220.127.116.11. Landscape or Regional Scales
Numerous techniques exist and are under development to infer mesoscale fluxes
of CO2 from spatial variations in atmospheric concentrations. At the smallest
scales are methods that are based on measuring the build-up of CO2 concentrations
above an ecosystem during temperature inversions (usually nocturnal) (Woodwell
and Dykeman, 1966; Gallagher et al., 1994). At a larger scale (upward
of 50 km2), measurements of concentration are made from tall towers or balloons
to approach or reach the top of the boundary layer (Bakwin et al., 1995,
1997). The use of aircraft allows flux to be inferred at still larger scales
(Choularton et al., 1995; Hollinger et al., 1995). Simultaneous
measurement of CO2 concentrations and other tracers (e.g., radon) and past trajectories
of air for continuous measurement sites may also be used to infer fluxes of
CO2 and non-CO2 gases.
An example of a technique that infers flux at this intermediate scale is the
convective boundary layer (CBL) budgeting approach. The concentrations
and fluxes of CO2 in the atmosphere, higher than a few meters above the land
surface, are the result of integrated individual contributions from the elements
of the landscape at scales on the order of 50-100 km (Stull, 1988). The CBL
budget approach exploits the natural integrating properties of the well-mixed
atmospheric boundary layer, allowing average surface fluxes to be obtained over
relatively large regions for time periods on the order of days rather than hours.
One of the main limits to widespread use of the CBL budgeting technique is
the relatively high cost involved, because of the need to employ aircraft to
sample above the top of the daytime CBL; these costs generally are on the order
of US$1,000 per day. Another limitation is that the same air mass theoretically
should be followed as it traverses the landscape. Practical experience has shown
that, even with sophisticated weather forecasting tools, this tracking is difficult.
Particularly when advection of air masses with different concentrations of the
entity of interest occurs, failure to effectively track an air mass can lead
to large errors in regional flux calculations using the CBL method. Additional
limitations include that fact that the measurements are for relatively short,
discontinuous periods and that the resulting flux includes an undefined area
of land and all fluxes of CO2 (fossil as well as biotic).