A 13-year (1990--1996, 1999--2004), hourly air CO2 record measured on a 40 m tower in northern Canada is analyzed against interpolated marine boundary layer CO2 data representing the free troposphere above the tower. In warmer years, the planetary boundary layer was more depleted with CO2, suggesting that the land area (103--104 km2) upwind of the tower sequestered more carbon. After using a novel approach to derive the photosynthetic flux from the air CO2 diurnal variation pattern, it is confirmed that boreal ecosystem photosynthesis increased more than ecosystem respiration in warmer years.