The atmospheric concentration of fluorocarbon 11 (F-11) has increased steadily since it was first put into commercial use in the late 1930s and early 1940s. The observed trends, however, have two periods of dramatic declines. The first occurred around 1974, when trends started falling from their all time high of 13.8 pptv/yr down to around 7.8 pptv/yr (1pptv=10-12 parts by volume) in 1982. This decline occurred at first, probably, because of market conditions and later because of the ban in the United States on inessential uses of F-11, particularly in aerosol spray cans. In the meantime, other uses of F-11, such as blowing foams, increased, causing an increasing trend once again until around 1987, when the atmospheric trends reached 11 pptv/yr (measured 1986--1988, inclusive). After this time, however, the trends have fallen dramatically and are now only 4.6 pptv/yr (measured 1990--1992) and even lower in the middle northern latitudes (about 2 pptv/yr). The recent decline of trend is attributed to the effect of the Montreal Protocol and subsequent agreements that are designed to ban worldwide production before the turn of this century.

The atmospheric trends reflect estimated emissions, which reached highest values in 1974 (340-355 Gg/yr;1 Gg=109 g) and again in 1988 (314--380 Gg/yr). The observed concentrations and trends agree extremely well with those calculated from independent estimates of emissions from the various sources. Atmospheric concentrations of F-11 are calculated from a mass balance theory involving seven reservoirs and their interactions. Three of the reservoirs are at the Earth's surface arising from the uses of F-11 (quick-release applications such as open cell foams and aerosols, nonhermetically sealed refrigeration, and rigid polyurethane foams). We estimate that of the 9150 Gg of F-11 that have been produced throughout its history, at present about 1040 Gg are tied up in rigid foams (90 Gg and 24 Gg are tied up in refrigeration and quick-release applications, respectively). There are four environmental reservoirs (deep oceans, the ocean mixed layer, the troposphere, and the stratosphere). F-11 is dynamically exchanged between these reservoirs and is destroyed principally by photolysis in the stratosphere and by soils at the Earth's surface. At present there are about 5360 Gg in the troposphere, 740 Gg in the stratosphere, and small amounts in the oceans (27 Gg and 6 Gg in the mixed layer and deep oceans, respectively). About 81% of the 9 Tg of F-11 produced is still in the surface or environmental reservoirs (1 Tg=1012g). The future concentrations of F-11 depend on leakage rates from the surface reservoirs and the atmospheric lifetime. It seems that concentrations are not likely to reach the peaks expected earlier and are likely to decline faster than previously thought. The peak concentration is expected to be about 275 pptv and may occur within the next 2 to 3 years. This abstract, the figures, and their captions form a self-contained condensed description of our results. ¿ American Geophysical Union 1993