Determining the validity of the boundary layer budget method for estimating greenhouse gas fluxes over complex terrain using idealized simulations

Knowledge of surface fluxes of greenhouse gases and air pollutants is important for policy and regulatory action. Typically the concentrations of these gases are measured, using towers or aircraft, from which the associated emissions have to be back-calculated. A relatively simple approach is the boundary layer budget method (BLB), which has been shown to work well in flat and homogeneous terrain. Most studies have applied the BLB method to determine fluxes from a fixed location by monitoring temporal changes in concentration (one-dimensional Eulerian approach); other studies have applied the BLB method to airmasses that are tracked over time, using e.g. aircraft measurements (Lagrangian approach).

The difficulty with the Lagrangian approach is the ability to track an airmass. Changes in scalar concentration in the airmass are due not only to surface fluxes but also to horizontal advection in and out of the sampled airmass and from entrainment or detrainment at the top. Because observations of horizontal and vertical advection are typically unavailable or highly inaccurate, these terms are not usually calculated and the change in airmass concentration is assumed to be caused by surface fluxes only. This assumption works well for flat, homogeneous terrain, but in mountainous terrain horizontal and vertical advection is likely not negligible due to the presence of thermally-driven wind systems.

Here we assess the violation of the BLB assumptions when applying this method to passive trace gas fluxes such as CO2 and methane in mountainous terrain. We use idealized simulations in which trace gas surface fluxes are prescribed. We then calculate the surface fluxes using the BLB method and compare these to the prescribed surface fluxes. We show that the BLB method has the potential to work over mountainous terrain as long as a suitable sampling strategy is used.