Measurements of N2O and SF6 for use in Inverse Model Studies

NOAA CMDL makes measurements of atmospheric nitrous oxide (N₂O) and sulfur hexafluoride (SF₆) mole fractions in discrete air samples from ∼60 surface sites and ∼10 vertical profile sites for use in inverse modeling. Measurements are made by gas chromatography with electron capture detection (ECD) relative to standard scales developed in CMDL's Halocarbons and Other Atmospheric Trace Species (HATS) group. Uncertainties in the standard scales (95% confidence limits) are 0.8 nmol mol^-1 for N₂O and 0.05 pmol mol^-1 for SF₆. For N₂O, the ECD response is characterized monthly by a second-order polynomial with a suite of 6 secondary standards covering the range 242-343 nmol mol^-1. An instrument response function prepared from measurements of the secondary standards relative to a reference cylinder is used to quantify samples. SF₆ in air samples is quantified with the reference by assuming a linear response with zero intercept in samples that are free of SF₆. Reproducibility of the measurements, based on agreement between two samples collected in series, is ∼0.4 nmol mol^-1 for N₂O and ∼0.04 pmol mol^-1 for SF₆. So far, all measurements have been made on a single analytical system. The measurements impose important constraints on the budgets of N₂O and SF₆ based on observed trends, changes in trends, and spatial gradients. These parameters can be used to determine source/sink imbalances, changes in emission rates over time, and the distribution of emissions. In addition, Peters et al. [JGR, in press, 2004] have used the SF₆ measurements to evaluate transport in the two-way nested chemistry transport model, TM5. They found that TM5 captures vertical gradients and synoptic variability in SF₆ well at both MBL and continental sampling sites, but it over-estimates the meridional gradient by ∼19%. The SF₆ data also suggest that global SF₆ emissions increased by ∼20% during late-2002.