Measurements of N2O and SF6 mole fraction between 1977 and 1998 in archived air samples from Cape Meares, Oregon

Abstract:

The quantification of greenhouse gas concentrations in the atmosphere is important for monitoring imbalances in their global budgets between sources and sinks and their changes in time. Nitrous oxide (N₂O) is a strong radiative trace gas with a GWP of ~300 times CO₂ over a 100 year period and an atmospheric lifetime of ~100 years. The preindustrial revolution background concentration of N₂O was ~270 ppb. Today, the concentration is ~330 ppb. Sulfur hexafluoride (SF₆) is another potent greenhouse gas with a long lifetime (800 to 3200 years) and very large GWP (~23000 times CO₂ over a 100 year period). Its current atmospheric concentration is low (~8 ppt today). Direct measurements of N₂O and SF₆ in air prior to the mid-1990s are few. Over 200 archived atmospheric gas samples collected at Cape Meares, Oregon between 1977 and 1998 were analyzed for their N₂O and SF₆ concentrations using an Agilent (model 6890 N) gas chromatograph fitted with an electron capture detector using a two column “heart-cut” technique. Precision of measurement of N₂O and SF₆ is calculated at 0.13% (1σ) and 1.35% (1σ) respectively. N₂O concentrations in the late 1970s and early 1980s average around 303 ppb, rising to 309 ppb in the early 1990s. Between 1980 and 1990, the increase in N₂O concentrations is found to be ~0.5 ppb/yr. SF₆ concentrations during the late 1970s and early 1980s average around 0.9 ppt and rise slowly, reaching 1.6 ppt in the 1990s. We find that the increase in SF₆ between 1980 and 1990 to be ~0.07 ppt/yr. We also discuss sample integrity in storage and observed temporal trends of N₂O and SF₆.