Tropospheric ozone, a dangerous atmospheric immission

Tropospheric ozone (O3) or ground-level ozone is a gas found in the lowest layer of the Earth’s atmosphere, the troposphere, which extends up to 10 kilometres.

Also known as surface or ambient ozone, it becomes a harmful air pollutant when it comes into contact with the biosphere. It even becomes part of the dreaded smog that affects large cities and industrial areas.

Although tropospheric ozone shares a name with beneficial ozone, the ozone found in the stratosphere (the layer of the atmosphere that rises up to 50 kilometres above the Earth’s surface) does not play as important a role. Ozone is formed when intense light breaks down oxygen molecules (O2) and they recombine to form ozone (O3). 

Ozone reaches its highest density 20 kilometres above the Earth’s surface. This is where it forms the shield that protects life on Earth from harmful ultraviolet (UV) radiation from the sun.

Tropospheric ozone has properties and effects that differ significantly from atmospheric or good ozone. For example, it is not a gas that is emitted directly into the air, but originates through the interaction between pollutants and solar radiation

In addition, tropospheric ozone acts negatively on ecosystems as it damages vegetation by reducing its capacity for photosynthesis and contributes to climate change as it is a greenhouse gas that causes global warming at the earth’s surface.

Tropospheric ozone- Climate & Clean Air Coalition - Kunak

Tropospheric ozone – Climate & Clean Air Coalition

How tropospheric ozone is formed

It is formed in the troposphere when three oxygen atoms combine with nitrogen oxides (NOx), methane (CH4) and volatile organic compounds (VOCs)from anthropogenic activities such as the burning of fossil fuels and industrial activities such as oil refining or electricity generation. 

Together, these molecules form an environmental soup that undergoes a photochemical reaction (activated by sunlight) when the sun shines intensely and for hours. This is why spring and summer are the seasons with the greatest increase in tropospheric ozone. These are the times when the characteristic summer smog, which usually occurs in late summer in the northern hemisphere, is formed.

In summary, the formation of tropospheric ozone, as it does not come from a specific emission source, is an immission that requires the combination of various environmental factors in order to combine with molecular oxygen, usually in urban and industrialised areas:

  • Solar radiation
  • Ambient temperature
  • Increase in the concentration of emissions:
      • nitrogen oxides
      • volatile organic compounds
      • methane

Similarly, the effect of wind can cause precursor emissions for the formation of ambient ozone to travel long distances from their point of origin, thus affecting not only urban but also rural environments.

Efectos del ozono a nivel del suelo-Kunak

The effects of ground-level ozone

Health and environmental impacts of ground-level ozone

Although colourless and odourless, tropospheric ozone can cause irritation to the eyes, mucous membranes and respiratory tract, even at low concentrations. As well as causing breathing difficulties, it can cause headaches and aggravate existing respiratory diseases such as asthma, emphysema and bronchitis.

 In addition to respiratory problems, inhaled ozone can also affect the cardiovascular system. Recent studies suggest a link between exposure to inhaled ozone and an increased risk of heart attack and stroke.

It is particularly dangerous for sensitive populations such as children and the elderly. It also has a direct effect on lung infections and increased premature mortality.

Prolonged exposure to ozone air pollution is linked to one million premature deaths per year due to respiratory diseases.Climate & Clean Air Coalition (UNEP).

In wildlife, the presence of tropospheric ozone can cause pulmonary oedema. It also damages vegetation by blocking the photosynthetic capacity of plant structures, a factor that directly affects agricultural and forestry productivity. It also reduces the ability of plants to sequester carbon dioxide from the atmosphere.

The direct link between tropospheric or ambient ozone and climate change is very important. Its presence as a greenhouse gas contributes to the warming of the troposphere, where life on Earth thrives. It is one of the gases that contribute most to global warming, after methane (CH4) and carbon dioxide (CO2).

Measures to reduce tropospheric ozone

Total ozone refers to the overall proportion of ozone in a vertical column of the atmosphere, from the Earth’s surface to the outer edge of the stratosphere. Although most of the total ozone is in the stratosphere, tropospheric ozone also contributes to the total amount of ozone

Therefore, measurements of total ozone can help scientists understand ozone concentrations in both the stratosphere and the troposphere. Despite its contribution to total ozone, tropospheric ozone is considered an air pollutant. It is therefore essential for our health and the environment to limit the formation of tropospheric ozone and to reduce our emissions of the pollutants that contribute to its formation. 

