Seven years of continuous methane observations at a remote boreal site in Ontario, Canada

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.