Chlorofluorocarbon Measurements

Click here for information about how halocarbons (CFCs, HFCs, HCFCs, and halons) are named.

CFCs were usually analysed in the first water sample collected from the 10-L bottles. Care was taken to co-ordinate the sampling of CFCs with other gas samples to minimize the time between the initial opening of each bottle and the completion of sample drawing. In most cases, helium, tritium, dissolved oxygen, TCO2, and TALK samples were collected within several minutes of the initial opening of each bottle. CFC samples were collected in 100-mL precision glass syringes and held immersed in a water bath until processing.

The CFC analytical system functioned relatively well during this expedition. The CFC system was installed in a specially designed laboratory van located on a deck and was isolated from possible contamination from the high levels of CFCs that are sometimes present in air inside the ship laboratories. Concentrations of CFCs in air inside this van were usually close to those of clean marine air.

Concentrations of CFC-11 and CFC-12 in air samples, seawater, and gas standards during the cruise were measured by shipboard electron capture gas chromatography, according to the methods described by Bullister and Weiss (1988). The concentrations of CFC-11 and CFC-12 in air samples, seawater samples, and gas standards are reported relative to the SIO 1986 calibration scale. CFC concentrations in air and standard gas are reported in units of mole fraction CFC in dry gas and are typically in parts-per-trillion (ppt) range. Dissolved CFC concentrations are given in unit of picomole CFC per kg seawater (pmol/kg). CFC concentrations in air and seawater samples were determined by fitting their chromatographic peak areas to multi-point calibration curves, generated by injecting known volumes of gas from a CFC working standard (PMEL cylinder 71489) into the analytical instrument. These concentrations of CFC-11 and CFC-12 in this working standard were calibrated versus a primary CFC standard (CC36743) before and after the cruise. No measurable drift in the working standard could be detected during this interval. Full range calibration curves were run at 1- to 2-day intervals. Single injections of a fixed volume of standard gas were run much more frequently (at intervals of 1 to 2 hours) to monitor short-term changes in detector sensitivity. The estimated reproducibility of the calibrations is about 1.3% for CFC-11 and 0.5% for CFC-12. A precision (1 standard deviation) for dissolved CFC measurements was ~1%, or 0.005 pmol/kg, whichever is greater.

Sample loops filled with CFC-free gas and syringe samples of CFC-free water (degassed in a specially designed glass chamber) were run to check sampling and analytical blanks. CFC-11 and CFC-12 concentrations measured in deep samples along the section were typically in the range of 0 to 0.007 pmol/kg, near the detection limit of the analytical system (~0.004 pmol/kg). Previous studies (Warner et al. 1996) of time-dependent tracers in this region of the Pacific indicate that waters at densities sigma > 27.4 should have CFC concentrations near zero at present. We attribute the low-level CFC signal in deep samples to the slow release of CFC from the walls and O-rings of the 10-L bottles into the seawater sample during storage and to contamination during the transfer and storage of the seawater samples in glass syringes prior to analysis. Based on the median concentrations observed in deep water samples along the section, the following blank corrections were applied to the seawater measurements:

Stations Blank Corrections Applied (in pmol/kg)
CFC-11
Stations 1 - 43 0.010
Stations 44 - 88 0.008
CFC-12
Stations 1 - 4 0.000
Stations 5 - 23 0.021
Stations 24 - 27 0.034
Stations 28 - 52 0.018
Stations 53 - 88 0.009

As a result of these blank corrections, some concentrations reported for deep samples are less than zero.

A number of water samples had anomalous high CFC-11 and/or CFC-12 concentrations relative to adjacent samples. These high values appeared to occur more or less randomly and were not clearly associated with other features in the water column (e.g., elevated oxygen concentrations). In most cases, only one of the two CFCs measured showed these anomalous high levels. This suggests that the high values were the result of analytical variability or isolated low-level contamination events. These samples are included in this report and are flagged as either "3" (questionable) or "4" (bad) measurements. Approximately 181 analyses of CFC-11 and 76 analyses of CFC-12 were given these flags.


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