Atmospheric ozone has undergone distinct changes in the stratosphere and troposphere during the second half of the 20th century, with depletion in the stratosphere and an increase in the troposphere.Liu, W., Hegglin, MI, Checa-García, R. et al. (2021).

To reduce tropospheric ozone levels and improve the quality of breathing air, it is crucial to reduce emissions of its precursors: nitrogen oxides, methane and volatile organic compounds. This implies implementing stricter emission control technologies, improving energy efficiency and promoting the use of cleaner energy.

In addition, it is essential to raise public awareness of the importance of adopting habits and behaviours that reduce the emission of these pollutants, such as using public transport, reducing the consumption of volatile chemicals at home and at work, and choosing products and services with a lower carbon footprint.

Through these actions, each individual can contribute to the reduction of ground-level ozone concentrations and the improvement of the quality of the air we breathe. With a comprehensive and collaborative approach, we can achieve cleaner and healthier air for all.

When and where tropospheric ozone is most prevalent

The concentration of ground-level ozone varies according to the time of day, season and geographical location. Ozone levels are generally highest during peak sunlight hours and in urban areas where air pollution is high. 

However, ozone has been shown to be transported by winds for tens of kilometres, which means that rural and remote areas can also be affected. It is therefore important to monitor tropospheric ozone levels and to take measures to reduce the formation of tropospheric ozone. 

Tropospheric ozone legislation and thresholds

Several countries have established regulations to control the concentration of tropospheric ozone in the open air. These regulations set limits expressed in Air Quality Index (AQI), which include total ozone, moderate ozone and other pollutants. Thresholds may vary according to geographic location, season and time of day.

Government environmental agencies are responsible for:

  • monitoring compliance with these regulations
  • conducting inspections
  • imposing penalties
  • promoting initiatives to improve air quality

However, it is a collective responsibility to ensure a healthy environment and to protect the population from the harmful effects of ground-level ozone. It is essential to understand that the protection of breathable air requires the active participation of the public and private sectors as well as citizens.

In the United States, legislation on tropospheric ozone is mainly regulated by the Clean Air Act. The United States Environmental Protection Agency (EPA) is responsible for setting national air quality standards for ozone to protect public health and the environment. 

Ground-level ozone may not exceed a concentration of 0.070 ppm (particles per million) in daily maximum eight-hour measurements averaged over three years.

The European Union regulates ozone in ambient air through Directive 2002/3/EC of the European Parliament and of the Council. It is important to ensure effective protection against harmful effects on human health from exposure to ozone. The harmful effects of ozone on vegetation, ecosystems and the environment as a whole should be reduced as far as possible. The transboundary nature of ozone pollution requires action at Community level.

Overview of threshold and target values and long-term objectives for atmospheric ground-level ozone

Target value for the protection of human health Target value for the protection of vegetation Long-term objective
for the protection of human health
Long-term objective
for the protection of vegetation
Information threshold
for the protection of human health
Alert threshold for the protection of human health
Maximum daily 8-hour mean: 120 µg/m3 on more than 25 days per calendar year averaged over three years AOT40* from May to July: 18 000 µg/m3 x h averaged over five years Maximum daily 8-hour mean within a calendar year: 120 µg/m3 AOT40* from May to July: 6 000 µg/m3 x h 1-hour concentration: 180 µg/m3 1-hour concentration: 240 µg/m3

* AOT40 (µg/m3 x hours) is the sum of the difference between hourly concentrations greater than 80 µg/m3 and 80 µg/m3 over a given period using only the 1-hour values measured between 8.00 and 20.00 Central European Time (CET) each day

The maximum permissible daily ambient ozone values are 120 µg/m3 averaged over a period of 25 days per calendar year averaged over three years. 

The 2024 revision of the EU Air Quality Act provides for a reduction of these measurements. It aims to achieve that by 2030 this reference value (120 µg/m3) will be considered for an average of 18 days per calendar year.

Niveles de O3-Organización Mundial Salud-Kunak

Tropospheric ozone levels – World Health Organization (WHO) Air Quality Guidelines

However, the World Health Organisation (WHO) sets a recommended level of 60 μg/m3 during the “high season”.  That is, the six consecutive months with the highest moving average ozone concentration. Within this time frame, average ozone concentrations are averaged over a daily maximum of 8 hours.