Challenges in Linking Atmospheric CO₂ Concentrations to Fluxes at Local and Regional Scales

Abstract

We describe relationships between atmospheric CO₂ concentration variations and CO₂ source-sink distributions, at two important scales between the single plant and the whole earth: the vegetation canopy and the atmospheric planetary boundary layer. For both these scales, it is shown how knowledge of turbulence and scalar dispersion can be applied to infer CO₂ source-sink distributions or fluxes from concentration measurements.

At the canopy scale, the turbulent transfer of CO₂ and other scalars is non-diffusive close to any point source or sink in the canopy, but diffusive at greater distances. This distinction leads to a physically tenable description of turbulent transfer, and thence to an 'inverse method' for finding the vertical profiles of sources and sinks in the canopy from measured concentration profiles. The method is tested with data from a wheat crop.

At the scale of the planetary boundary layer, we consider the daily CO₂ concentration drawdown (the depression of the near-surface CO₂ concentration below the free-atmosphere value) of typically 20-40 ppm. This is determined by both the regionally averaged CO₂ uptake at the surface and the growth of the daytime convective boundary layer (CBL). It is shown that, for a column of air which fills the CBL and is moved across the landscape by the mean wind, the net cumulative surface CO₂ flux (in mol m^-2) is given to a good approximation by h(t)[C_m(t) - C+]/V, where h(t) is CBL depth, C_m(t) the CO₂ concentration in the CBL column in mol mol^-1, C+ the concentration above the CBL, V the molar volume and time t is measured from the time at which C_m = C+ in the morning, typically about 0800 hours local time. The resulting CO₂ flux estimates are regionally averaged over the trajectory followed by the column. This 'CBL budget method' for inferring surface fluxes is compared with direct measurements of CO₂ fluxes, with satisfactory results. The technique has application to scalars other than CO₂.