Atmospheric Ozone

An Information Statement of the American Meteorological Society
(Adopted by AMS Council on 7 January 2018)


Ozone is one of the most important trace gases in our atmosphere that both benefits and harms life on Earth. High ground-level ozone amounts contribute to poor air quality, adversely affecting human health, agricultural productivity, and forested ecosystems. Ozone absorbs infrared radiation, and is most potent as a greenhouse gas in the cold upper troposphere located 8–15 km above the surface. In the stratosphere, between approximately 15 and 50 km above the Earth’s surface, a layer rich in ozone serves as a “sunscreen” for the world by shielding the Earth’s surface from harmful ultraviolet radiation. This absorption of solar energy also affects atmospheric circulation patterns and thus influences weather around the globe. Moreover, throughout the atmosphere, ozone is the key ingredient that initiates chemical cleansing of the atmosphere of various pollutants, such as carbon monoxide and methane, among others, which could otherwise accumulate to harmful levels or exert a stronger influence on climate. Therefore, changes to ozone anywhere in the atmosphere can have major impacts on the Earth.

A major challenge in developing both a quantitative scientific understanding of atmospheric ozone and its changes, and effective policies to mitigate the consequent harmful aspects, is that ozone is not directly emitted to the atmosphere. Instead, ozone abundances are mainly controlled by emissions of other trace gases and a suite of chemical reactions.  The relative importance of individual reactions varies with location and season. Human activities associated with energy production, transportation, and industry have altered the chemical reactions that create and destroy ozone throughout the atmosphere, leading to net increases in some regions and net decreases in others. As the multiple roles of ozone described above depend upon its location in the atmosphere, the overall societal impacts depend on where in the atmosphere ozone changes occur. The perturbations to ozone abundances by human activities have been substantial, and have, on the whole, enhanced its negative impacts.

Significant scientific progress has been made in understanding the factors controlling atmospheric ozone distributions and their temporal changes in different regions of the atmosphere, as well as in understanding the impacts of those changes on the planet. This progress has been made through a combination of many in situ and remote-sensing observations, fundamental laboratory-based process studies, and theoretical and numerical modeling. Several national and international governmental and scientific organizations produce regular assessments of scientific understanding and policy related to these ozone changes. The American Meteorological Society’s (AMS) position on atmospheric ozone, summarized below, is based on these assessments and the broader scientific literature that underpins such assessments.

Ozone in the stratosphere, profoundly important for life on land and in surface waters, can be depleted by industrial chemicals. Within the lower half of the stratosphere, between about 15 and 30 km above the surface, ozone concentrations reach their largest values in the entire atmosphere, several times higher than in the troposphere. This “ozone layer”, which evolved naturally, absorbs the majority of harsh ultraviolet (UV) radiation from the sun and shields life on land and in surface waters from radiation capable of damaging DNA, skin, and eyes. Decreases in stratospheric ozone therefore lead directly to increased exposure to UV radiation. Natural and controlled experiments have demonstrated that greater exposure to UV radiation increases rates of certain skin cancers and cataracts in humans, and decreases photosynthetic productivity in terrestrial and marine ecosystems as well as agricultural crop yields. In the 1970s, it was recognized that the ozone layer was threatened by the use of ozone-depleting substances (ODSs), chemicals such as chlorine-containing chlorofluorocarbons (used for refrigeration, air conditioning, and other applications), bromine-containing Halons (used for fire suppression), and many other chemicals containing chlorine and bromine. These human-emitted chemicals have their most dramatic impact in the annual Antarctic ozone hole, a phenomenon that started in the late 1970s and recognized in the early 1980s, where now more than half of the ozone above Antarctica is depleted each year in late September through October. Globally, the ozone layer has exhibited decreases of a few percent since 1980.

It is now known with high confidence that the world avoided a major catastrophe of global ozone layer depletion by taking deliberate actions. A series of international measurement campaigns by multiple agencies and universities to study stratospheric ozone chemistry provided undeniable evidence linking industrial ODSs to the severe stratospheric ozone depletion during Antarctic spring and around the globe. As the scientific understanding of ozone depletion increased and the risks of continued ODS production were realized, people of the world agreed to take action to minimize the ozone layer depletion and enable recovery in the future. This action was codified in the Montreal Protocol for the protection of the ozone layer in 1987, the only universally ratified international environmental treaty. Because of the compliance of all the countries with this treaty, the ozone layer depletion is no longer worsening and indications are that ozone is even beginning to recover. The current scientific understanding, supported by numerical model projections, suggests the ozone layer should return to pre-ODS levels of 1980 in the late 21st century with continued compliance with the Protocol, barring unforeseen events. This result stands as a shining example for the societal impacts of basic scientific discoveries and research, and for the application of research results to policy development.

