Ground-level ozone and photochemical smog

There is only one ozone, but it can have two different effects on humans and the environment. The ozone in the ozone layer protects life on earth from UV radiation from the sun. Ground-level ozone, on the other hand, damages people, plants, and ecosystems and is one of the main components of photochemical smog, also known as summer smog.

Photo by Andreas Strandman on Unsplash

Formation of ground-level ozone

Ozone is the result of various conversion processes of precursor substances such as nitrogen oxides and volatile organic compounds (VOCs). When exposed to UV radiation, nitrogen dioxide (NO2) breaks down into nitrogen monoxide (NO) and an oxygen atom (O). This oxygen atom then combines with the oxygen in the air, which occurs molecularly as O2 and thereby forms ozone (O3).

VOCs such as olefins or aromatics enable the reaction of NO to NO2, which can then form ozone again. At the same time, this reaction also runs in the other direction, where ozone is broken down so that an equilibrium is established that varies depending on the region and the composition of the air. The process is promoted by strong solar radiation and high temperatures. A good example of a city struggling with high levels of ozone and summer smog in Los Angeles in sunny California, where the main source of nitrogen oxides is car traffic.

Ozone concentrations can vary greatly depending on the geographical location

The ozone concentration is often low on roads with heavy traffic because although there are many precursor substances there, there is also enough NO, which breaks down ozone. Ozone concentrations are particularly higher in areas with little traffic, in city parks, and on the outskirts, as less is broken down there and more VOCs are in the air due to plants or air currents, which promote ozone formation. This is why attention is paid to plants with low VOC emissions in urban greening.

Health impact

Ozone is reactive and irritates the mucous membranes, eyes, and lungs. This is also reflected in decreased lung function. Above all, long-term physical exertion should be avoided at high concentrations, as these strongly irritate the respiratory tract. To avoid the problem, the activity can be postponed to another time of day where the concentrations are lower.

Measures against ozone formation

In urban greening, the production of precursor substances can be reduced through the targeted selection of plant species that emit a small amount of VOCs.
Lower-emission or zero-emission vehicles, but also public transport, cycling, and walking reduce the emissions of the precursor substances. But it is also up to politics to create better alternatives to private transport and to regulate industrial emissions.

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