SF 6 ground-based infrared solar absorption measurements: long-term trend, pollution events, and a search for SF 5CF 3 absorption

Abstract

Infrared solar spectra recorded with the Fourier transform spectrometer in the McMath solar telescope complex on Kitt Peak (31.9°N latitude, 111.6°W, 2.09 km altitude), southwest of Tucson, Arizona, have been analyzed to retrieve average SF 6 tropospheric mixing ratios over a two-decade time span. The analysis is based primarily on spectral fits to absorption by the intense, unresolved ν 3 band Q branch at 947.9 cm −1 . A best fit to measurements recorded with SF 6 near typical background concentrations yields a SF 6 increase in the average tropospheric mixing ratio from 1.13 pptv (10 −12 per unit volume) in March 1982 to 3.77 pptv in March 2002. The long-term increase by a factor of 3.34 over the time span is consistent with the rapid growth of surface mixing ratios measured in situ at Northern Hemisphere remote stations, though the infrared measurements show a large scatter. Average tropospheric mixing ratio enhancements above background by 2–3 orders of magnitude have been identified in spectra recorded on 5 days between November 1988 and April 1997. These spectra were individually analyzed in an attempt to detect the strongest 8– 12 μm band of SF 5CF 3, a molecule recently identified with an atmospheric growth that has closely paralleled the rise in SF 6 during the past three decades. Absorption by the strongest SF 5CF 3 band was predicted to be above the noise level in the Kitt Peak spectrum with the highest average mean tropospheric SF 6 mixing ratio, assuming the reported atmospheric SF 5CF 3/SF 6 ratio and a room temperature absorption cross sections reported for the SF 5CF 3 903-cm −1 band. An upper limit of 8×10 15 molecules cm −2 for the SF 5CF 3 total column was estimated for this case. We hypothesize that the highly elevated SF 6 levels above Kitt Peak resulted from a local release experiment rather than production via electrochemical fluoridation of intermediate products, the proposed source of atmospheric SF 5CF 3. The absence of the SF 5CF 3 feature in the spectra with elevated SF 6 is consistent with the absence of SF 5CF 3 reported in a pure SF 6 sample.