Environmental Pollution: Primary and Secondary Pollutants

Environmental pollution is defined as any undesirable change in the physical, chemical or

biological characteristics of air, water or soil (or in the levels of energy such as heat, noise

or radiation) that is harmful to living organisms, including humans. The substance responsible for

such a change is called a pollutant. A pollutant can be a solid, liquid or gas, and it is

considered a pollutant only when its concentration in the environment is high enough to cause

harm -- many substances that are pollutants at high concentration (for example, carbon dioxide or

nitrogen compounds) occur naturally at low, harmless levels.

Pollutants are classified into two broad categories based on how they enter the environment.

Primary pollutants are substances that are emitted directly into the environment from an

identifiable source and persist in the same chemical form in which they were released. Common

examples are carbon monoxide ($\text{CO}$) from vehicle exhaust, sulfur dioxide ($\text{SO}_2$)

from burning sulfur-containing coal, oxides of nitrogen ($\text{NO}, \text{NO}_2$) from

high-temperature combustion, unburnt hydrocarbons, and particulate matter released from chimneys

and vehicle tailpipes.

Secondary pollutants are not emitted directly at all. They are formed when primary pollutants

undergo chemical reactions in the atmosphere -- typically oxidation, hydrolysis, or reactions

driven by sunlight (photochemical reactions). Ozone ($\text{O}_3$) formed near ground level and

peroxyacetyl nitrate (PAN), both products of sunlight acting on $\text{NO}_x$ and hydrocarbons, are

classic secondary pollutants, as is sulfuric acid ($\text{H}_2\text{SO}_4$) formed when

$\text{SO}_2$ is oxidised and dissolved in atmospheric moisture.

This primary/secondary distinction matters for pollution control: reducing a primary pollutant at

its source (for example, removing sulfur from diesel before it is burned) directly reduces both

that primary pollutant and every secondary pollutant that would otherwise have formed from it

downstream. The rest of this chapter looks in turn at pollution of the three environmental

compartments -- air, water and soil -- the chemistry behind each, and the practical strategies used

to control and prevent them, including the modern approach of green chemistry.