Ten years of atmospheric methane observations at a high elevation site in Western China

In this paper, the continuous (1994–2001) and discrete air sample (1991–2001) measurements of atmospheric CH4 from the Waliguan Baseline Observatory located in western China (36°17′N, 100°54′E, 3816 m asl) are presented and characterized. The CH4 time series show large episodic events on the order of 100 ppb throughout the year. During spring, a diurnal cycle with average amplitude of 7 ppb and a morning maximum and late afternoon minimum is observed. In winter, a diurnal cycle with average amplitude of 14 ppb is observed with an afternoon maximum and morning minimum. Unlike most terrestrial observational sites, no obvious diurnal patterns are present during the summer or autumn. A background data selection procedure was developed based on local horizontal and vertical winds. A selected hourly data set representative of “baseline” conditions was derived with approximately 50% of the valid hourly data. The range of CH4 mixing ratios, annual means, annual increases and mean annual cycle at Waliguan during the 1992–2001 were derived from discrete and continuous data representative of “baseline” conditions and compared to air samples collected at other Northern Hemisphere sites. The range of CH4 monthly means of 1746–1822 ppb, average annual means of 1786.7±10.8 ppb and mean annual increase of 4.5±4.2 ppb yr−1 at Waliguan were inline with measurements from sites located between 30° and 60°N. There were variations observed in the CH4 annual increase patterns at Waliguan that were slightly different from the global pattern. The mean CH4 annual cycle at Waliguan shows an unusual pattern of two gentle peaks in summer and February along with two small valleys in early winter and spring and a mean peak-to-peak amplitude of 11 ppb, much smaller than amplitudes observed at most other mid- and high-northern latitude sites. The Waliguan CH4 data are strongly influenced by continental Asian CH4 emissions and provide key information for global atmospheric CH4 models.