CHLOROFLUOROCARBONS AS TIME- DEPENDENT TRACERS IN THE OCEAN

C H L O R O F L U O R O C A R B O N S (CFCs) are a group ofallthropogenic compounds which are widely used as refrigerants, aerosol propellants, plastic foam blowine~ a~ents~ and solvents. Althotmh~ often popularly referred to by' the Dupont tradename "'freons,'" these compounds are produced worldwide by a number of manufacturers. CFCs have low toxicity. are not very reactive under normal conditions, and have a variety of useful physical properties. Produclion of two of these compounds, dichlorodifluoromethane (F-12) and trichlorofluoromethane (F-I 1), began in the early 1930s. Industrial production ot F11 and F12 accelerated during the following three decades as demand for these compounds grew. CFCs are volatile, and most of the F11 and F12 produced eventually enters the atmosphere. In some applications, such as the use of F-I1 and F-12 as aerosol propellants, the typical delay between production and release of the CFC to the atmosphere is relatively short, but for other uses, such as in herlnetically settled refrigeration systems, the average delay until release can be a decade or longer. Sensitive anah, tical techniques for measuring CFCs in the environment were not available during the first four decades of release. Pioneering measurements of CFCs in the troposphere were made by, Lovelock ( 1971 ) using electron-capture gas chromotography. Since then, a number of programs have been established to monitor the increases of F11 and F-12 in the earth's atmosphere as a function of location and time. For the period prior to the start of monitoring programs, the increase of CFCs in the atmosphere can be modeled based on production records and release scenarios, ;vifll corrections for l o s s e s d u e to p l lo to lys i~ , o f lhe,,,e COml)ound~, in f i le stratosphere. More than 90c,~ of production and release of F-11 and F12 occurs in the Northern Hemisphere. Due to rapid mixing rates within the lower atmosphere, and the chemical stability of CFCs, the latitudinal distributions of these compounds in the earth's troposphere are relatively uniform. In 1987, the gradient between mid-latitude concentrations of these CFCs in the mmhern hemisphere and in remote regions in

By John L. Bullister

CHLOROFLUOROCARBONS
(CFCs) are a group ofallthropogenic compounds which are widely used as refrigerants, aerosol propellants, plastic foam blowine~ a~ents~ and solvents.Althotmh~ often popularly referred to by' the Dupont tradename "'freons,'" these compounds are produced worldwide by a number of manufacturers.CFCs have low toxicity.are not very reactive under normal conditions, and have a variety of useful physical properties.Produclion of two of these compounds, dichlorodifluoromethane (F-12) and trichlorofluoromethane (F-I 1), began in the early 1930s.Industrial production ot F-11 and F-12 accelerated during the following three decades as demand for these compounds grew.CFCs are volatile, and most of the F-11 and F-12 produced eventually enters the atmosphere.In some applications, such as the use of F-I1 and F-12 as aerosol propellants, the typical delay between production and release of the CFC to the atmosphere is relatively short, but for other uses, such as in herlnetically settled refrigeration systems, the average delay until release can be a decade or longer.
Sensitive anah, tical techniques for measuring CFCs in the environment were not available during the first four decades of release.Pioneering measurements of CFCs in the troposphere were made by, Lovelock ( 1971 ) using electron-capture gas chromotography.Since then, a number of programs have been established to monitor the increases of F-11 and F-12 in the earth's atmosphere as a function of location and time.For the period prior to the start of monitoring programs, the increase of CFCs in the atmosphere can be modeled based on production records and release scenarios, ;vifll corrections for losses due to pllotolysi~, of lhe,,,e COml)ound~, in file stratosphere.
More than 90c,~ of production and release of F-11 and F-12 occurs in the Northern Hemisphere.Due to rapid mixing rates within the lower atmosphere, and the chemical stability of CFCs, the latitudinal distributions of these compounds in the earth's troposphere are relatively uniform.In 1987, the gradient between mid-latitude concentrations of these CFCs in the mmhern hemisphere and in remote regions in the Southern Hemisphere was only about 7¢{.
Fig. 1 shows models of the increases of F-11 and F-12 ill the troposphere over the Weddell Sea ill Antarctica for the period 1930-1987.Data in this figure for the period 1976-1986 are based on a time series of measurements at the South Pole (Rasmussen and Khalil, 1986) and are normalized to 1987 lneasurements made in the Weddell Sea using the Scripps lnsititution of Oceanography cal ibration scale IBullister, 1984).For the period 1930-1975, estimates of accumulation of F-II and F-12 in the atmosphere are based on annual production and release data (Chemical Manufacturers Association. 1985) with corrections applied for stratospheric removal processes.Both F-11 and F-12 have increased rapidly in the atnlosphere flom 1930 to the present.During the period 1930-1975, the ratio of F-1 l/F-12 in the atmosphere also increased monotonically as a function of time.Since the mid-19708.the F-1 I/F-12 ratio in the atmosphere has remained about constant.

