Regional-scale CO2 fluxes over central Sweden by a boundary layer budget method

Measurements of CO2 flux and mixing ratio were made on a 100 m tower in an area of mixed pine and spruce forest near Uppsala in central Sweden. Free flight radiosondes were used to measure the height of the convective boundary layer (CBL) over four days in summer 1997. Mass budget equations were applied to calculate fluxes of CO2. The method exploits temporal variations in CO2 mixing ratio in the CBL which is well mixed and treats the CBL as a large chamber within which conservation of mass applies. The method yields cumulative surface fluxes over the trajectory of an air column crossing the landscape, with source areas of the order of 100 to 1000 km(2), depending on the time interval used. These flux estimates were compared with the ecosystem-scale fluxes measured by eddy covariance at the same site. Good agreement was found on two days when the airmass was maritime in origin, and the CO2 mixing ratio above the CBL (C-t) could be assumed to be equivalent to values measured at oceanic sites. Very poor agreement was found on two later days, when the airmass had passed over continental Europe over a number of days, and this assumption seemingly did not hold. We conclude that the CBL budget method has potential where C-t can be estimated reliably, using measurements either from aircraft or oceanic sites. In the latter case, it is necessary to consider air mass back-trajectories. As well as measuring CBL growth using radiosondes, it is useful to use a surface flux-driven model of CBL growth as a diagnostic tool to verify that the entrainment and subsidence terms have been quantified correctly. Further tests of the method are needed, preferably against aircraft flux measurements. The method is potentially valuable as an independent measure of regional-scale surface fluxes, and provides a check on the representativeness of eddy covariance measurements as a sample of the wider region. (C) 1999 Elsevier Science B.V. All rights reserved.