Stratospheric ozone influences weather, and this influence is detectable due to the ozone hole. Through the influence of stratospheric ozone on radiative heating and cooling, there are couplings between ozone and atmospheric circulation. Increasing levels of greenhouse gases in the atmosphere have cooled the stratosphere. This cooling in turn affects the rates of chemical reactions governing stratospheric ozone abundances. Such effects of ozone concentrations on radiation, and therefore temperature and moisture budgets, and the associated feedbacks with climate, are becoming routinely included in climate models as well as operational weather prediction systems for improved simulations of short- and long-term variations in atmospheric circulation. Observational and computer modeling studies demonstrate that the Antarctic ozone hole has led to a delay in the seasonal breakdown of the stratospheric polar vortex, which influences both the recovery of the ozone hole and the lower atmospheric circulation in the Southern Hemisphere summer.  

In contrast to the stratosphere, ozone concentrations in the lower atmosphere have increased since preindustrial times, often most profoundly in and downwind of large urban areas, degrading human and ecosystem health as well as agricultural crop yields.  Human activities associated with industrialization and modernization, such as power generation and transportation, have dramatically increased emissions of ozone precursors such as nitrogen oxides (NOx) and volatile organic compounds (VOC). Research in the 1950s showed that together with sunlight, these pollutants catalyze the rapid formation of ozone in the air — a process known as photochemical smog formation of which ozone and secondary aerosol particulate matter are the main byproducts. Prevalent in major cities and surrounding areas around the world, high ozone concentrations in photochemical smog can adversely affect human health, the built environment, ecosystems, and agricultural yields. For example, epidemiological studies show an increase in asthma-related hospital visits following enhanced ozone exposure. Forests downwind of regions with high surface ozone show decreased productivity and visible leaf and needle damage. High- ozone episodes lead to deterioration of common polymers. The exposure of soy plants to elevated ozone leads to decreased crop yields, costing an estimated $1billion per year in lost productivity at current surface ozone levels.  For these and other reasons, many localities now aim to regulate ozone concentrations at the surface to remain below specific threshold values.

The recognition that chemical processes, especially those influenced by human actions, contribute greatly to ozone concentrations in the lower atmosphere was the foundation for policy action.  Effective policies to reduce surface and lower atmospheric ozone concentrations must incorporate an understanding of meteorological processes which might lead to elevated concentrations of ozone, natural and anthropogenic activities which lead to emissions of ozone precursors, and the atmospheric chemical reactions that form ozone. Ozone concentrations respond nonlinearly to changes in emitted precursor gases, with some precursors being more influenced by human activities than others. Moreover, each locale has a different background ozone concentration set by circulation patterns and pollution sources upwind, which can vary significantly from day to day. Furthermore, ozone in a given location can increase as the result of  influences beyond the control of that region, for example, due to ozone transport from the stratosphere, production from wildfires, and from international precursor emission. Therefore, exemptions are included in U.S. air quality regulations for influences beyond the control of an air management agency.

In the United States and other nations that have enacted air pollution control policies, daytime maximum surface ozone concentrations have decreased over the past decades; unfortunately, surface ozone levels are increasing in other regions of the world.  Ozone precursor emissions have decreased severalfold across the United States in response to regulations, even while energy production and automobile use have continued to increase.  Thus, there has been a major success in cleaning U.S. near-surface air across much of the nation. Other industrialized nations have enacted similar controls, approaching similar levels of success. Air quality policies continue to evolve as scientific understanding improves. For example, nonmethane VOC emission controls effectively reduced the highest ozone levels in Los Angeles, CA. However, the highest warm season ozone levels began to decline only after NOx reductions were phased in under the Clean Air Act in U.S. urban regions with abundant supplies of biogenic VOC emissions from vegetation. Yet, ozone pollution is increasing in populated areas of rapidly developing countries where severe events occur analogous to those that occurred in U.S. or European cities during the mid to late 20th century. Because of the scientific advances, technological improvements, and lessons learned in cleaning up ozone pollution in developed countries, there is now an opportunity for a more rapid transition to cleaner air in these areas while increasing prosperity.

Ozone and its precursors are transported throughout the lower atmosphere, adding a hemispheric dimension to local air quality.  Ozone produced as part of photochemical smog, and its precursors such as methane, carbon monoxide, and nitrogen oxide reservoirs that produce ozone, can be transported through the atmosphere for days. The global spread of ozone and its precursors resulting from pollution have led to increases in ozone concentrations on large regional and even hemispheric scales throughout the troposphere, the region between the surface and the stratosphere. Increases in tropospheric ozone have been largest in the northern hemisphere where anthropogenic emissions of ozone precursors are the largest. This is documented in long-term monitoring network observations. Air quality regulations continue to aim for lower ozone concentrations, motivated by improved understanding of ozone’s negative effects on public health and ecosystems. Yet, increases in the regional or even hemispheric ozone “background” concentrations are generally outside of local control, and may even arise from international or intercontinental transport of ozone formed from precursors emitted in another country. As such, to attain compliance with more stringent regulations on surface ozone concentrations, a locality may require even more restrictive emission control strategies if global tropospheric ozone continues to increase.

