North American influence on atmospheric carbon dioxide data collected at Sable Island, Canada

Continuous and flask measurements of atmospheric CO₂ taken at Sable Island from August 1992 to April 1993 are presented and characterised as a function of air mass origin. The atmospheric environment over Sable Island (43°56ʹN, 60°01ʹW) is continuously influenced by the complex meteorology of synoptic systems moving off North America. This makes the interpretation of the Sable Island CO₂ data difficult. However, trajectory analysis shows distinct quantitative differences between the statistics of CO₂ measurements associated with air masses from “North America” (regions of high anthropogenic and terrestrial biospheric fluxes associated with much of the United States and the southern half of Canada) and of those associated with air masses from the “Arctic/North Atlantic” (regions of few terrestrial fluxes and oceans associated with the northern half of Canada and the Atlantic Ocean). When the continuous CO₂ data are segregated into these two trajectory sectors for the period of observation, air masses originating in the North American sector show a higher CO₂ mixing ratio by ~2 ppm in winter and lower by ~3 ppm in summer, compared to air masses arriving from the other sector. Furthermore, the continuous Sable Island CO₂ measurements show a detectable monthly mean (August/September) diurnal cycle with an amplitude of ~2 ppm, with a minimum occurring on average around noon local time. Given the timing of the observed diurnal minimum and the lack of vegetation on the island, this indicates that the diurnal pattern observed at Sable Island is a diffused remnant of diurnal cycles transported from the main North American continent. These characteristic details are not captured by the discrete flask sampling program on the island.