[CH4 concentrations and the variation characteristics at the four WMO/GAW background stations in China].

Background CH4 concentrations were continuously measured at the 4 WMO/GAW stations [Waliguan in Qinghai (WLG), Lin'an in Zhejiang (LAN), Shangdianzi in Beijing (SDZ), and Longfengshan in Heilongjiang (LFS)] by Cavity Ring Down Spectroscopy system. From 2009 to 2010, the diurnal cycle of hourly average CH4 concentration at LAN was found to be similar in all four seasons, with the highest level detected at 05:00 (Beijing Time) and the lowest at about 14:00. Similar CH4 diurnal cycles were observed at LFS in the summer time. However, the daily amplitude was much higher than that at LAN and reached 216. 8 x 10(-9) (molar ratio). For SDZ station, there were similar trends in spring, autumn and winter. The daily average concentration in the summer was much higher than those of the other seasons and reached the highest at about 20:00. No apparent CH4 diurnal cycle was observed at the WLG station during the whole year. The seasonal variations were obvious at the three regional stations (LAN, SDZ, LFS). The background concentration was the lowest in July at LAN while reached the highest level in August at LFS. The yearly background concentration variation at LFS displayed a "W" pattern. At LFS and SDZ, the wintertime CH4 concentrations were higher than those in spring and autumn. WLG represented a clean area and its CH4 value was the lowest among the four stations with the monthly average amplitude to be about 11.5 x 10(-9). At all three regional stations, non-background data accounted for more than 70% of the whole data. Cluster analysis of 3 day backward trajectories corresponding to the high CH4 concentration (WLG: CH4 > 1 870 x 10(-9), LFS: CH4 > 2100 x 10(-9), LAN: CH4 > 2 150 x10(-9), SDZ: CH4 > 2050 x 10(-9)) data points suggested that the high CH4 level measured in summer might be associated with the air mass transportation.