Seasonal and latitudinal variation of atmospheric methane: a ground-based and ship-borne solar IR spectroscopic study

Column-averaged volume mixing ratios of CH4 were retrieved with a precision of better than 0.5% from infrared solar absorption spectra obtained at Ny-Alesund (Spitsbergen, 79 N) between 1997 and 2004 and during two ship cruises (54 N–34 S) on the Atlantic in 2003. The retrieval has been performed in a spectral region available to all operational FTIR (Fourier Transform InfraRed) spectrometers performing solar absorption measurements. The seasonality and the long-term increase of the tropospheric volume-mixing ratio, derived from the infrared measurements agree well with data from surface sampling at this site. The latitudinal variation of shipborne measurements between 54 N and 34 S is in agreement with inverse model simulations which are optimized vs. the global NOAA/ESRL measurements.