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Chlorofluorocarbon (CFC) Measurements on au0806

Responsible investigator

Associate Professor Mark J. Warner
email: warner@u.washington.edu
University of Washington
School of Oceanography
Seattle, WA 98195-7940
USA

Data contributor

Emily Lemagie
University of Washington
School of Oceanography
Seattle, WA 98195-7940
USA

Laboratory of analysis

University of Washington, School of Oceanography

Originator's Protocol for Data Acquisition and Analysis

Samples for the analysis of dissolved CFC-11, CFC-12, and CFC-113 were drawn from 1148 of the Niskin water samples collected during the expedition. When taken, water samples for CFC analysis were the first samples drawn from the 10-liter bottles. Care was taken to co-ordinate the sampling of CFCs with other samples to minimize the time between the initial opening of each bottle and the completion of sample drawing. In most cases, dissolved oxygen, alkalinity and dissolved inorganic carbon samples were collected within several minutes of the initial opening of each bottle. To minimize contact with air, the CFC samples were drawn directly through the stopcocks of the 10-liter bottles into 100-ml precision glass syringes equipped with 3-way plastic stopcocks. The syringes were immersed in a holding bath of seawater until analyzed.

For air sampling, a ~300 meter length of 3/8 inch OD Dekaron tubing was run from the portable laboratory to the bow of the ship. A flow of air was drawn through this line into the CFC van using an Air Cadet pump. The air was compressed in the pump, with the downstream pressure held at ~1.5 atm. using a back-pressure regulator. A tee allowed a flow (100 ml min-1) of the compressed air to be directed to the gas sample valves of the CFC analytical systems, while the bulk flow of the air (>7 l min-1) was vented through the back pressure regulator. Air samples were generally analyzed when the relative wind direction was within 100 degrees of the bow of the ship to reduce the possibility of shipboard contamination. The pump was run for approximately 30 minutes prior to analysis to insure that the air inlet lines and pump were thoroughly flushed. the average atmospheric concentrations determined during the cruise (from a set of 5 measurements analyzed when possible, n=33) were 241.4 +/- 0.9 parts per trillion (ppt) for CFC-11, 536.5 +/- 2.7 ppt for CFC-12, and 77.5 +/- 1.8 ppt for CFC-113.

Concentrations of CFC-11 and CFC-12, and CFC-113 in air samples, seawater and gas standards were measured by shipboard electron capture gas chromatography (EC-GC) using techniques modified from those described by Bullister and Weiss (1988). For seawater analyses, water was transferred from a glass syringe to a fixed volume chamber (~30 ml). The contents of the chamber were then injected into a glass sparging chamber. The dissolved gases in the seawater sample were extracted by passing a supply of CFC-free purge gas through the sparging chamber for a period of 4 minutes at 80 ml min-1. Water vapor was removed from the purge gas during passage through an 18 cm long, 3/8 inch diameter glass tube packed with the desiccant magnesium perchlorate. The sample gases were concentrated on a cold-trap consisting of a 1/8 inch OD stainless steel tube with a ~10 cm section packed tightly with Porapak N (60-80 mesh). A vortex cooler, using compressed air at 95 psi, was used to cool the trap, to approximately 20°C. After 4 minutes of purging, the trap was isolated, and the trap was heated electrically to ~100°C. The sample gases held in the trap were then injected onto a precolumn (~25 cm of 1/8 inch O.D. stainless steel tubing packed with 80-100 mesh Porasil C, held at 70°C) for the initial separation of CFC-12, CFC-11 and CFC-113 from other compounds. After the CFCs had passed from the pre-column into the main analytical column (~183 cm of 1/8 inch OD stainless steel tubing packed with Carbograph 1AC, 80-100 mesh, held at 70oC) of GC1 (a HP 5890 Series II gas chromatograph with ECD), the flow through the pre-column was reversed to backflush slower eluting compounds.

The analytical system was calibrated frequently using a standard gas of known CFC composition. Gas sample loops of known volume were thoroughly flushed with standard gas and injected into the system. The temperature and pressure was recorded so that the amount of gas injected could be calculated. The procedures used to transfer the standard gas to the trap, precolumn, main chromatographic column and EC detector were similar to those used for analyzing water samples. Two sizes of gas sample loops were used. Multiple injections of these loop volumes could be made to allow the system to be calibrated over a relatively wide range of concentrations. Air samples and system blanks (injections of loops of CFC-free gas) were injected and analyzed in a similar manner. The typical analysis time for seawater samples was 11.5 min., and for gas samples was ~10.5 minutes.

