Dataset: Nitrous oxide and transient tracer concentrations in the tropical Pacific Ocean
Deployment: RB1503_leg1

Water samples were collected in bottles designed with a modified end-cap to minimize the contact of the water sample with the end-cap O-rings after closing. Stainless steel springs covered with a nylon powder coat were substituted for the internal elastic tubing provided with standard Niskin bottles. When taken, water samples collected for dissolved CFC/SF6/N2O analyses were the first samples drawn from the bottles. To minimize contact with air, the CFC/SF6/N2O samples were drawn directly through the stopcocks of the bottles into 250-milliliter (ml) precision glass syringes equipped with three-way plastic stopcocks. The syringes were immersed in a holding tank of clean surface seawater held at ~10 degrees Celsius (°C) until 20 minutes before being analyzed. At that time, the syringe was placed in a bath of surface seawater heated to 32°C.

Concentrations of CFC/SF6/N2O in seawater and gas standards were measured by shipboard electron capture detector gas chromatography (ECD-GC) using techniques modified from those described by Bullister and Wisegarver (2008), as outlined below. For seawater analyses, water was transferred from a glass syringe to a glass-sparging chamber (volume 200 ml). The dissolved gases in the seawater sample were extracted by passing a supply of CFC/SF6/N2O-free N2 through the sparging chamber for a period of 6 minutes at 150 milliliters per minute (ml min-1). Water vapor was removed from the purge gas during passage through a Nafion drier. Carbon dioxide was removed with an 18-centimeter (cm) long, 3/8-inch diameter glass tube packed with Ascarite and a small amount of magnesium perchlorate desiccant. The sample gases were concentrated on a cold-trap consisting of a 1/16-inch OD stainless steel tube with a 2.5 cm section packed tightly with Porapak Q, a 15 cm section packed with Carboxen 1000 and a 2.5 cm section packed with MolSieve 5 angstrom (Å). A Neslab Cryocool CC-100 was used to cool the trap to -65°C. After 6 minutes of purging, the trap was isolated, and heated electrically to 170°C. The sample gases held in the trap were then injected onto a precolumn (~61 cm of 1/8-inch OD stainless steel tubing packed with Porasil B, held at 80°C) for the initial separation of CFC-12, CFC-11, SF6 from later eluting peaks.

After the SF6 and CFC-12 had passed from the pre-column and into the second pre-column (26 cm of 1/8-inch OD stainless steel tubing packed with MolSieve 5Å, 160°C) and into the analytical column #1 (174 cm of 1/8-inch OD stainless steel tubing packed with MolSieve 5Å and 60 cm Porasil C held at 80°C), the outflow from the first pre-column was diverted to the second analytical column (180 cm 1/8-inch OD stainless steel tubing packed with Porasil B held at 80°C). The gases remaining after CFC- 11 had passed through the first pre-column, were backflushed from the precolumn and vented. After CFC-12 had passed through the second pre-column, a flow of Argon:Methane (95:5) was used to divert the N2O to a third analytical column (30 cm of MolSieve 5Å, 150°C). Column #3 and the second pre-column were held in a Shimadzu GC8AIE gas chromatograph with an electron capture detector (ECD) held at 330°C. Columns #1, and the first pre-column were in another Shimadzu GC8AIE gas chromatograph with ECD. The column #2 was also in a Shimadzu GC8AIE gas chromatograph with the ECD held at 330°C.

The analytical system was calibrated frequently using a standard gas of known CFC/SF6/N2O 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, pre-columns, main chromatographic column, and 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. System blanks (injections of loops of CFC/SF6/N2O-free gas) were injected and analyzed in a similar manner. The typical analysis time for seawater, standard or blank samples was ~12 minutes. Concentrations of CFC-11, CFC-12, and N2O in seawater samples and gas standards are reported relative to the SIO98 calibration scale (Prinn et al., 2000). Concentrations of SF6 in seawater samples and gas standards are reported relative to the SIO-2005 calibration scale (Bullister and Tanhua, 2010). Dissolved CFC concentrations are given in units of picomoles per kilogram seawater (pmol kg-1), SF6 concentrations in femtomoles per kilogram (fmol kg-1), and N2O concentrations in nanomoles per kilogram (nmol kg-1). CFC/SF6/N2O concentrations in 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 into the analytical instrument. The response of the detector to the range of moles of CFC/SF6/N2O passing through the detector remained relatively constant during the cruise. Full-range calibration curves were run at several times during the cruise and partial curves were run as frequently as possible, usually while sampling. 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 purging efficiency was estimated by re-purging a high-concentration water sample and measuring the residual signal. At a flow rate of 150 ml min-1 for 6 minutes, the purging efficiency for SF6 and both CFC gases was >99%. The efficiency for N2O was typically about 96%.