Ground Level Ozone: The Facts

Ground-level ozone is a major constituent of photochemical smog, a toxic haze which forms mostly in summer in warm urban areas across the globe. Smog is particularly severe in India, Pakistan and China, many of whose cities are notorious for their unhealthy air quality. Ground level ozone is a secondary pollutant, meaning it is not emitted, but instead forms in the air when certain airborne gases react together. Motor vehicles are a major source of these gases, which is why the introduction of electric vehicles (EVs) may significantly reduce this type of air pollution.
Ground Level Ozone from car fumes
Transport fumes contribute to ground level ozone pollution. Photo: CC0 Public Domain

In this article we look at ground level ozone – a type of air pollution associated with summer smog – and explain what it is, where it comes from, and what health effects it has on humans and the environment.

What is Ground Level Ozone?

Ground level ozone (O3) should not be confused with the “good” ozone layer in the stratosphere that shields us from high-energy, ultraviolet solar radiation. Although it has the same chemical make-up (three oxygen atoms) as its sister, ground level ozone is seen as “bad” because it’s a very unhealthy pollutant. It contributes significantly to air pollution in cities and other industrialized areas around the world, notably during the summer months, or periods of heat.

In addition, it shares some of the heat-trapping qualities of greenhouse gases like carbon dioxide (CO2). Ground level ozone is sometimes called tropospheric ozone because it remains in the troposphere, the layer of air closest to the earth’s surface. It has nothing to do with the Montreal Protocol which is designed to protect stratospheric ozone from destruction by fluorocarbon gases.

Where Exactly is the Troposphere?
The troposphere is the lowest layer of air surrounding the planet. Its average height around the globe is roughly 13km (8 miles), although this varies from the equator to the poles. According to the NOAA, it’s about 18-20 km (59,000-65,600 ft) high at the equator; and roughly (4 miles/21,000 ft) high at the poles. 1

Ground level ozone forms just above the earth’s surface (up to about 2 miles above ground). However, it only accounts for about 10 percent of the global total. Almost 90 percent resides much higher up in the stratosphere and there is little ozone exchanged between the two zones.

Where Does Ozone Come From?

A tiny amount of ozone occurs naturally at ground level and a small amount falls from the stratosphere. But neither of these sources is significant. The vast majority of the ozone that is found near the ground comes from the burning of fossil fuels in cars, industrial factories, power plants and refineries. 2 In fact, since 1900, the amount of ground-based ozone has more than doubled due to the increased activity of automobiles and industry. 3

But ozone is not emitted directly into the air from this fossil fuel burning. Instead, it results from chemical reactions that take place between nitrogen oxides (NOx gases), hydrocarbons (including methane and carbon dioxide), carbon monoxide, and volatile organic compounds (VOCs). These gaseous compounds come from vehicle tailpipes, industrial chimneys, oil/gas infrastructure, wood-burning, including bushfires as well as the burning of crop residues, and mix together in the air.

When this mixture of man-made pollutants interacts with sunlight – particularly ultraviolet light – ozone is created. Which is why ozone levels rise during the hotter months.

Ozone levels typically reach peak concentration in urban areas around mid-afternoon, once vehicle exhaust fumes from the morning rush-hour have had a chance to react with the sunlight. However, as the sunlight fades, the production of ozone begins to subside. So peak levels rarely endure for more than two to three hours.

The COVID-19 shutdowns during the spring and summer of 2020 caused such a reduction in traffic, that many cities across the Northern Hemisphere, became smog-free. For more, see: Effect of COVID-19 on Climate Change.

Diagram of Photochemical Smog
Nitrous oxide from vehicle exhaust fumes reacts with sunlight and other chemicals in the lower troposphere, such as volatile organic compounds (VOCs) and ozone, to form photochemical smog – a common feature of the developing Anthropocene epoch. Image: Li-Wei Chao (CC BY-SA-4.0)

Ozone as a Key Component of Photochemical Smog

The typical conditions that lead to the formation of ozone include: (a) abundant car fumes; (b) smokestack emissions, with accompanying black carbon particles of partially combusted coal; (c) a cocktail of other highly reactive gaseous pollutants; (d) sunlight and heat; (e) little or no wind to scatter the gases.

These five components, along with the ozone they create, are the key components of photochemical smog, a phenomenon well known to cities around the world. Fortunately, the introduction of electric vehicles is likely to significantly reduce urban air pollution, since they emit no pollutants whatsoever.

It wasn’t until the early 1950s that the reactions between fossil fuel gases, aerosols, sunlight and ozone, and their creation of photochemical smog, were properly understood. In the end it was thanks to the research of Dutch chemist Arie Haagen-Smit (1900-77) in southern California, that identified ozone as an important component.

Photochemical smog forms in all modern cities, but is more prevalent in cities with sunny (chemically friendly), dry (rain disperses smog) climates, and a large number of vehicles, including cars, trucks and motorbikes – the older the better. Such cities include: Beijing, Delhi, Denver, Ho Chi Minh City, Lahore, Los Angeles, Mexico City, Santiago, and Teheran, to name but a few. The density of smog is also affected by smoke and aerosols from burning crop stubble and other biomass. Thus, rural areas can contribute to urban smog, which can be blown over the neighboring countryside as well.

Cities found in basins surrounded by mountains may have particularly bad smog problems because the polluted air is trapped in the valley and cannot be easily dispersed by wind. Los Angeles, California, and Mexico City, Mexico, both have high smog levels partly because of these topographical features.

Is Ground Level Ozone a Greenhouse Gas?

