Procedure which can Modify Noticeable CFC Many years

Contour dos shows reconstruction of percentages away from CFC-11/CFC-twelve and you may CFC-113/CFC-several into the remote North american heavens. The fresh ratio out of CFC-11/CFC-12 for the air enhanced substantially throughout the late 1940s in order to regarding the 1977, which regarding CFC-113/CFC-twelve got a small boost inside the mid-eighties.

Dating together with other Atmospheric Smoke, Sulfurhexafluoride

The slowing and turnover of CFC atmospheric mixing ratios in the 1990s has resulted in ambiguity in CFC-based apparent age. Sulphur hexafluoride (SFsix) has been developed as an alternative to CFCs for gas-solubility dating of water (Busenberg and Plummer, 1997). Significant production of SF6 began in the 1960s for use in high voltage electrical switches. SF6 is extremely stable, with an estimated atmospheric lifetime of 800 (Morris et al., 1995) to 3200 years (Ravishankara et al., 1993). SF6 can be analysed by GC-ECD techniques to a precision of 1-3% (Wanninkhof et al., 1991; Law et al., 1994), so, even though atmospheric mixing ratios are small (currently about 10 pptv), dating is possible from about 1970 to the present.

While we can use Henry’s law to help you interpret apparent CFC many years to own individual CFC ingredients, there are also differences in the brand new ratios out of atmospheric CFC levels that can be used getting relationships

The earliest reported measurement of the atmospheric concentration of SF6 was 0.03 pptv in 1970 (Lovelock, 1971). SF6 is accumulating rapidly in the atmosphere with a current growth rate of more than 7% per year. The historical atmospheric mixing ratio of SF6 has been reconstructed from production records and atmospheric measurements (Ko et al., 1993; Elkins et al., 1996; Maiss et al., 1996; Geller et al., 1997; Levin and Hesshaimer, 1996), and retrieved from concentrations measured in seawater (Law et al., 1994) and in previously-dated groundwater (Busenberg and Plummer, unpub. data). Figure 1 shows a preliminary reconstruction of northern hemisphere SF6 mixing ratios. As atmospheric CFC concentrations fall, an even more sensitive dating tool will be the ratio of SF6 to, for example, CFC-12 (Figure 2). However, because of the low solubility of SF6 (Mroczek, 1997), apparent ages can be very sensitive to excess air, so this needs to be accurately determined.

Busenberg and Plummer (1997) used SF6 successfully to date shallow groundwater on the Delmarva Peninsula, Maryland, and water from springs in the Blue Ridge Mountains of Virginia. Although SF6 is mostly of anthropogenic origin, concentrations between one and two orders of magnitude higher than that possible for modern air-water equilibrium were measured in groundwater from volcanic rift zones in New Mexico and Idaho (Busenberg and Plummer, 1997). There is likely a natural, igneous source of SF6 that will complicate dating in some environments. SF6 has also been used as an artificial tracer (Wilson and Mackay, 1996; Schlatter et al., 1997) that is injected into groundwater or surface water. Clark et al. (1995b) found that wastewater discharged to the Hudson River at New York City was contaminated with SF6. Busenberg and Plummer (1997) found minor contamination of SF6 in Rio Grande water near an outfall from a sewage treatment plant. All other surface waters had SF6 concentrations near equilibrium with air.

Of many procedure can be found throughout the recharge plus in the groundwater ecosystem one could affect CFC concentrations past the individuals set by heavens-h2o equilibrium, and consequently connect with translation from visible age. Table dos lists the most important process, the effect on noticeable CFC years, and you will relative advantages.

Profile 3 Density off CFC-eleven, CFC-12, and you may CFC-113 in the water recharged between 1940 and you can 1998 from the temperatures off 5 in order to twenty five °C on sea level.

magnitude of age problems you to definitely originate from uncertainties for the cost temperature. For example, in the event the CFC-eleven attention try 600 pg kilogram -1 in a particular water sample you to energized in the fifteen°C, however, charge temperatures was incorrectly thought to be 5°C, an evident many years that’s specific several age too-old carry out feel assigned, predicated on CFC-eleven. A suspicion within the charge temperature out of ± 2°C contributes to age uncertainties out-of 36 months or shorter to have water billed just before in the 1990 (Busenberg et al., 1993). Over-estimate off demand temperature contributes to obvious age too-young, and you may below-estimation brings many years that will be biased dated. Suspicion when you look at the recharge temperatures out-of ± 2°C causes uncertainty when you look at the visible CFC chronilogical age of one year or shorter to own liquid charged before the middle-70s. However, since the heavens collection ratios peaked on mid- and you will late-1990’s, CFC noticeable years became really sensitive to concerns within the charge temperature.