Changes in tropospheric ozone influence as well as respond to atmospheric composition and climate. As noted above, ozone is radiatively active, acting as a greenhouse gas in the upper troposphere. The increases in upper tropospheric ozone since preindustrial times, due to anthropogenic emissions of its precursors, have contributed significantly to the positive radiative forcing of climate (warming), with a magnitude similar to that from changes in methane concentrations, though the spatial and temporal patterns of these forcings are not necessarily comparable. If the ozone precursor emissions are reduced, the atmosphere would respond quickly and reduce ozone. As ozone is the dominant source of the hydroxyl radical, our atmosphere’s main cleansing agent, increases in global tropospheric ozone have altered the atmosphere’s ability to cleanse itself. Changes in tropospheric ozone thus affect the abundance of other greenhouse gases, such as methane and certain halocarbons, as well as that of aerosol particles, while changes in methane and aerosol particles influence tropospheric ozone concentrations. These chemical effects thereby couple the fates of ozone and several climate forcing agents and their impacts on radiative forcing in ways that continue to be the focus of continued research.

Ozone changes remain a significant environmental challenge, requiring a broad set of actions across the world to monitor, evaluate, and mitigate the impacts. There are still many important gaps in knowledge and policy actions related to ozone throughout the atmosphere.  They include ground-level ozone pollution with its major health and ecological disbenefits, increases in tropospheric ozone and the consequent climate impacts, and ozone layer depletion and its consequences for living things at the surface.  Therefore, long-term trends in ozone abundances need to be documented around the world, monitored, and their causes determined to ensure that policy actions are working, to establish new scientific understanding, and to develop new policy. This need in turn calls for expanded in situ and remote-sensing observational capabilities of both chemical and meteorological variables.

Natural variations in atmospheric ozone make it difficult to identify changes in ozone caused by anthropogenic activities, yet being able to make such distinctions is essential for policy decisions. Changes in climate and other atmospheric and oceanic events, such as wildfires, contribute to changes in ozone levels in various parts of the atmosphere through transport and chemical processes. In addition, even though the scientific understanding of the various controls on atmospheric ozone is well advanced, large perturbations to those controls by human activities continue.  As the levels of human-induced emissions decrease( e.g., ODS levels or tropospheric ozone precursor levels respond to regulations) natural variations play an even larger role in year-to-year variations.

This situation is further complicated because emissions from some regions of the globe continue to increase, and this pollution undergoes international transport, with the impacts on other regions varying with changes in atmospheric circulation. Yet, parsing the contribution of natural variations from human activities and specific regional activities is essential for policy action. Therefore, an integrative approach that accounts for all possible causes of ozone trends over time is required. Such an approach necessitates not only observations of the changes, which are currently sparse, but also developing and testing mechanistic explanations of those changes, which rely on a hierarchy of computer models ranging from process-based to fully coupled climate-chemistry Earth-system models. Scientific understanding and technical advances across the range of spatial and temporal scales represented in current computer models are enabling such attributions on regional and global scales, but these so far remain relatively uncertain. Thus, developing a deeper scientific understanding of the natural and anthropogenic processes affecting ozone, and the variability therein, and improving the representation of such understanding in models, will remain crucial activities for future policy decisions.

Significant scientific effort over the past 50 years has led to recognition of the multiple roles of atmospheric ozone and substantial progress in identifying the processes shaping ozone trends in different parts of the atmosphere, as well as the associated impacts on air quality, ecosystem viability, weather, and climate. Based on these advances, society has taken concerted actions to mitigate the negative impacts of ozone trends, successfully addressing the causes of ozone depletion in the stratosphere and slowing or reversing increases of the highest summertime surface ozone events near populated regions in North America and Europe. Indeed, there are clear signs of progress in moving ozone abundances toward their natural levels in many regions. International cooperation in responding to the challenge of stratospheric ozone depletion, and clean air regulations in developed nations decreasing surface ozone in urban areas, are two of the great environmental success stories of the twentieth century. While many facets of ozone's atmospheric behavior are understood, scientific uncertainties and policy challenges remain. Their resolution will require combined efforts by the meteorological, chemical, and atmospheric physics communities. The AMS strongly supports these efforts focused on obtaining a better understanding of ozone and its behavior.

[This statement is considered in force until January 2023 unless superseded by a new statement issued by the AMS Council before this date.]