Observation of ambient CH4 variations using ground-based high resolution Fourier transform solar spectrometry

Concentration of CH4 in ambient atmosphere is quite low (similar to 1.8 ppmv) and mixed well. So observation of CH4 variation with high precision has to rely on high-precision of inversion technique. This paper demonstrates the observation of CH4 variations using high resolution (0.02 cm(-1)) Fourier transform solar spectrometry, in which a high precision and large scale of retrieval algorithm is investigated. In the CH4 VCD (vertical column density) retrieval, both a nonlinear least-square method and an iterative scheme are applied after the measured spectrum is modeled with various a-priori parameters. The X CH4 (column-averaged dry air mixing ratio) is calculated by using the O-2 VCD derived from the 7885 cm 1 O-2-A band. Fitting errors for both CH4VCD and X CH4 are less than 1%, and most X CH4 retrievals are within the < 0.5 % error threshold of the TCCON (total carbon column observing network). Furthermore, the daily CH4 variations are also investigated based on one typical daily observation. It shows that the daily CH4 VCD decreases with time and the variations of the X CH4 is within 0.02 ppmv.