7 YEARS OF CONTINUOUS METHANE OBSERVATIONS AT A REMOTE BOREAL SITE INONTARIO, CANADA

A 7-year record (1990-1996) of continuous atmospheric methane (CH4) me asurements is presented from a remote midcontinental monitoring statio n at Fraserdale, Ontario (49 degrees 53'N, 81 degrees 34'W). Ninety-si x air samples per day were measured with a fully automated gas chromat ograph with flame ionization detection. Five-day Lagrangian back traje ctories over the 7-year period were used to establish a climatology in the region of the station. The site is predominantly influenced by ai r flow from northern and high-latitude regions and therefore uniquely positioned to monitor wetland emissions. During winter, CH4 concentrat ion time series from Fraserdale often match the short-term variability observed at the high Arctic monitoring station at Alert, Northwest Te rritories (82 degrees 27'N, 61 degrees 31'W). During summer, due to di urnal changes of vertical mixing in the boundary layer, large diurnal cycles in CH4 mixing ratio up to 150 ppb are observed. The data select ed for the afternoon, when the boundary layer is well-mixed, are repre sentative of a larger spatial scale. The mean annual cycle of CH4 at F raserdale determined using these selected data is significantly differ ent from annual cycles at other mid- and high-northern latitude sites thus providing key information for global atmospheric CH4 models. In l ate summer the annual cycle at Fraserdale shows a distinct secondary m aximum in CH4. This is the result of advection of air with enhanced CH 4 due to emissions from the extensive wetland areas to the north and n orthwest. The average growth rate (using selected data) for the period was 5.6 ppb yr-l with a growth rate pattern that is slightly differen t and out of phase with growth rate changes observed at other high-lat itude observing sites by 2 to 6 months.