Assessing filtering of mountaintop CO₂ mole fractions for application to inverse models of biosphere-atmosphere carbon exchange

CO₂ fluxes over mountainous terrain are often difficult to measure due to unusual and complicated influences associated with atmospheric transport. Consequently, deriving regional fluxes in mountain regions with carbon cycle inversion of atmospheric CO₂ mole fraction is sensitive to filtering of observations to those that can be represented at the transport model resolution.

Using five years of CO₂ mole fraction observations from the Regional Atmospheric Continuous CO₂ Network in the Rocky Mountains (Rocky RACCOON), five statistical filters are used to investigate a range of approaches for identifying regionally representative CO₂ mole fractions. Test results from three filters indicate that subsets based on short-term variance and local CO₂ gradients across tower inlet heights retain nine-tenths of the total observations and are able to define representative diel variability and seasonal cycles even for difficult-to-model sites where the influence of local fluxes is much larger than regional mole fraction variations.