Ozone Destruction
Much of the interest in the accumulation of CFCs in the atmosphere has focused on their role in the destruction of stratospheric ozone.Due to their chemical inertness m the troposphere, these compounds are gradually carried upward into the stratosphere, where they undergo photolysis by high energy ultraviolet light.This photolytic process initiates a chain of reactions which result in the catalytic destruction of stratospheric ozone (Molina and Rowland, 1974).Intense public debate arose as the risks from reductions in the earth's ozone layer were weighed against the economic importance of CFCs.Concerns over possible danger to human health and the environment led to enactlnent of restrictions on some uses of these compounds in the US in 1976.Nevertheless.worldwide use of CFCs has remained high during the past decade.Recently, much attention has been tocused on studies which show that total cohunn ozone levels over Antarctica in early spring have declined sharply during the past decade (Farman el a/.. 1985 ).The observed decline of Antarctic ozone (the "'Ozone Hole") is much greater than that predicted by earlier models of global stratospheric chemistry, and has sparked renewed concern over the enviromnental x F-12 effects of these compounds.CFCs (along with other gases such as CO,, CH 4, and N,O) are also very efficient absorbers of infrared radiation, and increasing concentrations of CFCs in the atmosphere may make a substantial additional contribution to global wanning due to rising levels of atmospheric CO~ (Ramanathan, 1988 ).
The proposed 1988 Montreal Protocol treaty attempts to freeze CFC production at 1986 levels for the next few ~ears, then gradually cut production by 50c/~ during tile 1990s.Even al these lower production and release rates, ammspheric concentrations of F-11 and F-12 will continue to rise during this period.Proposals have been made recently for a complete ban on production of some CFCs (including F-11 and F-12) within a decade.A variety' of compounds with lower potential for ozone damage are now being tested as possible substitutes for CFCs.Even with the elimination of new CFC production, the very slow removal of these compounds fronl the atmosphere (only about 19/-per year via stratospheric photolysis), and the continued gradual release of existing stockpiles (including CFCs held in refrigeration systems, plastic foams, etc.) msures that these coinpounds will remain in the atmosphere at relatively high levels for many hundreds of years.

