Nocturnal vertical CO2 accumulation in two Amazonian ecosystems

Abstract

Observations of the nocturnal vertical profile of CO2, specific humidity, temperature, and winds over a forested and a deforested site in the Amazonian region are presented. This study aims to understand better how scalars accumulate vertically at these two distinct sites at night. The measurements also provide an alternate way to determine the nocturnal fluxes under extremely calm conditions that vex the eddy covariance method. A profiling system based on a low‐cost Russian CO2 sensor aboard a tethered balloon, developed for this study, is described. At the deforested site, accumulation of scalars is restricted to a 100‐m thick layer, above which background concentrations of CO2 and specific humidity remain unaffected during the night. Over the forest, most of the scalars accumulate within the canopy. At both sites, accumulation and cooling rates are larger during calm periods near the surface, with the scalars escaping to upper levels during windier episodes. At the deforested site, the surface fluxes estimated from the time change of the profiles are only comparable to those observed at the nearby micrometeorological tower under windier, less stable conditions. On extremely stable nights, CO2 accumulates uniformly throughout the accumulation layer, possibly indicating convergence from the nearby forest toward the deforested region.