Assessing filtering of mountaintop CO2 mixing ratios for application to inverse models of biosphere-atmosphere carbon exchange.

There is a widely recognized need to improve our understanding of biosphere-atmosphere carbon exchanges in areas of complex terrain including the United States Mountain West. CO₂ fluxes over mountainous terrain are difficult to measure often due to unusual and complicated influences associated with atmospheric transport in complex terrain. Using five years of CO₂ mixing ratio observations from the Regional Atmospheric Continuous CO₂ Network in the Rocky Mountains (Rocky RACCOON), five statistical (subsetting) filters are used to investigate a range of approaches for identifying regionally representative CO₂ mixing ratios. Test results from three filters indicate that subsets based on short-term variance and local CO gradients across tower inlet heights retain nine-tenths of the total observations and are able to define representative diurnal variability and seasonal cycles even for difficult-to-model sites where the influence of local fluxes is much larger than regional mixing ratio variations. Test results from two other filters that consider measurements from previous and following days using spline fitting or sliding windows are overly selective. Case study examples showed that even when standardized to common subset sizes these windowing-filters rejected measurements representing synoptic changes in CO₂, which suggests that they are not well suited to filtering continental CO₂ measurements. We present a novel CO₂ lapse rate filter that uses CO₂ differences between levels in the model atmosphere to constrain subsets of site measurements that are representative on model scales. [ABSTRACT FROM AUTHOR]