Assessing scalar concentration footprint climatology and land surface impacts on tall-tower CO2 concentration measurements in the boreal forest of central Saskatchewan, Canada

Reducing the large uncertainties in current estimates of CO2 sources and sinks at regional scales (10(2)-10(5) km(2)) is fundamental to improving our understanding of the terrestrial carbon cycle. Continuous high-precision CO2 concentration measurements on a tower within the planetary boundary layer contain information on regional carbon fluxes; however, its spatial representativeness is generally unknown. In this study, we developed a footprint model (Simple Analytical Footprint model based on Eulerian coordinates for scalar Concentration [SAFE-C]) and applied it to two CO2 concentration towers in central Canada: the East Trout Lake 106-m-tall tower (54A degrees 21'N, 104A degrees 59'W) and the Candle Lake 28-m-high tower (53A degrees 59'N, 105A degrees 07'W). Results show that the ETL tower's annual concentration footprints were around 10(3)-10(5) km(2). The monthly footprint climatologies in summer were 1.5-2 times larger than in winter. The impacts of land surface carbon flux associated with heterogeneous distribution of vegetation types on the CO2 concentration measurements were different for the different heights, varied with a range of +/- 5 % to +/- 10 % among four heights. This study indicates that concentration footprint climatology analysis is important in interpreting the seasonal, annual and inter-annual variations of tower measured CO2 concentration data and is essential for comparing and scaling regional carbon flux estimates using top-down or bottom-up approaches.