Tropospheric methane retrieved from ground-based near-IR solar absorption spectra

High-resolution near-infrared solar absorption spectra recorded between 1977 and 1995 at the Kitt Peak National Solar Observatory are analyzed to retrieve column abundances of methane (CH4), hydrogen fluoride (HF), and oxygen (O2). Employing a stratospheric ‘‘slope equilibrium’’ relationship between CH4 and HF, the varying contribution of stratospheric CH4 to the total column is inferred. Variations in the CH4 column due to changes in surface pressure are determined from the O2 column abundances. By this technique, CH4 tropospheric volume mixing ratios are determined with a precision of ~0.5%. These display behavior similar to Mauna Loa in situ surface measurements, with a seasonal peak-to-peak amplitude of approximately 30 ppbv and a nearly linear increase between 1977 and 1983 of 18.0 ± 0.8 ppbv yr-1, slowing significantly after 1990.