Concentrations of the CFCs in air, seawater samples and gas standards are reported relative to the SIO98 calibration scale (Cunnold, et. al., 2000). Concentrations in air and standard gas are reported in units of mole fraction CFC in dry gas, and are typically in the parts per trillion (ppt) range. Dissolved CFC concentrations are given in units of picomoles per kilogram seawater (pmol kg-1). CFC concentrations in air and seawater samples were determined by fitting their chromatographic peak areas to multi-point calibration curves, generated by injecting multiple sample loops of gas from a working standard (UW cylinder 45191 for CFC-11: 386.94 ppt, CFC-12: 200.92 ppt, and CFC-113: 105.4 ppt) into the analytical instrument. The response of the detector to the range of moles of CFC-12 and CFC-113 passing through the detector remained relatively constant during the cruise. The response of the detector to the upper range of CFC-11 amounts was found to slowly change during the cruise. Full-range calibration curves were run at intervals of 10 days during the cruise. These were supplemented with occasional injections of multiple aliquots of the standard gas at more frequent time intervals. Single injections of a fixed volume of standard gas at one atmosphere were run much more frequently (at intervals of ~90 minutes) to monitor short-term changes in detector sensitivity. The CFC-113 peak was often on a small bump on the baseline, resulting in a large dependence of the peak area on the choice of endpoints for integration. The height of the peak was instead used to provide better precision. The precisions of measurements of the standard gas in the fixed volume (n=450) were ± 0.61% for CFC-12, 0.89% for CFC-11, and 5.2% for CFC-113.

The efficiency of the purging process was evaluated periodically by re-stripping high concentration surface water samples and comparing the residual concentrations to initial values. These re-strip values were approximately 1% for all 3 compounds. A correction has been applied to the shipboard data.

The determination of a blank due to sampling and analysis of CFC-free waters was hampered by the lack of CFC-free waters. At several stations at the northern end of the section, CFCs in the deepest sample were measured to be less than 0.005 pmol kg-1 for CFC-11 and CFC-12. No sampling blank corrections have been made to this preliminary data set.

On this expedition, based on the analysis of 46 duplicate samples, we estimate precisions (1 standard deviation) of 0.75% or 0.003 pmol kg-1 (whichever is greater) for dissolved CFC-11, 0.30% or 0.003 pmol kg-1 for CFC-12 measurements, and 4.8% or 0.005 pmol kg-1 for CFC-113.

A very small number of water samples had anomalously high CFC concentrations relative to adjacent samples. These samples occurred sporadically during the cruise and were not clearly associated with other features in the water column (e.g. anomalous dissolved oxygen, salinity or temperature features). This suggests that these samples were probably contaminated with CFCs during the sampling or analysis processes. Measured concentrations for these anomalous samples are included in the preliminary data, but are given a quality flag value of either 3 (questionable measurement) or 4 (bad measurement).

A small amount of water vapor made its way onto the chromatographic column on April 10th and resulted in less than optimal performance of the analytical system for a few days. During that time CFC-113 peaks were located atop a broad contaminant peak and difficult to integrate. A large amount of CFC-113 data are flagged as bad (4) during this period. As the contamination cleared up over 2-3 days, this broad peak gradually disappeared. CFC-113 values have been flagged as questionable during this interval, until the baseline was flat. Although the baseline was very noisy, the data quality for CFC-11 and CFC-12 was only slightly worse than normal and was not flagged.

References Cited

Bullister, J.L., Weiss, R.F., 1988. Determination of CC13F and CC12F2 seawater and air. Deep-Sea Research, 25, 839-853.

Prinn, R. G., Weiss, R.F., Fraser, P.J., Simmonds, P.G., Cunnold, D.M., Alyea, F.N., O'Doherty, S., Salameh, P., Miller, B.R., Huang, J., Wang, R.H.J., Hartley, D.E., Harth, C., Steele, L.P., Sturrock, G., Midgley, P.M., McCulloch, A., 2000. A history of chemically and radiatively important gases in air deduced from ALE/GAGE/AGAGE. Journal of Geophysical Research, 105, 17,751-17,792

BODC Data Processing Procedures

Data received were loaded into the BODC database using established BODC data banking procedures.

Data was mapped to BODC parameter codes the mapping can be seen in the table below;

Data Quality Report

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