GE-I-9: Ozone burden | Umweltbundesamt

GE-I-9: Ozone burden

Ozone forms from nitrogen dioxide and volatile organic compounds at high temperature and sun irradiation. The emissions from those substances decreased distinctly since 1995; this led to a decline in peak concentrations of ozone. How­ever, at the same time, there was less ozone depletion caused by nitrogen monoxide, thus largely leading to ozone concentrations stagnating during the peak season. In 2003 and 2018, extreme weather conditions led to high ozone burdens.

Ozone burden caused repeatedly by weather patterns

According to scientific research, there is an increasing amount of evidence pointing to the influence of weather conditions on the increase in diseases and mortalities related to conditions of the respiratory tract. Apart from temperature, it is air humidity which is of particular relevance in respect of illnesses affecting the respiratory tract, as dry air dries up the mucous membranes and facilitates virus infections. However, high air humidity can also make breathing difficult, and it can increase the number of allergens such as dust mites and moulds in the air. Likewise, wind, air pressure and thunderstorms can cause respiratory complaints. There is also reliable evidence now for the influence of weather patterns on heart disease.

The relationships are more complex between meteorological factors and air-hygienic components and their impacts on human health. Interaction takes place between temperature, air humidity, air pressure and air pollutants such as nitrogen oxide, ozone and particulate matter. Owing to changed mixing processes in the troposphere, in other words, the lowest layer of the earth’s atmosphere, and photo-chemical reactions, there is a potential for the frequency of high air pollutant burdens increasing. Furthermore, high air temperatures increase the impact of air pollutants. Proteins adhering to particulate matter are transported more deeply into the respiratory tract. Likewise, ozone and nitrogen oxides increase the allergenicity of pollen (cf. Indicator GE-I-3).

The indicator focuses on the ozone as an air pollutant, given that weather-pattern related factors are of particular significance to the formation of ozone. Ozone occurring near the ground will not be released at once; in fact it is formed only as a result of intensive sun irradiation on account of complex photo-chemical processes produced by precursors − mostly nitrogen oxides and volatile organic compounds. This entails that ozone concentrations are likely to increase more in rural areas than in conurbations. Especially for the volatile organic compounds, vapours from deciduous and coniferous trees are relevant; biogenic nitrogen oxides emanate mostly from over-fertilised soils. The emissions from precursor substances have decreased since 1995; this has led to a distinct decline in peak concentrations of ozone. However, owing to a reduction in the titration effect (ozone depletion by nitrogen monoxide) this was associated with an increase in medium-high ozone concentrations.

High ozone concentrations are injurious to human health. They can reduce the functionality of lungs, cause inflammatory reactions in the respiratory tract as well as breathing difficulties. These impacts can become serious during physical exertion and intensified breathing. Sensitive individuals or those with pre-existing complaints such as asthmatic persons are particularly vulnerable. They are advised to avoid physical exertions outdoors while ozone levels are high. Given the fact that ozone is highly reactive, it is reasonable to assume that ozone might be carcinogenic.

In cases where the ozone values reach or exceed an hourly mean of 180 µg/m³ air, the population is informed accordingly and provided with behavioural recommendations; an hourly mean of 240 µg/m³ triggers warning messages. Furthermore, a target value has been set for the purpose of health protection. The maximum daily 8-hour value of 120 µg/m³ must not be exceeded on more than 25 days per calendar year. The mean of the days is calculated across three years. The WHO published new recommendations for air quality in 2021. These recommendations have supplemented the 8-hour value applied so far by a value for the so-called ‘ozone peak season’, thus facilitating the illustration of the long-term effect. The ‘ozone peak season’ is defined as the phase of six consecutive months with the highest ozone concentrations. For the DAS Monitoring Indicator, this season was fixed at April to September, as in Germany increased ozone concentrations near the ground occur mainly during that time. Beyond this half-year period, the mean of the daily maximum 8-hour values has been calculated. According to the WHO, the critical threshold value is set at a concentration of 60 µg/m³.

The Länder and the UBA measure ozone concentrations within a nationwide measuring network comprising more than 260 stations. The measuring stations are divided into the categories ‘urban background’, ‘rural background’, ‘industrial background’ and ‘mountain background’, as these are characterised by different preconditions for ozone formation. At the mountain stations – stations at altitudes from 900 metres above sea level – and at rural stations, the ozone concentrations measured are typically higher than at urban stations. This is due to the fact that the concentrations of nitrogen oxide contained in exhaust gases emitted by car exhausts are lower in rural areas. Nitrogen monoxide reacts with ozone and leads to ozone depletion, especially during the night. In fact, ozone precursor substances are transported by wind beyond urban areas, thus contributing to the formation of ozone at some distance from their actual source. The concentration values measured at industrial stations are comparable to those from urban stations.

The concentrations measured during the ‘ozone peak season’ have remained almost unchanged since 1995 whilst they even decreased in the case of the mountain stations. This is due to the fact that the emissions from the ozone precursor substances were regressive. However, the time series indicates fluctuations from year to year which can be attributed, above all, to specific weather patterns occurring in summer. For example, the ‘ozone summer’ of 2003, during a particularly long period of sunny summer weather with above-average temperatures and long sunshine periods, allowed the formation of high ozone concentrations. Quite the opposite happened in the summer of 2018, indicating new high temperature records, especially in July and August. Although the mean concentration values were increased, high peak concentration values were few and far between. This demonstrates that despite regressive concentrations of the ozone precursor substances, extreme weather patterns can still lead to distinctly increased ozone concentrations. With higher temperatures and more intensive sun irradiation, climate change drives the formation of ozone near the ground, which can potentially lead to recurring stresses on health owing to excessive ozone concentrations.