Air Pollution Ozone

1. What is Ozone (O₃)

Ozone (O₃) is a gas that can form and react under the action of light and that is present in two layers of the atmosphere: the stratosphere and the troposphere.

In the stratosphere, ozone forms a layer that shields the Earth from ultraviolet rays.

However, in the lower atmosphere (troposphere), ozone (O₃) is the most important photochemical oxidant. There, it is a secondary pollutant formed when precursor pollutants such as nitrogen oxides (NO_x) and volatile organic compounds react under the action of light.

Near strong emission sources of nitrogen oxides (NO_x), where there is an abundance of NO, ozone is “scavenged” as it reacts with NO. As a result its concentrations are often low in busy urban centres and higher in suburban and adjacent rural areas. However, ozone is also transported long distances in the atmosphere and is therefore considered a trans-boundary problem.

Because the formation of ozone requires light, ozone concentrations fluctuate depending on season and time of day, with higher concentrations in the summer and in the afternoons.

Controlled exposure studies on humans and animals have provided evidence that ozone can cause adverse health effects. However, more research is needed, especially addressing the spatial and seasonal patterns of ozone exposure and related health effects. More...