How is ozone gas created


Lexicon> letter O> ozone

Definition: a chemically aggressive gas

More general term: air pollutant

Molecular formula: O3

English: ozone

Category: Ecology and Environmental Technology

Author: Dr. Rüdiger Paschotta

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Original creation: 08/19/2014; last change: May 31, 2020


Ozone (chemical formula: O3) is a chemically very aggressive gas. It consists of molecules that each contain three oxygen atoms - in contrast to ordinary oxygen gas (O2), whose molecules each consist of only two oxygen atoms. Ozone molecules easily give one oxygen atom to another, oxidizing the latter and turning the ozone into ordinary oxygen.

Ozone occurs naturally in the atmosphere. In the lower (near-earth) layers (in the troposphere), however, its concentration is usually very low. In contrast, considerable natural ozone concentrations occur in the stratosphere.

The ozone in deep atmospheric layers is very harmful to many living things (both animals and plants) because of its aggressive oxidizing effect (see below). On the other hand, stratospheric ozone is very important for life on earth, as ozone strongly absorbs ultraviolet light from the sun and thus protects living beings from this aggressive light. Above all over the Antarctic, the concentration of stratospheric ozone is greatly reduced in certain times of the year due to the influence of ozone-depleting substances that were previously emitted in large quantities. This effect is called Ozone hole designated. Certain fluoro-chloro-hydrocarbons play a particularly important role. B. from propellants, refrigerants and fire extinguishing agents. Since the emissions of such substances have now been greatly reduced worldwide through international agreements (Montreal Protocol), their concentration in the stratosphere is slowly decreasing again. As a result, the ozone hole is likely to close again within the next few decades.

Ozone also acts as a greenhouse gas. Increased ozone concentrations due to human activities therefore also contribute to a certain extent to climate hazards.

Formation of ozone

Ozone can be created directly from oxygen at very high temperatures. This is e.g. B. the case in lightning and in other electrical discharges, e.g. B. with corona discharges on high-voltage lines. Similar to the formation of nitrogen oxides, ozone largely breaks down again when it is slowly cooled down after the strong heating. However, in the event of sudden cooling, such as B. in the case of lightning, a substantial part of the ozone is retained.

Irradiation of air with ultraviolet light (UV-C radiation) z. B. from the sun, ozone can be created from oxygen. In this process, oxygen molecules are first broken down into individual oxygen atoms, and these then partially react with other oxygen molecules to form ozone.

Nitrogen oxides contribute significantly to the formation of ozone, although they can even cause ozone depletion locally. They often develop their ozone-forming effects far from their sources.

In addition, ozone is created in complex chemical reactions from other pollutants such as nitrogen oxides, which z. B. arise in large industrial combustion systems (including power plants) and in internal combustion engines of vehicles. If volatile hydrocarbons (also from exhaust gases or from natural sources) are also present, this promotes the formation of ozone. Ultraviolet light is also involved in such reactions; therefore, high ozone levels often occur in fine summer weather (Summer smog, photochemical smog). On the other hand, ozone can also be broken down again at high local nitrogen oxide concentrations. As a result, the ozone concentration in cities with high levels of air pollution is often not too high, but it can be greatly increased in the surrounding areas of such cities. Incidentally, ozone formation from nitrogen oxides is relatively slow and for this reason, to a large extent, only takes place outside the cities.

Air pollution control measures can alleviate the ozone problem, but even today they are not perfectly effective.

Various processes for cleaning exhaust gases counteract the formation of ozone. For example, modern power plants have effective denitrification systems, and the nitrogen oxide emissions from cars are now greatly reduced thanks to the catalytic converters used. However, the catalysts are z. B. largely ineffective before reaching the necessary working temperature, so that a lot of nitrogen oxides are still produced, especially in short-distance traffic.

Ozone is also produced industrially or in smaller plants for various purposes. For example, it is used in water treatment because of its disinfecting effect. The release of intentionally produced ozone only contributes marginally to air pollution; Energy technology processes are dominant here.

Ozone is also created when so-called ionizers are operated, which are intended to increase the quality of the air in rooms. However, this could lead to health disadvantages [2].

Health effects of ozone

In humans and animals, ozone mainly damages the respiratory tract. It gets into the lungs, where it oxidizes various substances and in this way causes irritation. This in turn can cause inflammation, which can have further adverse consequences. Lung function is also impaired and physical performance is reduced.

Further reductions in ozone pollution would be very desirable for health reasons.

Acute symptoms such as irritation of the mucous membranes, irritation of the throat and lacrimation occur with ozone concentrations of about 200 μg / m3 on. Frequent or long-lasting concentrations at this level lead to an increase in respiratory diseases such as asthma and also to premature death. The susceptibility to allergies (e.g. to grass pollen) also seems to increase due to increased ozone pollution. For these reasons, it would be very desirable for the ozone concentration in the air we breathe to stay below 100 μg / m3 remains. Unfortunately, in Germany too, ozone levels often occur which are considered harmful to health. Particularly at risk are people who already suffer from respiratory diseases, infants and toddlers and people who z. B. exercise physically demanding activities outdoors for work-related reasons.

In closed rooms, the ozone concentrations are usually significantly lower than in the open air, as the ozone reacts on walls and objects and breaks down in the process.

In Germany, an information threshold of 180 μg / m applies to ozone3 and an alarm threshold of 240 μg / m3 (both as a 1-hour value). If the information threshold is exceeded, the population is informed via the media and asked to behave appropriately. For example, you should avoid vigorous physical exertion outdoors. The target value of 120 μg / m3 the maximum 8-hour value for a day may be exceeded on a maximum of 25 days per year, averaged over 3 years. However, this is not achieved in many places. In many places there are annual mean values ​​between 50 and 75 μg / m3, and the summer peak values ​​are much higher. In principle, ozone can start at 40 μg / m3 can be recognized by its smell, but the smell will soon no longer be perceived through habituation.

Plants are also damaged by ozone. This results in reduced yields in agriculture.

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See also: exhaust gas, nitrogen oxides, refrigerants, unburned hydrocarbons
as well as other articles in the category ecology and environmental technology