2.6. Nitrous Oxide and Sulfur Hexafluoride
The ongoing collaboration between CCG and NOAH on measurements of N2O and SF6 from at least a subset of the sites in the Carbon Cycle Group's cooperative air sampling network continued during 1996 and 1997. Data from Bermuda East are plotted in Figure 2.20.
Fig. 2.20. Nitrous oxide (in nmol) and SF6 (in pmol; abbreviated ppt) mole fractions from Bermuda East. Measurements were edited for sampling and analysis errors and selected for background conditions. One sample flagged as nonbackground is shown for N2O. Measurements made since August 1997 were made on the MAGICC analytical system; measurements made prior to that were from the Dragons GC [Tans et al., 1996, section 2.5].
A significant change in measurement procedures occurred in mid-1997. Since then, samples from all CCG network sites have been analyzed for N2O and SF6 using the electron capture detector (ECD) channel of the new CCG analysis system, MAGICC. The chromatographic method is the same as used previously, but the analytical instrument has changed leading to significantly improved precision. Typical precision for N2O, determined from repeated injections of standard gas, is 0.2 ppb or 0.06% relative. Samples are now bracketed by a single standard gas and N2O mole fractions are calculated from an instrument response curve that accounts for the highly nonlinear nature of the ECD. This curve was determined in a separate automated procedure, and it is based on three well-calibrated standards from the NOAH Group. These standards have N2O mole fractions of 300.9, 311.8, and 335.2 ppb. The nonlinearity correction for samples is large, 0.3 ppb per 1 ppb difference from the assigned standard value. Samples are quantified for SF6 using a single point calibration assuming linear extrapolation through zero.