Abstract: Methane (CH4) is the second most significant greenhouse gas. The atmospheric CH4 concentration was measured weekly by flask sampling and the gas chromatography method at four national baseline observatories, including Waliguan (WLG), Shangdianzi (SDZ), Lin'an (LAN), and Longfengshan (LFS) from September 2006 to August 2008. The analysis results of the CH4 concentration data of the two years show that the annual CH4 concentrations increased by 9.1×10-9, 3.8×10-9, 21.8×10-9 and 8.2 ×10-9 on the average, at WLG, SDZ, LAN, and LFS observatories, respectively. The growth trend for CH4 concentration at WLG observatory was very similar to that of the global average, and representative of the atmospheric baseline level of the Eurasian continent. At SDZ observatory, the value of CH4 concentration was lower and the seasonal amplitude was also much smaller. However, both at LFS and LAN observatories, the CH4 concentrations obviously varied seasonally, with the maximum concentration in winter and the minimum concentration in summer.

Key words: atmospheric CH4, flask sampling, background concentration, regional differences