Yes. Ozone is a greenhouse gas, because it absorbs infrared energy emitted by the planet. So ground level ozone is harmful to Earth’s climate system as well as to human health. Stratospheric ozone, however, is not seen as harmful, however, due to its protective qualities (in shielding us from UV radiation) which outweigh any contribution it has to the greenhouse effect and to climate change.

What is the Global Warming Potential of Ozone?

Ozone’s annual global warming potential (GWP) is between 918 and 1022, which means that it traps roughly 1,000 times more heat than carbon dioxide. However, due to its instability, it has quite a short active life in the atmosphere – typically lasting for about 3 weeks – and thus decays much more quickly than carbon dioxide. So, its GWP over 20 years is only about 62-69. Because of this, ground level ozone does not have a significant global effect, but it can have very strong radiative effects on a regional level – up to one and a half times that of carbon dioxide.

Overall, tropospheric ozone is seen as an unhealthy pollutant rather than a greenhouse gas, although on warm summer days its effects on temperature in urban areas can be dramatic.

Seasonal Ground Ozone Levels
Image showing the mean seasonal concentrations of ground level (tropospheric) ozone (1979-2000). Colour code: Red (very high). Yellow (high). Green (medium). Blue (low). In the June to August image, notice how photochemical ozone production peaks over the East Coast of the USA and China. All measurements based on data from Total Ozone Mapping Spectrometer (TOMS) and SBUV satellite instrumentation. Image: NASA.

What Are the Health Effects of Ground level Ozone?

Due to its powerful oxidizing properties, ozone has a range of harmful impacts on human health, whether or not photochemical smog is present. It can react with almost all biological tissues, resulting in a variety of adverse health effects.

To begin with, it can be intensely irritating to the eyes, throat and airways. If it is inhaled, it can trigger a variety of health problems including coughing, nausea, chest pain and breathing difficulties. It aggravates bronchitis and lung problems, and is especially dangerous for people with respiratory conditions such as emphysema, and asthma, as well as cardiopulmonary problems.

The microscopic types of particulate matter in smog can cause serious damage to the human body. Particles known as PM2.5, or smaller, can reach the deepest parts of the lungs and even the brain.

Long-term exposure to ground level ozone has been shown to increase the risk of respiratory failure and lung cancer. A study of 450,000 people living in U.S. urban areas showed a clear correlation between ozone levels and respiratory illness over an 18-year follow-up period. The study revealed that inhabitants of cities with high ozone levels, such as Los Angeles or Houston, had more than a 30 percent increased risk of dying from lung disease.

In another study, published in Nature magazine, photochemical smog in the eastern Chinese city of Jinan, during the period 2011–15, were linked to a 5.8 percent increase in the rate of overall mortality. 4 See also: Health Effects of Air Pollution.

Smog Comparison, within 10 Days: China
Comparison: Smog levels are high (left) and lower (right) 10 days later. Fanhe, China (2019). Photo: © CC BY-SA 4.0

What are Unhealthy Levels of Ozone?

The U.S. Environmental Protection Agency (EPA) has developed an air quality index designed to help explain air pollution levels to the general public. Under initial guidelines, 8-hour average ozone concentrations of 85-104 ppbv (parts per billion by volume) were classified as “Unhealthy for Sensitive Groups”; concentrations of 105-124 ppbv were plain “unhealthy”; and concentrations of 125-404 ppb were “very unhealthy”. The most recent guidelines say that 8-hour average ozone concentrations of 70 ppbv (equivalent to an AQI value greater than 100) are “unhealthy”. 5 For the latest guidelines about safe exposure to ground level ozone, visit the EPA. 6

How Does Ground-level Ozone Affect the Environment?

According to the U.S. EPA the same high oxidizing potential that causes ozone to harm humans, causes damage to mucous and respiratory tissues in animals, and also to tissues in plants, at least above concentrations of about 100 ppbv. Too much ozone can damage plants and soil in various ways. It can reduce the rate of photosynthesis; increase the risk of disease; lower yields for timber and other crops, such as soybeans and winter wheat.

As far as ecosystems are concerned, exposure to excessive ground level ozone can lead to loss of diversity (less variety of species of plants, animals, insects, and fish), and cause changes to water and nutrient cycles.

How Does Climate Change Affect Ground-Level Ozone?

Global warming stimulates the formation and concentration of ozone in the atmosphere in several ways. 7

Firstly, by changing humidity and wind conditions, leading to a reduction in the frequency of surface cyclones. This results in more stagnant atmospheric conditions which prevents the dispersion of nitrogen oxides and other chemicals, while prolonging the time available for the chemical reactions necessary to produce ozone.

Second, these chemical reactions are typically enhanced by rising temperatures. For example, by the year 2050, warming alone may increase the number of high ozone days across the eastern United States by 68 percent.

Third, climate change is also prolonging the ozone season. For example, high ozone levels usually occur in the United States during the summer. However, autumn ozone over the southeastern United States reached summer levels on several occasions during the 2000s and in 2010.

To understand more about the timeline of our planet and its greenhouse gases, see: History of Earth in One Year (Cosmic Calendar).


  1. National Weather Service. NOAA. []
  2. Ground-level Ozone Basics.” U.S. EPA. []
  3. Ozone in the Troposphere.” UCAR. []
  4. Ambient air pollution, smog episodes and mortality in Jinan, China.” Zhang, J., Liu, Y., Cui, L. et al. Sci Rep 7, 11209 (2017). []
  5. Climate Central.” []
  6. Ozone (O3) Air Quality Standards.” []
  7. Ozone Pollution: A Major Health Hazard Worldwide” Junfeng (Jim) Zhang, Yongjie Wei, Zhangfu Fang. Front Immunol. 2019; 10: 2518. Oct 31, 2019. []
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