The Entry of CFCs into the Ocean
At equilibrium, the concentration of a dissolved CFC in the surface ocean is a function of its concentration in the overlying atnaosphere and of the salinity and temperature of the seawater.A global model of tile equilibrium concentrations of F-11 and F-12 in the inixed layer of the ocean, as a function of latitude and time, can be derived using models of sea surface temperature and salinity and of the increases ofF-11 and F-12 in the troposphere.
From the surfiice layer, these dissolved compounds can be mixed downward in the water column.Using inodels of CFC inpul functions, and measurements of the distribution of these dissolved coinpounds in the water column, information can be obtained on the rates at which tile ocean can take up gases flom the atmosphere, and on mixing and circulation within the interior of the ocean.
Techniques have been developed for measuring the concentrations of dissolved F-11 and F-12 in seawater.A 24-sample water column profile of the concentrations of dissolved F-11 and F-12 can be obtained on board ship within eight hours of the completing of a station.These rapid analytical techniques allow CFC distributions to be obtained along conductivity-temperature-depth sections with reso-  (Bullister and Weiss, 1988).Typical modern near-surface concentration are about three orders of magnitude higher, allowing a wide dynamic range in the ocean for these tracers.Additional modifications in sampling and analyrical techniques may allow more rapid and automated processing of CFC salnples.The continued transfer of CFCs fl'om the surface layer into the interior of the ocean, together with improvements in the limits of detection lor these compounds, will allow useful measurements of CFCs to be extended in the future to deeper and more isohlted regions of the ocean.
The accurate determination of the concentrations of dissolved F-1 I and F-12 in seawater is extremely susceptible to the entry of trace levels of contamination during sample collection and processing.CFCs can be present at very high levels in laboratory and shipboard air, and in almost all materials used in the walls and seals of sampling containers.Efforts are being taken to reduce such sample contanfination, but this problem remains a m~\ior obstacle to the more routine determination of trace levels of dissolved CFCs in seawater.
A nnmber of groups are involved in the study of the entry of CFCs into the ocean.Detailed vertical profiles of F-I1 and F-12 in the North Pacific (Gammon et al.. 1982) demonstrated the value of these compounds as time-dependent tracers in the upper ocean.CFCs have been used to study deep water formation rates in the Greenland-Norwegian Seas (Bullister and Weiss, 1983), the sub-surface transport of these compounds m the North Atlantic via deep western boundary currents (Weiss el al., 1985: Fine andMolinari, 1988) Sections of dissolved CFC concentrations were obtained on a cruise in the North Atlantic oil lhe R/V Ocea#ms during July-Sept 1988.This section extended flom Iceland to 3°S along about 20°W (Fig. 2a).About 2000 seawater samples were analyzed on board ship for F-12 and F-1 I during this cruise.A section showing F-12 concentrations is shown in Fig. 2b.Dissolved F-I1 and F-12 concentrations in the near-surface layer along this summerlime section were within a few percent of atmospheric equilibrium at most stations.Due to the strong dependence ofF-11 and F-I 2 solubilities on temperature, highest concentrations of dissolved F-12 (and F-11 ) were tbund in the cold ( 10-I I°C) surface waters along the northern end of this section.The overflow of cold, dense water from north of/he Iceland-Scotland Ridge system can be detected as a deep maximum (>2000 m) in dissolved F-12 concentration at station 7-14.This overflow is an important component in the formation of North Atlantic Deep Water (NADW).Elevated levels of dissolved CFC,,, and other tracers carried in recently ventihlled NADW (derived fl'om combined overflow of dense waters fbrmed to the north of the Greenhuld-lceland-Scotland Ridge system) can be traced in deep western boundary currents lk)r thousands of kilometers southward in the western basin of the North Atlantic.
A sub-surface tongue of high-CFC water centered at a depth of about 1700 m extends southward along the section to about 40°N.This maximum is associated with a relatively homogeneous, well-ventilated layer of cool, low salinity water formed in the region of the Labrador Sea.The sharp decrease in mid-depth F-12 concentrations south of about slation 50 hi Fig. 2b is associated with the transition to a regime of warmer, saltier water more strongly influenced by outflow flom the Mediterranean.At several stations along tile cruise track, welldefined, anomously high temperature and salinity features were observed in the mid-depth water column.These features are thought to be parcels of relatively undiluted Mediterranean outflow water ("Meddies') which are transported rapidly from their formation region to this area.These features also contain elevated levels of dissolved CFCs relative to the background field.The deepening of the 0.5 x 10 -~-' mole kg ~ F-12 contour at station 79 (see Fig. 2b), for example, reflects the presence of a CFC maximum associated with the Meddy observed at this station.The rapid transport and decay' of these short-lived features may have a significant role in determining the mean salinity, temperature and tracer fields in this region of the North Atlantic, The concentrations of dissolved F-1 I and F-12 are at or below detection limits in water deeper than about 500m at stations south of about 20°N.The ventilation of the mid-depth and deep waters of this region of the eastern North Atlantic are slow relative to time history of CFC release into the environment.
Of particular interest is the maximum in dissolved F-12 centered near the equator at a depth of about 1700m (Fig. 2b).Earlier CFC studies during the Transient Tracers in tile Ocean-Tropical Atlantic Study program detected tile presence of mid-depth maxima in dissolved F-11 and F-12 extending along tile equator fl'om the western boundary near Brazil eastward toward the nrid-Atlantic Ridge (Weiss el al.. 1985).Tile F-12 maximum shown in Fig. 2b is thought to represent the continuation of this deep, eastward equatorial transport to at least 25°W in the Atlantic.

