Trends in N₂O and SF₆ mole fractions in archived air samples from Cape Meares, Oregon (USA), 1978–1996

Abstract

Abstract. Quantifying historical trends in atmospheric greenhouse gases (GHGs) is important to understanding changes in their budgets and for climate modeling, which simulates historic and projects future climate. Archived samples analyzed using updated measurement techniques and calibration scales can reduce uncertainties in historic records of GHG mole fractions and their trends in time. Here, we present historical measurements of two important GHGs, nitrous oxide (N2O) and sulfur hexafluoride (SF6), collected at the midlatitude Northern Hemisphere station Cape Meares, Oregon (USA, 45.5∘ N, 124∘ W), between 1978 and 1996 in archived air samples from the Oregon Health and Science University – Portland State University (OHSU–PSU) air archive. N2O is the third most important anthropogenically forced GHG behind carbon dioxide (CO2) and methane (CH4). SF6 has a low abundance in the atmosphere, but is one of the most powerful GHGs known. Measurements of atmospheric N2O made during this period are available for select locations, but before mid-1990 they have larger uncertainties than more recent periods due to advancements made in gas chromatography (GC) methods. Few atmospheric SF6 measurements exist pre-1990, particularly in the Northern Hemisphere. The GC system used to measure N2O and SF6 mixing ratios in this work is designed to be fully automated, and is capable of running up to 15 samples per batch. Measurement precision (1σ) of N2O and SF6 is 0.16 % and 1.1 %, respectively (evaluated at 328.7 ppb and 8.8 ppt). Samples were corrected for detector response nonlinearity when measured against our reference standard, with the corrections determined to be 0.14 ppb ppb−1 in N2O and 0.03 ppt ppt−1 in SF6. The mixing ratio of N2O in archived samples is found to be 301.5±0.3 ppb in 1980 and rises to 313.5±0.3 ppb in 1996. The average growth rate over this period is 0.78±0.03 ppb yr−1 (95 % CI). The seasonal amplitude is statistically robust, with a maximum anomaly of 0.3 ppb near April and a minimum near November of −0.4 ppb. Measurements of N2O match well with previously reported values for Cape Meares and other comparable locations. The mixing ratio of SF6 in analyzed samples is found to be 0.85±0.03 ppt in 1980 and rises to 3.83±0.03 ppt in 1996. The average growth rate over this period is 0.17±0.01 ppt yr−1 (95 % CI). The seasonality is statistically robust and has an annual peak amplitude of 0.04 ppt near January and a minimum amplitude of −0.03 ppt near July. These are unique SF6 results from this site and represent a significant increase in the SF6 data available during the 1980s and early 1990s. The mixing ratio and growth rate of SF6 measured compares well to other Northern Hemisphere measurements over this period. From these N2O and SF6 measurements, we conclude that sample integrity is generally robust in the OHSU-PSU air archive for N2O and SF6.