutants (primary pollutants) react in the atmosphere.

Secondary pollutants are pollutants which form in the atmosphere. These pollutants are not emitted directly from a source (like vehicles or power plants). Photochemical smog is made up of various secondary pollutants like ozone, peroxyacyl nitrates (PANs), and nitric acid (seen in Figure 2).

Nitrogen dioxide, when prevalent in the air, appears as a reddish-brown haze. Nitrogen dioxide and other nitrogen oxides react with other chemicals in the air to form other pollutants, known as secondary pollutants. These secondary pollutants include ozone, particulate matter, acid rain, and other toxic chemicals.

PANs are secondary pollutants, which means they are not directly emitted as exhaust from power plants or internal combustion engines, but they are formed from other pollutants by chemical reactions in the atmosphere.

Examples of a secondary pollutant include ozone, which is formed when hydrocarbons (HC) and nitrogen oxides (NOx) combine in the presence of sunlight; NO2, which is formed as NO combines with oxygen in the air; and acid rain, which is formed when sulfur dioxide or nitrogen oxides react with water.

Peroxyacyl nitrates or PANs, which are also referred to as acyl peroxy nitrates or APN, are a component of photochemical smog, produced in the atmosphere when oxidized volatile organic compounds combine with nitrogen dioxide ().

The pollutants have been classified into primary and secondary categories. The major primary pollutants are Oxides of Sulphur, Oxides of Nitrogen, Oxides of Carbon, Particulate Matter, Methane, Ammonia, Chlorofluorocarbons, Toxic metals etc. The secondary pollutants are not emitted directly.

Primary air pollutants: Pollutants that are formed and emitted directly from particular sources. Examples are particulates, carbon monoxide, nitrogen oxide, and sulfur oxide. Secondary air pollutants: Pollutants that are formed in the lower atmosphere by chemical reactions.

Primary pollutants are directly generated from human activities like from car, from ships, from construction work etc. Secondary pollutants form due to mixing of primary pollutants and natural resources such as water, air etc. They are harmful like mixing of sulphuric acid susoended in air with rain to form acid rain .

Pollution may muddy landscapes, poison soils and waterways, or kill plants and animals. Humans are also regularly harmed by pollution. Long-term exposure to air pollution, for example, can lead to chronic respiratory disease, lung cancer and other diseases.

Photochemical smog is produced when sunlight reacts with nitrogen oxides and at least one volatile organic compound (VOC) in the atmosphere. Nitrogen oxides come from car exhaust, coal power plants, and factory emissions. When sunlight hits these chemicals, they form airborne particles and ground-level ozone—or smog.

Primary pollutants come directly from sources such as industrial facilities, automobiles and forest fires. These include sulfur and nitrogen compounds, particulate matter, volatile organic compounds such as paint fumes and solvents, and toxic metals such as mercury.

Mercury is a persistent, bioaccumulative, toxic pollutant. When released into the environment, it accumulates in water laid sediments where it converts into toxic methylmercury and enters the food chain.

UNDERSTANDING THE MERCURY CYCLE Mercury is emitted by natural sources, such as volcanoes, geothermal springs, geologic deposits, and the ocean. Human-related sources primarily include coal combustion, waste incineration, industrial uses, and mining.

A secondary pollutant is a gaseous product of a reaction involving a primary pollutant. The EPA has set standards for formaldehyde levels in the air. Nothing is being done to control methane emissions. The EPA has some voluntary programs to reduce methane, but nothing mandatory.