Atmospheric Ozone


(Adopted by AMS Council on 5 September 2003)
Bull. Amer. Met. Soc.85, 297—299

Ozone is an important trace gas in our atmosphere that has both beneficial and damaging aspects. Naturally occurring in both the troposphere and stratosphere, stratospheric ozone has a beneficial effect for life on earth as a filtering agent for damaging ultraviolet radiation. When photochemically produced in the troposphere to sufficiently high levels, however, ozone can be toxic and can result in significant physiological and ecological damage.

Human activities are causing changes in ozone levels in much of the atmosphere. By the use of stratospheric ozone depleting chemicals, humankind has caused a decrease in stratospheric ozone. Combustion of fossil fuels in motor vehicles and in stationary power plants has led to increases in nitrogen oxides and volatile organics emissions into our troposphere. Interacting together in sunlight, nitrogen oxides and hydrocarbons are causing increases in tropospheric ozone. This is especially noticeable during pollution episodes in urban centers as photochemical smog events. Tropospheric ozone increases on regional and global scales can lead to agricultural loss and ecological damage. Increases at the global scale can also contribute to global warming. While many facets of ozone's atmospheric behavior are well understood, a large number of important uncertainties remain, whose resolution will require substantial combined efforts by the meteorological and chemical communities. The American Meteorological Society (AMS) strongly supports interactions between these communities focused on obtaining a better understanding of ozone and its behavior.

1) The AMS recognizes that human activities are affecting atmospheric ozone by depleting stratospheric ozone and by increasing ground-level ozone worldwide, especially in polluted urban centers. Stratospheric ozone depletion leads to increased amounts of damaging ultraviolet radiation reaching the earth's surface. This is detrimental to the atmosphere, ecosystems, and humankind, and has led to the Montreal Protocol banning stratospheric ozone-depleting chemicals. Increased ground-level ozone concentrations have direct health effects on plants, animals, and humans. Concerns here have led to the Clean Air Act Legislation aimed at the reduction of tropospheric ozone precursor emissions. Tropospheric ozone and its precursors can also have an impact on greenhouse warming.

2) The AMS notes that the chemical, radiative, and dynamical components of ozone's behavior are coupled and complex. This complexity adds substantial uncertainty to many of the currently available assessments of ozone's impacts. Despite these uncertainties, however, ample evidence substantiates that atmospheric ozone has been affected in important and even critical ways by human activity. Stratospheric depletion has been established by observations of the Antarctic ozone hole, and our evidence that it results from human-produced halocarbons is overwhelming. Anthropogenic activities also significantly influence tropospheric ozone. Photochemical smog is well documented in most urban centers of the world, and in many cities leads to ozone levels approximately 10 times higher than occurs naturally. Air pollution from the combustion of fossil fuels has caused increases in ground-level ozone throughout the Northern Hemisphere, where average ozone levels have increased by 50% or higher. Although it is uncertain how specific emissions of pollutants affect tropospheric ozone, it is certain that the increases in tropospheric ozone are associated with emissions of nitrogen oxides (NOx) and volatile organic compounds (VOCs).

The effects in both the stratosphere and troposphere are sufficiently profound to mandate substantial concern, both on a local and global basis.

3) While human activity clearly causes changes in the atmospheric concentration of ozone, there remain important gaps in our understanding of ozone's complex behavior. This is particularly the case for tropospheric ozone, where the NOx and VOC interactions are still being unraveled and the importance of long-range transport and mixing is being resolved. In order to satisfactorily forecast future ozone trends in our atmosphere and provide a firm basis for policy analysis and associated policy actions, the lack of understanding needs to be addressed.

4) Many uncertainties arise because of the strong couplings among chemistry, radiation, and atmospheric dynamics. Thus, resolution of the uncertainty will require coordinated effort among scientists having chemical and meteorological backgrounds. The AMS actively supports continuing forums for this scientific interaction and welcomes interactions with other scientific organizations for this purpose.

Current ozone-control legislation and international agreements (such as the Montreal Protocol) tend to reflect the uncertainties noted above. Nonetheless, the actions associated with the Montreal Protocol and its associated amendments have been based on the best science available.

Implementation of the U.S. Clean Air Act is burdened by uncertainties about the relative impact of emissions of NOx and VOCs on tropospheric ozone during pollution events, and by the failure to use observation-based investigations of ozone and ozone precursors to evaluate the impact of control strategies. Directed research should address these issues. Research also needs to identify the increasing impact of international and intercontinental transport of tropospheric ozone and its precursors, as further industrialization leads to increases in ground-level ozone worldwide.

With regard to the Montreal Protocol, it is encouraging to note that halocarbon limitations under this agreement appear to have resulted, recently, in decreases of some of the shorter-lived halogen-containing species and an end to increases of chlorofluorocarbons. Increasing reliance on replacement compounds should lead to the recovery of the stratospheric ozone system in the latter part of the twenty-first century. Owing to the noted uncertainties and complexities associated with stratospheric ozone depletion, however, the effects of this and other international agreements must be monitored continuously and carefully to confirm the expected recovery and establish the basic understanding required for more effective maintenance in future years.

[This statement is considered in force until September 2013 unless superseded by a new statement issued by the AMS Council before this date.]