CFCs in the Southern Ocean
Unlike a number of bomb-produced tracers which were deposited primarily in the Northern Hemisphere (e.g.tritium), the input functions for F-11 and F-12 are similar for both northern and southern hemispheres.CFCs may be especially useful as trac-ers of ventilation of interrnediate and deep waters in the Southern Ocean.
A study of dissolved F-I 1 and F-12 in Antarctica was made during the austral summer of 1987 on the West German R/V Polarstern.This study included detailed salnpling on the shelf and slope region near the Filchner Ice Shelf in the southern Weddell Sea, and stations across the Weddell Gyre (Fig. 3a, p. 16).
Figures 3b and 3c (p.16) show dissolved F-I1 concentrations and F-1 l/F-12 ratios along a section crossing the Weddell Sea.The highest levels of dissolved F-11 and F-12 measured along this section were present in near-surface waters, and at ice-free stations were close to equilibrium with the overlying atmosphere.F-1 l/F-12 ratios were about 2.2 in nearsurface samples, and reflect tile F-I I/F-12 ratio of modern air.Concentrations of dissolved CFCs decrease rapidly through the shallow ( 100 m) temperature minimum zone, which is a remnant flom the base of a cold, deep mixed layer which forlns in this region in winter.Lower but non-zero concentrations ofF-11 and F-12 are present throughout the deep waters along this section.F-1 l/F-12 ratios <2 in deeper waters are characteristic of atmospheric ratios prior to 1975, and reflect the isolation of this region from rapid atmospheric exchange.
The near bottorn maximum in dissolved F-12 in Fig. 3b is associated with the presence of a layer of cold, low-salinity new Weddell Sea Bottom Water (WSBW).The highest levels of dissolved CFCs (>1.5 x 10 *-~ mole kg t) in this layer were present in the the coldest WSBW banked along the western edge of the Weddell Sea near the tip of the Antarctic Peninsula (station 128).F-1 l/F-12 ratios in the core of the WSBW are essentially modem and indicate the rapid transport (less than ten years) of surface derived waters into this region.
New WSBW is believed to be produced as the result of the outflow and mixing of cold, dense shelfderived waters with surrounding deep waters on the

Future Studies
A better understanding of boundary conditions for CFCs during water mass fonnation processes is essential to modeling distributions observed within the interior of the ocean.Although much of the surface ocean is in close equilibrium with the CFCs in the atmosphere, some processes (such as bottom water formation in Antarctica) involve components which are significantly undersaturated in dissolved F-11 and F-12.Studies are needed to detennine the degree of disequilibrium lot" CFCs in waters undergoing rapid deep convective mixing processes and the boundary conditions for CFCs during subduction of water along isopycnal surfaces into the interior of the ocean.In multi-tracer studies, a thorough understanding of the differences in boundary conditions for various time-dependent tracers, and the implications of these differences, `s'ill improve out ability to obtain quantitative infornaation from the observed fields, There are a number of other CFCs which may prove useful as time-dependent ocean tracers.The concentration of CCI+ FCCIF, IF-113) has been increasitw very rapidly in the atmosphere durino the past decade, and measurements of dissoh, ed F-113 in seawater I Wisegarver and Gammon, 1988) may prove a valuable tool for studying processes occurring on this time scale.Halocarbons such as carbon tetrachloride and methyl chloroform, which have longer histories of release into the atmosphere than do the CFCs, may also prove to be valuable time-dependent tracers (Krysell and Wallace, 1988).
It is likely that dissolved CFC measurements will be included in the future along with nlore traditional tracer measurements (such as dissolved oxygen and nutrients) in many hydrographic programs, Together with other time-dependent tracers, such as ~He-tritium and ~+C, CFC studies ,sill probably be an important component of global programs such as the World Ocean Circulation Experiment (WOCE).The Scientific Plan for WOCE (1988) includes in its goals the study of the existing and evolving tracer fields in the world ocean by means of a one-time global survey and selected repeat sections.
Large numerical models will incorporate the in-formation obtained by' these global, multi-tracer studies to improve our understanding of the present rates and pathways of ocean circulation and mixing processes, and of the response of the ocean to changes in global climate.
Fig. 25." Dissolved F-I2 concentrations ( x 10/-' mole kg: ) al(mg North Atlantic section.The limits o/deteclion .fi~rF-12 are less than 0.005 x 104: mole kg :.More than 2000 water samples were analyzed.forCFCs along this section.Due to the relatively coarse contourin., intervals used in this illustration, mare" details of the F-12 distrihlttion ohserved along t/lis seeti(m are not shown.Samples were taken during Oceanus e/'uise 202. , and the ventilation of the Arctic Ocean (Wallace el a/.. 1987: Smethie et a/., 1988).The following discussion gives examples of two recent CFC sitidies.
CFCs in the North Atlantic CFCs iE/brmed along the southern margi;zs and trcmsl~orted thr(mqh the deep Weddell Sea.continental slope north of the Filchner Ice Shelf.Dense varieties of shelf waters are produced by the rapid fonnation and export of sea ice in this region and by cooling of water circulating beneath the Filchner Ice Shelf.Measurements made during the 1987 expedition show that, due to restricted air-sea exchange in this ice-covered region, CFC concentrations in these shelf waters are about 40% undersaturated relative to the overlying atmosphere.Mixing of these shelf waters with low CFC deep waters produce new WSBW, which at the time of its formation is significantly undcrsaturated in CFCs.Such information on the initial CFC content in new WSBW is needed to interpret the CFC signal present in WSBW and to estimate the rates of formation and transport of this water mass in the deep Weddell Sea.