Climate science at a glance
- Surface-level ozone is a key air pollutant and greenhouse gas.
- Surface ozone depends heavily on weather conditions, and is therefore subject to climate change influences.
- Climate change affects the emissions of the pollutants (known as precursors) needed to create ozone, the chemical and physical processes that create ozone, and the transport of ozone and its precursors.
- Changes in temperature and precipitation can increase air quality risks from surface-level ozone.
- Climate warming generally increases surface-level ozone across the United States.
What is surface ozone and is it different from ozone in the upper atmosphere?
Ozone is a gas composed of three atoms of oxygen (O3). Ozone can be good or bad, depending on where it is found. Ozone occurs both in the Earth's upper atmosphere (in the stratosphere) and at ground-level (in the troposphere). Unlike stratospheric ozone, which forms naturally in the upper atmosphere and protects people from the sun's harmful ultraviolet rays, surface-level ozone is harmful to human health.
How does surface ozone form?
Surface ozone is created through chemical reactions when pollutants react in the presence of sunlight. The majority of tropospheric ozone formation occurs when nitrogen oxides (NOx) and volatile organic compounds (VOCs) react in the atmosphere in the presence of sunlight. NOx and VOCs are called ozone precursors. They are the primary pollutants that enable the formation of the secondary pollutant, ozone.
There are primary pollutants that are emitted and there are secondary pollutants that form in the air. Ozone is one of these [secondary pollutants]. You can't go to a smokestack and measure the ozone coming out. You'll get precursors or other compounds that form it but never ozone itself.
Cristina Archer, professor at the University of Delaware's College of Earth, Ocean and Environment
Ozone concentrations depend on emissions of NOx and VOCs as well as weather conditions such as temperature, humidity, cloud cover, and winds. Emissions of the ozone precursors NOx and VOCs come from a variety of human sources. NOx emissions occur when cars or power plants burn fossil fuels such as coal and gasoline. VOCs are also human-made and derive from a variety of sources, including cars and gasoline-burning engines, paints, insecticides, cleaners, industrial solvents, and chemical manufacturing. There are also natural processes, such as wildfires and wetland methane emissions, that produce ozone precursors.
How does climate change affect surface ozone?
There are three dominant pathways that climate change affects surface ozone: climate change affects the emissions of the pollutants (known as precursors) needed to create ozone, the chemical reactions that create ozone, and the transport of ozone and its precursors.
(1) Climate change impacts on ozone precursors
A large amount of ozone precursors are emitted from climate-sensitive natural sources such as wildfires and lightning. Increased wildfire activity in the western US affects surface ozone.
(2) Climate change impacts on chemical and physical processes that affect ozone
Changes in air temperature and water content affect the air's chemistry and the rates of chemical reactions that create and remove ozone. Many chemical reaction rates increase with temperature and lead to increased ozone production. Projections of future ozone changes driven by climate change show that increasing temperature and water vapor in the atmosphere will likely increase surface ozone in polluted regions such as the eastern US.
In particular, increasing temperatures increase the decomposition of the pollutant known as peroxyacetylnitrate (PAN), which is a major reservoir species for long range transport of the ozone precursors NOx and HOx. This means increasing temperature decreases the lifetime of PAN, altering the long-range transport of ozone pollution.
(3) Climate change impacts on surface ozone transport
Ozone and its precursors can remain in the atmosphere from weeks to months, which means weather patterns affect how they travel in the atmosphere. Changes in large scale circulation patterns, such as the widening of the the Hadley Cell or changes in the strength and location of the jet stream, may increase surface ozone risks in mid-latitude regions such as the United States.
What are the health impacts of surface ozone?
Surface ozone has large negative impacts on health, especially on the cardiopulmonary and respiratory systems. Surface ozone can lead to coughing, irritation of the throat and chest, exacerbation of asthma, inflammation of lung cells, aggravation of chronic lung diseases, and ozone even reduces the disease-fighting capabilities of the immune system. On days where ozone levels are high enough, prolonged exposure can even lead to permanent lung damage. Ozone is regulated as a pollutant by the EPA because of ozone's hazardous nature.
US surface ozone trends and climate change
- (Gaudel et al. 2020): Ozone profiles taken by commercial aircraft in North America from 1994 to 2016 show that ozone levels increased 5% per decade on average above all 11 regions studied.
- (Archer et al. 2019): Increasing temperatures due to climate change shifts climatic conditions, resulting in worse air quality in the US mid-Atlantic by increasing the number of days with high concentrations of ozone.
- (Schnell et al. 2017): The weather factors that drive heat waves also contribute to intensified surface ozone and air pollution episodes. These extreme, multiday events tend to cluster and overlap, worsening the health impacts beyond the sum of their individual effects.
- (Lin et al. 2017): Model simulations looking at air quality over 1980–2014 indicate that climate warming generally increases surface-level ozone across the United States, but results differ spatially and in the magnitude of the projected signal.
- (Fu et al. 2015): Warmer August temperatures in the Southeast from 1988 to 2011 were associated with increased human sensitivity to surface-level ozone.