Long-term validation of tropospheric column-averaged CH4 mole fractions obtained by mid-infrared ground-based FTIR spectrometry - Sepulveda; M. Schneider; F. Hase; O. E. García; A. Gomez-Pelaez; S. Dohe; T. Blumenstock; and J. C. Guerra (Atmospheric Measurement Techniques, 2012)