What are secondary pollutants and how are they formed?

Pollutants may be primary (active on emission) or secondary (arising from primary pollutants undergoing physical or chemical change).

Primary pollutants include:

•        Nitrogen oxides (NOx): released by industrial processes and vehicles

•        Sulfur dioxide (SO2): mainly emitted from burning coal

•        Particulate matter: dust & ash from burning fossil fuels, wood etc.

•        HC – hydrocarbons (organic compounds consisting of only hydrogen and carbon atoms). Hydrocarbons occur naturally in crude oils, like petroleum and natural gas, which are made from fossilised/decomposed organic matter, so it is abundant in hydrogen and carbon atoms.

Secondary pollutants are created when a synergistic reaction (interaction) occurs between primary pollutants (e.g. NOx/SO2) and/or other substances e.g. ozone, sunlight, water

Secondary pollutants include:

• ·        Acid rain

• ·        Tropospheric ozone
• ·        Photochemical smog

Acid rain – SO2 or NOx react with water vapour  → creating sulphuric acid or nitric acid = this makes the rain acidic hence the term acid rain!

Tropospheric ozone  - formed from the photochemical breakdown (photolysis) of primary pollutants (NOx, CO -carbon monoxide, and HC)   

NO₂ + UV-A = NO + O

O + O2 = O3 (ozone)

Equation explanation: Nitrogen dioxide is broken down by UV to create Nitrogen monoxide (NO) and ozone (O); O is very reactive so combines with O2 to form Ozone (tropospheric ozone).

So tropospheric ozone is ozone that forms in the troposphere from the breakdown of nitrogen dioxide and nitrous oxide (primary pollutants) – tropospheric ozone is harmful to animals, humans & plants!

Remember – ozone should only exist in the stratosphere (where it filters harmful UV); in the troposphere it is a secondary pollutant).

Photochemical smog – NOx (from vehicles) reacts with O3 (tropospheric ozone), hydrocarbons and UV  → forming peroxyacetyl nitrates (PAN’s) or photochemical smog – a harmful secondary pollutant that causes many respiratory issues in humans & animals.

NOx + O3 + HC + UV-A = PAN's (photochemical smog)

To understand more about ozone, read this article – what is ozone?