Using boundary-layer equilibrium to reduce uncertainties in CO2 flux inversions

Description

The relationship between CO2 concentration gradients and time scales 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 diagnostic of vertical CO2 gradients is developed based on equilibrium boundary-layer concepts and applied to observations from the U.S. Southern Great Plains Atmospheric Radiation Measurement Climate Research Facility and to a global data assimilation system based on Transport Model version 5 (TM5). The finite time scale over which vertical concentration gradients relax toward equilibrium is diagnostic of mixing rates between the boundary layer and free-troposphere and can be applied to observations and model simulations of conserved boundary-layer tracers 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 CO2 vertical gradients in terms of modeled mixing rates. Boundary-layer depth is found to be an unreliable indicator of mixing rates, which depend on but are not necessarily proportional to boundary-layer depth. Seasonality in transport and mixing can be reversed at some sites, diminishing in summer when the boundary layer is deepest. 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 the proposed errors would only further confound inverse estimates.