Using Boundary Layer Equilibrium to Reduce Uncertainties in CO2 Flux Inversions

A strong timescale dependence of atmospheric CO2 budgets is demonstrated in high frequency observations from the U.S. Southern Great Plains Atmospheric Radiation Measurement Climate Research Facility, and in a global data assimilation system. The relationship between CO2 concentration gradients and timescales of transport and mixing is explored and used to test the feasibility of previously proposed hypotheses for errors in CO2 flux inversions and atmospheric transport models. A simple model for CO2 vertical concentration gradients is developed based on the approximate equilibrium between surface fluxes and vertical and horizontal transport over seasonal and longer timescales. The finite timescale over which concentration gradients relax toward equilibrium is a diagnostic of the rate at which surface CO2 is exchanged with the free-troposphere, and can be applied to observations and model simulations of any conserved boundary layer tracer with surface sources and sinks. This diagnostic does not require dynamical variables from the transport models, and is independent of the prior- and post-inversion seasonal surface fluxes that may have complicated previous interpretations of concentration gradients in terms of modeled mixing rates. Results indicate that observations frequently cited as evidence for systematic biases in atmospheric transport models are insufficient to prove that such biases exist, and in some cases model errors proposed to reconcile carbon inventory and inverse estimates of global carbon sinks could further confound these estimates.