Editing Chlorofluorocarbon (section)

==Tracer of ocean circulation==
Since the time history of CFC concentrations in the atmosphere is relatively well known, they have provided an important constraint on ocean circulation. CFCs dissolve in seawater at the ocean surface and are subsequently transported into the ocean interior. Because CFCs are inert, their concentration in the ocean interior reflects simply the convolution of their atmospheric time evolution and ocean circulation and mixing.

The entry of CFCs into the ocean makes them extremely useful as transient tracers to estimate rates and pathways of ocean circulation and mixing processes.<ref name=PlummerBusenberg>{{cite book |last1=Plummer |first1=L. N. |last2=Busenberg |first2=E. |chapter=Chlorofluorocarbons in aquatic environments |pages=1–8 |title=Use of Chlorofluorocarbons in Hydrology: A Guidebook |date=2006 |publisher=International Atomic Energy Agency |isbn=978-92-0-100805-3 |url=https://www-pub.iaea.org/MTCD/publications/PDF/Pub1238_web.pdf }}</ref><ref name=BullisterWisegraver>{{cite journal |last1=Bullister |first1=John L. |last2=Wisegarver |first2=David P. |title=The shipboard analysis of trace levels of sulfur hexafluoride, chlorofluorocarbon-11 and chlorofluorocarbon-12 in seawater |journal=Deep Sea Research Part I: Oceanographic Research Papers |date=August 2008 |volume=55 |issue=8 |pages=1063–1074 |doi=10.1016/j.dsr.2008.03.014 |bibcode=2008DSRI...55.1063B }}</ref> However, due to production restrictions of CFCs in the 1980s, atmospheric concentrations of CFC-11 and CFC-12 has stopped increasing, and the CFC-11 to CFC-12 ratio in the atmosphere have been steadily decreasing, making water dating of water masses more problematic.<ref name=BullisterWisegraver/> Incidentally, production and release of sulfur hexafluoride ({{chem2|SF6}}) have rapidly increased in the atmosphere since the 1970s.<ref name=BullisterWisegraver/> Similar to CFCs, {{chem2|SF6}} is also an inert gas and is not affected by oceanic chemical or biological activities.<ref>{{cite journal |last1=Watanabe |first1=Yutaka W. |last2=Shimamoto |first2=Akifumi |last3=Ono |first3=Tsuneo |title=Comparison of Time-Dependent Tracer Ages in the Western North Pacific: Oceanic Background Levels of (SF6, CFC-11, CFC-12 and CFC-113 |journal=Journal of Oceanography |date=2003 |volume=59 |issue=5 |pages=719–729 |doi=10.1023/B:JOCE.0000009600.12070.1a |bibcode=2003JOce...59..719W }}</ref> Thus, using CFCs in concert with {{chem2|SF6}} as a tracer resolves the water dating issues due to decreased CFC concentrations.

Using CFCs or {{chem2|SF6}} as a tracer of ocean circulation allows for the derivation of rates for ocean processes due to the time-dependent source function. The elapsed time since a subsurface water mass was last in contact with the atmosphere is the tracer-derived age.<ref name=Fine>{{cite journal |last1=Fine |first1=Rana A. |title=Observations of CFCs and SF6 as Ocean Tracers |journal=Annual Review of Marine Science |date=15 January 2011 |volume=3 |issue=1 |pages=173–195 |doi=10.1146/annurev.marine.010908.163933 |pmid=21329203 }}</ref> Estimates of age can be derived based on the partial pressure of an individual compound and the ratio of the partial pressure of CFCs to each other (or {{chem2|SF6}}).<ref name=Fine/>

