and Roether, 1980). Using a hydrological model and tritium data in precipitation observed at numerous stations of the WMO/IAEA network, Weiss and Roether estimated the delivery of tritium to the oceans as a function of time and space. Using a complex atmospheric circulation model, Koster et al. (1989) derived tritium fluxes to the ocean similar to those estimated by Weiss and Roether (1980). Dreisigacker and Roether (1978) converted the flux boundary condition given by Weiss and Roether (1980) into a concentration boundary condition for the North Atlantic (between 20 and 60°N). This "Dreisigacker/Roether function" (Figure 1) was used either directly or in modified form in many tracer studies, among them Sarmiento (1983), Thiele et al. (1986), Smethie et al. (1986), Thiele and Sarmiento (1990), Heinze et al. (1990), and Schlosser et al. (1991 a). Recently, Doney et al. (1992) reevaluated the IAEA tritium data using a factor analysis. They derived tritium concentrations in precipitation as a function of time and space with an estimated error of ± 3 to ± 10 percent. These data can be used to derive a relatively accurate flux boundary condition for tritium.

Tritiogenic 3He is exchanged between the atmosphere and the ocean on a time scale of the order of I week to 1 month. Since the atmospheric helium concentration (5.24 ppm; Glueckauf and Paneth, 1945) as well as the 3He/4He ratio (1.384 X 10–6; Clarke et al., 1976) are constant for practical purposes, the 3He boundary condition for ocean surface water in equilibrium with the atmosphere is well defined. However, in areas of deep convection or in ice-covered regions, significant 3He excesses above solubility equilibrium can be observed (see, e.g., Fuchs et al., 1987; Schlosser et al., 1990).

Halocarbons

The first accurate measurements of CFC-11 and CFC-12 in the atmosphere were made in the mid-1970s. Since

FIGURE 1

Tritium concentration in North Atlantic surface water as a function of time. (From Dreisigacker and Roether, 1978; reprinted with permission of Elsevier Science Publishers.)

1978, high-quality continuous measurements of both substances have been made at a number of different locations around the earth (see, e.g., Cunnold et al., 1986). McCarthy et al. (1977) have shown that the amount of CFC-11 and CFC-12 released to the atmosphere can be determined from the industrial production data, and atmospheric concentrations prior to the mid-1970s can be estimated from the amount of CFCs released. The cumulative amount of CFC-11 and CFC-12 in the atmosphere can be calculated as a function of time from this release data and the atmospheric lifetimes of CFC-11 and CFC-12. The atmospheric lifetimes have been estimated to be 57 to 105 years with a best estimate of 111 years for CFC-12 (Cunnold et al., 1986). The CFC inventory as a function of time can be calculated from

where I is the atmospheric CFC inventory in January of a given year, R is the amount of CFC released during a given year, ? is the reciprocal of the atmospheric lifetime of CFCs, and i denotes the year. Inventories are converted to concentrations by normalizing them to concentrations measured after the mid-1970s.

Smethie et al. (1988) used the procedure described above to estimate the CFC-11 and CFC-12 atmospheric concentrations as a function of time. The Chemical Manufacturers Association (1983) global release data were used; they are thought to be the most reliable up to 1976 because they contain Russian data for the 1968-to-1975 period. The normalization was done using 1976 data based on the SIO 1986 concentration scale. (Normalizations could have been done using data from later years, but the inventories are less accurate in later years because Russian data are not available after 1975.) The calculation was made using the extreme values for the atmospheric lifetimes, but this had only a small effect on estimated concentrations. The maximum difference in concentrations was 2.3 percent in 1950 decreasing to 0.3 percent in the mid-1970s for CFC-11, and 5 percent decreasing to 0.4 percent for CFC-12.

CFC-11 and CFC-12 concentrations for the Northern Hemisphere, based on the best estimate of the atmospheric lifetimes prior to 1976 and measurements after 1976, are shown in Figure 2. The production and use of CFCs occur mainly in the Northern Hemisphere, and concentrations of both CFC-11 and CFC-12 are 7 to 8 percent higher in the Northern Hemisphere than in the Southern Hemisphere (Cunnold et al., 1986), with a sharp decrease in concentration across the intertropical convergence zone. CFC-11 and CFC-12 are well mixed in both hemispheres, although concentrations above coastal waters can be a few percent higher than elsewhere in the atmosphere remote from land, particularly near the northeastern United States and Europe (Prather et al., 1987). The annual increase in CFC-11 and CFC-12 atmospheric concentrations is between 4 and 5 percent per



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