Regional source/sink impact on the diurnal, seasonal and inter-annual variations in atmospheric CO2 at a boreal forest site in Canada

Time series of in-situ CO₂ data from Fraserdale (50°N, 81°W) in the northern Ontario boreal forest is described, together with an analysis of observed variations occurring on daily to interannual timescales. Atmospheric CO₂ measurements at Fraserdale reflect a complex interaction between the daily cycle of the vegetative carbon flux and the daily evolution of the boundary layer mixing dynamics. This is particularly evident during the growing season, when CO₂ concentrations are influenced strongly by local and regional biospheric activities. In addition, the atmospheric CO₂ measurements at Fraserdale are greatly influenced by the direction of atmospheric transport, and reflect the complex heterogeneous distribution of ecosystem types around the site. Averaged over the 7-yr period from 1990 to 1996, the seasonal cycle associated with air from west and northwest of the site shows an amplitude of ~19 ppm, while that associated with air from the south and southwest shows an amplitude of ~23 ppm; the seasonal minimum, on average, occurs about a week earlier in the latter case than in the former. This is reflective of the fact that many of the deciduous trees are located to the south and southwest of Fraserdale. Furthermore, its location in the boreal forest causes the seasonal minimum to occur on average in early August at Fraserdale, compared to late August observed at Alert and at many other background stations in the high-latitude Northern Hemisphere. At the Fraserdale site there is no statistically significant indication, during the 1990–1996 study period, of changes in the length of the growing season (as measured by zero crossing points in the seasonal cycle).