Applying the Virtual Tall Towers Approach to CO2 Data in the Amazon Basin.

The Virtual Tall Towers (VTT) approach, a method used to extrapolate mixing ratios vertically within the convective boundary layer (CBL), was applied to afternoon surface layer CO₂ mixing ratios at two sites in the Amazon Basin to estimate CO₂ within the CBL throughout 2017. Measurements in the CBL (or mixed layer) are removed from strong surface sources and sinks, providing a more spatially representative measure of CO₂ and allowing more meaningful comparison with large-scale models of the atmosphere. Surface layer measurements of CO₂ mixing ratios taken from an eddy-covariance flux tower in Tambopata, Peru (TBP, 45m) were corrected using tower eddy covariance measurements and an estimated CBL height from a top-down “gradient function” derived in Wisconsin, USA. The VTT correction was evaluated using in situ CO₂ measurements within both the surface layer (79m) and mixed layer (325m) from the Amazon Tall Tower Observatory (ATTO) in Brazil on diurnal, seasonal, and annual cycles to test the application of the concept in a tropical ecosystem. Using meteorological and mixing ratio data from ATTO, the gradient function was then tuned for the tropical environment. At TBP, the VTT adjusted afternoon CO₂ by an average of 0.75ppm in the dry season and 1.25ppm in the wet season, while the ATTO afternoon VTT adjustment averaged 0.7ppm throughout the year. As the first application of the VTT approach outside of a temperate forest ecosystem, this method can be used to extend existing CO2 data sets back in time and estimate CO2 measurements from shorter towers in other parts of the Amazon.