Atmospheric Ozone

(Accepted by the AMS Council 28 January 1996)

Ozone is a minor but important constituent of the earth's atmosphere. While it is essential for life as we know it today, it is also a toxic gas that can result in significant physiological and ecological damage, if exposures exceed critical limits. In both the stratosphere and troposphere, ozone concentration levels depend on multitudes of linked chemical and meteorological mechanisms, which vary profoundly with space and time. Human pollutant emissions often perturb these linkages, resulting in significant increases or decreases in ozone concentration. 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) intends to assist in the provision of a forum for such future joint interactions.

1. The AMS recognizes ozone as an atmospheric constituent that has a number of important beneficial as well as detrimental effects on the atmosphere, on surface ecosystems, and on humankind. These include effects on both ultraviolet and long-wave radiation, resulting effects on atmospheric wind systems, and direct impacts on plants and animals. Moreover, ozone is recognized as the dominant progenitor of much of the trace-gas chemistry that occurs in both the troposphere and the stratosphere. The AMS also notes that extensive couplings exist among these chemical, radiative, and dynamical components of ozone's behavior, and that these couplings act to complicate ozone's composite behavior and add substantial uncertainty to many of the currently available assessments of ozone's impact.

2. Despite many uncertainties, ample evidence exists to substantiate that atmospheric ozone has been affected in important and even critical ways by human activity. In the case of the stratosphere, the existence of the Antarctic ozone hole is unquestionable, and our evidence that it results from human-produced halocarbons is overwhelming. Ozone depletion in the midlatitude stratosphere is less dramatic; however, the general agreement of the large numbers of available column and profile measurements makes it reasonable to speculate that such depletion is indeed occurring and that it is largely human induced.

   Anthropogenic activities also significantly influence tropospheric ozone. Humankind is directly responsible for the excessive ozone levels that often occur near the surface in and downwind from populated areas. These effects are also felt throughout significant regions of the free troposphere.

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

3. While the above manifestations of human impact on ozone are clear, there remain important gaps in our understanding of ozone's complex behavior. For example, currently we cannot quantitatively explain the the midlatitude observations of insignificant surface UV increases. Impacts of tropospheric ozone buildup on the free troposphere's chemistry are also in a highly speculative state. These issues and others must be resolved in order to satisfactorily forecast potential future manifestations and provide a firm basis for policy analysis and associated policy actions.
4. Many of the noted uncertainties arise because of the strong couplings between chemistry, radiation, and atmospheric dynamics described above. Because of this, their resolution will require substantial combined effort by scientists having chemical and meteorological backgrounds. The AMS intends to actively provide one component of a continuing forum for this scientific interaction, and act in conjunction with other like-minded organizations for this purpose.
5. Current ozone-control legislation (such as the U.S. Clean Air Act, with 1990 amendments) and international agreements (such as the Montreal Protocol) tend to reflect the uncertainties noted above. The State Implementation Plan approach of the current U.S. Clean Air Act is burdened by older and largely invalid concepts of chemistry and atmospheric transport, which make ozone-standard attainment difficult if not impossible at many locations. Future amended versions of this act should incorporate more manageable and scientifically valid approaches to ozone control, and substantial current meteorological and chemical research should be directed to determining effective strategies for this purpose.

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. 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 ensure their effectiveness and to establish the basic understanding required for more effective maintenance during future year

© 1996 American Meteorological Society