Why is ozone a major component in photochemical smog?

Ozone is a major component in photochemical smog due to its formation from sunlight-driven reactions involving nitrogen oxides and volatile organic compounds.

Ozone (O3) is not directly emitted into the atmosphere, but is formed through a series of complex chemical reactions. These reactions are driven by sunlight, hence the term 'photochemical'. The primary precursors for ozone are nitrogen oxides (NOx) and volatile organic compounds (VOCs). These compounds are released into the atmosphere from various sources, including vehicle exhaust, industrial emissions, and natural sources such as vegetation.

When NOx and VOCs are emitted into the atmosphere, they can react with each other under the influence of sunlight to produce ozone. This process begins with the photolysis (breakdown by sunlight) of nitrogen dioxide (NO2) to form nitric oxide (NO) and a single oxygen atom. This oxygen atom can then react with an oxygen molecule (O2) to form ozone. VOCs can enhance this process by reacting with NO to regenerate NO2, which can then undergo photolysis again to produce more ozone.

The formation of ozone is a key aspect of photochemical smog, a type of air pollution that is characterised by a hazy, brownish-yellow colour and reduced visibility. Photochemical smog typically occurs in urban areas with high levels of vehicle emissions and strong sunlight. It can have harmful effects on human health, particularly on the respiratory system, and can also damage crops and other vegetation.

In addition to ozone, photochemical smog can contain other harmful pollutants, such as particulate matter and sulfur dioxide. However, ozone is often considered the primary pollutant in photochemical smog due to its high reactivity and the role it plays in the formation of other pollutants. For example, ozone can react with VOCs to form secondary organic aerosols, which contribute to the particulate matter in smog.

In summary, ozone is a major component in photochemical smog due to its formation from sunlight-driven reactions involving nitrogen oxides and volatile organic compounds. Its high reactivity and the role it plays in the formation of other pollutants also contribute to its prominence in photochemical smog.