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Detailed, full water column vertical profiles of the concentrations of two dissolved chlorofluorocarbons (CFC-11 and CFC-12) and dissolved sulfur hexafluoride (SF₆) were collected on 14 cruises to the HOT site during the period 2005 to 2011. Vertical profiles of the concentration of dissolved carbon tetrachloride (CCl₄) were collected on 5 cruises to the HOT site during the period 2009-2011.

Seawater samples at the HOT site were collected on hydrocasts using a 24 position CTD/rosette frame equipped with 10 liter PVC bottles. The 10 liter bottles used were based on a NOAA-PMEL bottle design, which uses a modified end-cap to minimize contact of the end-cap O-rings with the seawater sample after closing. For bottles where seawater samples were collected for dissolved CFC-11, CFC-12, CCl₄ and SF₆ ('CFC/SF₆') analysis, these samples were the first drawn from the bottles after the return of the CTD/rosette to the deck of the ship. Care was taken to coordinate the CFC/SF₆ sampling with other seawater sampling to minimize the time between the initial opening of each bottle and the completion of sample drawing. Samples easily impacted by gas exchange (dissolved oxygen, DIC and pH) were collected within a few minutes of the initial opening of each bottle. To minimize contact with air, the seawater samples for CFC/SF₆ analysis were drawn directly through the stopcocks of the 10 liter bottles into 250 ml precision glass syringes equipped with three-way plastic stopcocks. The syringes were then immersed in a holding tank of clean surface seawater held at ~10^oC until ~20 minutes before being analyzed. At that time, the syringe was placed in a bath of surface seawater heated to ~30^oC.

About 200 cc of seawater from a syringe was transferred to a purge and trap system and analyzed simultaneously for CFC-11, CFC-12 and SF₆ on board ship based on methods described by Bullister and Wisegarver (2008). On 5 cruises, an additional valve and chromatographic column were added to the analytical system to allow for the analysis of CCl₄.

The analytical system was calibrated frequently using a gas standard of known CFC-11, CFC-12, CCl₄ and SF₆ composition. Gas sample loops of known volume were thoroughly flushed with the gas standard and injected into the system. The temperature and pressure were recorded so that the amount of gas injected could be calculated. The procedures used to transfer the gas standard to the trap, precolumn, main chromatographic column, and into the Electron Capture Detector (ECD) were similar to those used for analyzing water samples. Four 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/SF₆ free gas) were injected and analyzed in a similar manner. The typical analysis time for seawater, air, standard or blank samples was ~11 minutes.

CFC/SF₆ concentrations in air and seawater samples were determined by fitting their chromatographic peak areas to multi-point calibration curves for each gas, generated by injecting multiple sample loops of a gas standard into the analytical instrument. The response of the detector to the range of moles of CFC/SF₆ passing through the detector remained relatively constant during each cruise. Full-range calibration curves were run on each cruise. Single injections of a fixed volume of gas standard at one atmosphere pressure were run much more frequently (at intervals of ~90 minutes) to monitor short-term changes in detector sensitivity.

CFC-11, CFC-12 and CCl₄ concentrations are reported on the SIO-98 calibration scale and SF₆ values are reported on the SIO-05 scale (Bullister and Tanhua, 2010). Dissolved CFC and CCl₄ concentrations are reported in units of picomoles per kilogram seawater (pmol kg^-1), where 1 pmol = 10^-12 mole. SF₆ concentrations are given in units of femtomoles per kilogram seawater (fmol kg^-1 ), where 1 fmol = 0.001 pmol = 10^-15 mole).

Based on analysis of replicate samples, the estimated analytical precision of the dissolved CFC-11 and CFC-12 measurements is approximately ±1% or ±0.002 pmol kg^-1, whichever is greater. The estimated precision of the dissolved CCl₄ measurements is ±3% or ±0.003 pmol kg^-1, whichever is greater. The estimated precision of the dissolved SF₆ measurements is ±2% or ±0.02 fmol kg^-1, whichever is greater. The estimated overall error in the reported concentrations is estimated to be about a factor of 2 larger and is a function of a number of factors, including the absolute accuracy of the calibration of the gas standards, the accuracy of the volumetric calibrations of the sample gas loops and purge chamber, errors in fits to the calibration curves, errors in the volumes of water and gas standard analyzed and errors in the recorded temperature and pressure of the gas standards.

Blank levels were estimated based on analysis of deep seawater samples (from 2000-3000 meters depth) at the HOT site thought to be essentially tracer free, and on tests of the rate of change with time of dissolved CFC/SF₆ concentrations in the closed, 10 liter bottles. Blank corrections were applied separately to the data for each HOT cruise. On average the applied blank corrections were ~0.002 pmol kg^-1 for CFC-11, ~0.001 pmol kg^-1 for CFC-12, ~0.003 pmol kg^-1 for CCl₄ and 0.0 fmol kg^-1 for SF₆.

A small number of water samples had anomalously high CFC-11, CFC-12, CCl₄ or SF₆ concentrations relative to adjacent samples. These samples occurred sporadically during the cruises 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 or SF₆ during the sampling or analysis processes. Measured concentrations for these anomalous samples are included in the data file, but are given a WOCE quality flag (Swift, 2010) value of either 3 (questionable measurement) or 4 (bad measurement).

Additional information on the CFC and SF₆ observations at the HOT site is provided in Bullister et al. (2006).

Bullister, J.L., D.P. Wisegarver and R.E. Sonnerup, 2006. Sulfur Hexafluoride as a Transient Tracer in the North Pacific Ocean. Geophys. Res. Lett., vol 33, doi: 10.1029/2006GL026514, 2006

Bullister, J.L., and D.P. Wisegarver (2008): The shipboard analysis of trace levels of sulfur hexafluoride, chlorofluorocarbon-11 and chlorofluorocarbon-12 in seawater. Deep-Sea Res. I, 55, doi: 10.1016/j.dsr.2008.03.014, 1063-1074.

Bullister, J.L., and T. Tanhua (2010): Sampling and measurement of chlorofluorocarbons and sulfur hexafluoride in seawater. In The GO-SHIP Repeat Hydrography Manual: A Collection of Expert Reports and Guidelines., E.M. Hood, C.L. Sabine, and B.M. Sloyan (eds.), IOCCP Report Number 14, ICPO Publication Series Number 134, Available online at http://www.go-ship.org/HydroMan.html.

Karl, D. M., and R. Lukas (1996), The Hawaii Ocean Time-series (HOT) program: Background, rationale and field implementation, Deep Sea Res., Part II, 43, 129-156.

Swift, J.H. (2010): Reference-Quality Water Sample Data: Notes on Acquisition, Record Keeping, and Evaluation. In The GO-SHIP Repeat Hydrography Manual: A Collection of Expert Reports and Guidelines., E.M. Hood, C.L. Sabine, and B.M. Sloyan (eds.), IOCCP Report Number 14, ICPO Publication Series Number 134, Available online at http://www.go-ship.org/HydroMan.html.