Measurement of horizontal and vertical advection of CO2 within a forest canopy

Abstract

Eddy covariance measurements often underestimate the net exchange of CO2 between forest canopies and the atmosphere under stable atmospheric conditions, when horizontal and vertical advection are significant. A novel experimental design was used to measure all terms in the mass balance of CO2 in a 50 m × 50 m wide, 6 m tall control volume (CV) located on the floor of a 40 m tall Eucalyptus forest to examine the contributions of the eddy flux, the change in storage and the horizontal and vertical advection terms. Horizontal flux divergences between the four vertical walls of the CV were determined using perforated tubing arranged parallel to the ground to measure CO2 mixing ratios. The change in storage was calculated using CO2 concentration profiles measured in the centre of the CV. Vertical advection was calculated using these profiles, combined with vertical velocities,