The age of a water parcel can be estimated by the CFC partial pressure (pCFC) age or {{chem2|SF6}} partial pressure (p{{chem2|SF6}}) age. The pCFC age of a water sample is defined as:
:<math>pCFC = \frac{[CFC]}{F(T,S)}</math>

where [CFC] is the measured CFC concentration (pmol kg<sup>−1</sup>) and F is the solubility of CFC gas in seawater as a function of temperature and salinity.<ref name=WarnerWeiss>{{cite journal |last1=Warner |first1=M.J. |last2=Weiss |first2=R.F. |title=Solubilities of chlorofluorocarbons 11 and 12 in water and seawater |journal=Deep Sea Research Part A. Oceanographic Research Papers |date=December 1985 |volume=32 |issue=12 |pages=1485–1497 |doi=10.1016/0198-0149(85)90099-8 |bibcode=1985DSRA...32.1485W }}</ref> The CFC partial pressure is expressed in units of 10–12 atmospheres or parts-per-trillion (ppt).<ref name=MinWarner>{{cite journal |last1=Min |first1=Dong-Ha |last2=Warner |first2=Mark J. |last3=Bullister |first3=John L. |title=Estimated rates of carbon tetrachloride removal in the thermocline and deep waters of the East Sea (Sea of Japan) |journal=Marine Chemistry |date=August 2010 |volume=121 |issue=1–4 |pages=100–111 |doi=10.1016/j.marchem.2010.03.008 |bibcode=2010MarCh.121..100M }}</ref> The solubility measurements of CFC-11 and CFC-12 have been previously measured by Warner and Weiss<ref name=MinWarner/> Additionally, the solubility measurement of CFC-113 was measured by Bu and Warner<ref name=BuWarner>{{cite journal |last1=Bu |first1=Xin |last2=Warner |first2=Mark J. |title=Solubility of chlorofluorocarbon 113 in water and seawater |journal=Deep Sea Research Part I: Oceanographic Research Papers |date=July 1995 |volume=42 |issue=7 |pages=1151–1161 |doi=10.1016/0967-0637(95)00052-8 |bibcode=1995DSRI...42.1151B }}</ref> and {{chem2|SF6}} by Wanninkhof et al.<ref>{{cite journal |last1=Wanninkhof |first1=Rik |last2=Ledwell |first2=James R. |last3=Watson |first3=Andrew J. |title=Analysis of sulfur hexafluoride in seawater |journal=Journal of Geophysical Research: Oceans |date=15 May 1991 |volume=96 |issue=C5 |pages=8733–8740 |doi=10.1029/91JC00104 |bibcode=1991JGR....96.8733W }}</ref> and Bullister et al.<ref>{{cite journal |last1=Bullister |first1=John L |last2=Wisegarver |first2=David P |last3=Menzia |first3=Frederick A |title=The solubility of sulfur hexafluoride in water and seawater |journal=Deep Sea Research Part I: Oceanographic Research Papers |date=January 2002 |volume=49 |issue=1 |pages=175–187 |doi=10.1016/S0967-0637(01)00051-6 |bibcode=2002DSRI...49..175B }}</ref>  Theses authors mentioned above have expressed the solubility (F) at a total pressure of 1 atm as:
:<math>\ln F = a_1 + a_2\left(\frac{100}{T}\right) + a_3\ln\left(\frac{T}{100}\right) + a_4\left(\frac{T}{100}\right)^2 + S\left[b_1 + b_2\left(\frac{T}{100}\right) + b_3\left(\frac{T}{100}\right)^2\right]</math>
where F = solubility expressed in either mol l<sup>−1</sup> or mol kg<sup>−1</sup> atm<sup>−1</sup>,
T = absolute temperature,
S = salinity in parts per thousand (ppt),
a<sub>1</sub>, a<sub>2</sub>, a<sub>3</sub>, b<sub>1</sub>, b<sub>2</sub>, and b<sub>3</sub> are constants to be determined from the least squares fit to the solubility measurements.<ref name=BuWarner /> This equation is derived from the integrated [[Van 't Hoff equation]] and the logarithmic Setchenow salinity dependence.<ref name=BuWarner />

It can be noted that the solubility of CFCs increase with decreasing temperature at approximately 1% per degree Celsius.<ref name=Fine/>

Once the partial pressure of the CFC (or {{chem2|SF6}}) is derived, it is then compared to the atmospheric time histories for CFC-11, CFC-12, or {{chem2|SF6}} in which the pCFC directly corresponds to the year with the same.  The difference between the corresponding date and the collection date of the seawater sample is the average age for the water parcel.<ref name=Fine/> The age of a parcel of water can also be calculated using the ratio of two CFC partial pressures or the ratio of the {{chem2|SF6}} partial pressure to a CFC partial pressure.<ref name=Fine/>