Processes that can Personalize Apparent CFC Years

Contour 2 shows repair of percentages out of CFC-11/CFC-a dozen and you may CFC-113/CFC-a dozen in the remote United states air. The brand new proportion off CFC-11/CFC-several inside air improved dramatically in the late 1940s so you can on 1977, and that off CFC-113/CFC-several had a moderate increase in the mid-eighties.

Relationships along with other Atmospheric Fumes, Sulfurhexafluoride

The slowing and turnover of CFC atmospheric mixing ratios in the 1990s has resulted in ambiguity in CFC-based apparent age. Sulphur hexafluoride (SF_six) 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.

Once we may use Henry’s legislation to help you interpret apparent CFC decades for private CFC ingredients, there are also differences in the fresh ratios out-of atmospheric CFC concentrations used to own 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 processes exist through the cost and in the brand new groundwater ecosystem one make a difference CFC density beyond the individuals place by the heavens-liquids equilibrium, and therefore apply to translation out-of apparent years. Desk 2 lists initial procedure, their affect visible CFC age, and cousin importance.

Profile step 3 Levels of CFC-11, CFC-12, and CFC-113 in water charged anywhere between 1940 and you can 1998 at temperatures regarding 5 so you’re able to 25 °C on sea level.

magnitude of age errors that come from uncertainties during the cost temperature. Such as for example, if the CFC-11 focus try 600 pg kg -one in a certain water decide to try one recharged from the fifteen°C, however, charge heat is incorrectly believed is 5°C, an obvious decades that’s certain twelve decades too-old carry out be assigned, considering CFC-eleven. An uncertainty for the recharge temperature regarding ± 2°C contributes to decades concerns out of three-years or shorter to own water recharged prior to throughout the 1990 (Busenberg mais aussi al., 1993). Over-estimate off cost temperatures causes noticeable many years too young, and lower than-quote offers age that will be biased dated. Suspicion into the cost temperatures out of ± 2°C leads to suspicion inside the visible CFC period of one year or smaller to own drinking water charged ahead of the middle-70s. not, just like the heavens combination ratios peaked about middle- and you may late-1990’s, CFC visible decades turned into very responsive to uncertainties inside the demand temperatures.