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In this paper, the continuous (1994–2001) and discrete air sample (1991–2001) measurements of atmospheric CH 4 from the Waliguan Baseline Observatory located in western China (36°17′N, 100°54′E, 3816 m asl) are presented and characterized. The CH 4 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 CH 4 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 CH 4 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 CH 4 annual increase patterns at Waliguan that were slightly different from the global pattern. The mean CH 4 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 CH 4 data are strongly influenced by continental Asian CH 4 emissions and provide key information for global atmospheric CH 4 models.

A 7-year record (1990–1996) of continuous atmospheric methane (CH4) measurements is presented from a remote midcontinental monitoring station at Fraserdale, Ontario (49°53′N, 81°34′W). Ninety-six air samples per day were measured with a fully automated gas chromatograph with flame ionization detection. Five-day Lagrangian back trajectories over the 7-year period were used to establish a climatology in the region of the station. The site is predominantly influenced by air flow from northern and high-latitude regions and therefore uniquely positioned to monitor wetland emissions. During winter, CH4 concentration time series from Fraserdale often match the short-term variability observed at the high Arctic monitoring station at Alert, Northwest Territories (82°27′N, 61°31′W). During summer, due to diurnal changes of vertical mixing in the boundary layer, large diurnal cycles in CH4 mixing ratio up to 150 ppb are observed. The data selected for the afternoon, when the boundary layer is well-mixed, are representative of a larger spatial scale. The mean annual cycle of CH4 at Fraserdale determined using these selected data is significantly different from annual cycles at other mid- and high-northern latitude sites thus providing key information for global atmospheric CH4 models. In late summer the annual cycle at Fraserdale shows a distinct secondary maximum in CH4. This is the result of advection of air with enhanced CH4 due to emissions from the extensive wetland areas to the north and northwest. The average growth rate (using selected data) for the period was 5.6 ppb yr−1 with a growth rate pattern that is slightly different and out of phase with growth rate changes observed at other high-latitude observing sites by 2 to 6 months.