Sulfur hexafluoride (SF₆) - Combined Dataset

Since 1995 two separate sampling programs in The Halocarbons and other Atmospheric Trace Species (HATS) group and one flask program in the Carbon Cycle Greenhouse Gasses group (CCGG) have measured sulfur hexafluoride (SF₆). The HATS programs include a flask system and an in situ program. Flask measurements started in 1995 with eight stations and continue sampling at 15 locations (13 background air locations). The HATS group developed a four channel gas chromatograph in 1998 (called CATS) to upgrade and expand the in situ measurement program. The CATS gas chromatographs are currently deployed at five field sites and make hourly measurements.

A combined dataset has been developed to incorporate all of the HATS monthly mean measurements made by each program; and SF₆ measurements from the CCGG network at the same HATS locations. The combined dataset is calculated by taking weighted averages of co-located measurements from background NOAA/GML air measurement programs (listed below). All programs are on the same NOAA scale (WMO X2014 scale for SF₆), and every attempt is made to correct known biases between measurement programs; however, there may be small differences that are not understood and characterized. When comparing measurements from station to station, be aware that not all locations are composed of the same measurement programs. The hemispheric and global means in the combined dataset are our best measure of long-term trends and are used for international and national assessments for background air measurements.

SF6 measurement programs	Start	Finish
HATS current flask instrument (OTTO)	1995	2013
GGGRN current flask instrument (MAGIC)	2001	Current
HATS in situ (CATS program)	1998	Current

Current GML programs (green rectangles) and discontinued programs (yellow rectangles) are used in the combined dataset.

The figure above shows the different measurement programs' calculated global means and illustrates the overlap amongst programs. Measured monthly means from the different programs are statistically combined to create a long-term NOAA/GML dataset (black line). The combined data is calculated by first interpolating missing data at sampling location for each measurement program. A weighted average is subsequently calculated where there are co-located measurements from two or more programs. The gap filled station measurement is then smoothed with the Savitzky-Golay algorithm.

Zonal means are calculated for four northern (solid lines) and three southern (dashed lines) bins.

The global SF₆ mean is plotted with 1-σ error bars (gray error bars). Improvements in measurement techniques and technology, as well as more sampling locations have reduced the global mean uncertainty since the mid 1990s. The red line is a linear fit to the global mean data demonstrating a fairly constant annual growth rate of 0.24 ppt per year. More recently the SF₆ growth rate has been increasing.

Global history of sulfur hexafluoride as a function of latitude (y-axis) and time (x-axis).

The Combined GML SF₆ Data file uses monthly data from the following programs:

• HATS flask program: Monthly, Global
• Global Greenhouse Gas Reference Network: Monthly
• CATS in situ program: Hourly, Daily, Monthly, Global

Publications:

• Elkins, J.W, and G.S. Dutton (2009), Nitrous oxide and sulfur hexafluoride [in 'State of the Climate in 2008']. Bull. Amer. Meteor. Soc, 90 S38-S39.
• Hall, B.D., G.S. Dutton, D.J. Mondeel, J.D. Nance, M. Rigby, J.H. Butler, F.L. Moore, D.F. Hurst and J.W. Elkins (2011), Improving measurements of SF₆ for the study of atmospheric transport and emissions. Atmos. Meas. Tech., 4 (11) 2441-2451, issn: 1867-1381, ids: 863AS, doi: 10.5194/amt-4-2441-2011.
• Rigby, M., et al., History of atmospheric SF₆ from 1973 to 2008, Atmos. Chem. Phys., 10, 10305-10320, 462 doi:10.5194/acp-10-10305-2010, 2010.