CIESIN Thematic Guides

Measurements and Trends in Ozone and Chlorofluorocarbon Levels

Ozone in the atmosphere can be measured by ground-based, satellite, and in-situ methods to provide information on total column ozone abundance and how it varies with altitude. Ground-based measurements of total column ozone have been carried out since 1957 by two devices: the Dobson spectrophotometer and the M83 filter ozonometer. Satellite measurement of column ozone has been conducted since the launch of the Nimbus-7 satellite in 1978. The Total Mapping Ozone Spectrometer (TOMS) on this platform has provided the most complete, long-term global data set of total ozone to date and has been used extensively for determining global trends and for conducting related atmospheric research. In the document Upper Atmosphere Research Satellite (UARS), the National Aeronautics and Space Administration (1989) describes several instruments on board the UARS that measure ozone and other chemical species. Scientists can use these measurements to gain a better understanding of how ozone is created and destroyed in the atmosphere. Mégie et al. (1989) provide an overview of surface and satellite devices and measurement techniques for ozone in the chapter "Global Trends" of the Scientific Assessment of Stratospheric Ozone.

The most widely used source of ozone data is the TOMS data set. In an analysis of 13 years of daily ozone measurements from 1979 to 1991, Stolarski et al. (1991) show statistically significant decreases in total column ozone at all latitudes outside the tropical regions in "Total Ozone Trends Deduced from Nimbus-7 TOMS Data." Greatest loss is observed at high latitudes due to the unique conditions that lead to polar ozone depletion. Losses in the Antarctic show a maximum downward trend of approximately 3 percent per year during the spring months over the course of TOMS observations. Ozone loss at mid-latitudes ranges from 0.2-0.8 percent decrease per year. More recent TOMS data analysis by Gleason et al. (1993) in "Record Low Ozone in 1992" shows globally averaged ozone levels reached all-time lows during 1992. Measurements from the National Aeronautic and Space Administration's Stratospheric Aerosol and Gas Experiment (SAGE) and ozonesonde launches have indicated that depletion has occurred primarily at low stratospheric altitudes, between 17 and 25 km. In the chapter "Ozone and Temperature Trends" of the Scientific Assessment of Ozone, Stolarski et al. (1992) discuss these trends, along with comparisons between satellite and surface data.

Several groups around the world have been monitoring CFC levels in the lower atmosphere since the mid- to late-1970s. These data give concentrations for the various CFCs and ozone-depleting substances at the Earth's surface. Stratospheric monitoring, on the other hand, focuses on reactive chlorine species, because these chemicals are quickly broken down in the presence of ultraviolet radiation.

Trends in CFCs have shown a nearly constant increase at all monitoring locations. The vast majority of CFC production is in the Northern Hemisphere but, due to their stability, CFCs become well-mixed in the troposphere. CFC-11 and CFC-12 have been increasing globally at a rate of approximately 3.7 to 4.0 percent per year from the late 1970s through the late 1980s. Fraser et al. (1992) discuss trend analysis for these and other CFCs and ozone-depleting substances in the chapter "Source Gases" of the Scientific Assessment of Ozone. In "Decrease in the Growth Rates of Atmospheric Chlorofluorocarbons 11 and 12," however, Elkins et al. (1993) indicate a slowdown in the increase of CFC-11 and CFC-12. Global rates have shown decreasing growth from 11+/-1 parts per trillion per year (ppt/yr) during the mid 1980s to 2.7 ppt/yr for CFC-11, and 19.5+/-2 ppt/yr in the mid 1980s to 10.5+/-0.3 ppt/yr for CFC-12. These trends coincide with industry reports of decreased production of these compounds. If such trends continue, peak levels of chlorine in the stratosphere may be reached before the turn of the century and a downturn may follow.