Carbon dioxide measurements in the nocturnal boundary layer over Amazonianforest.

Measurements of carbon dioxide concentration, temperature and windspeed wer e made in the nocturnal boundary layer over a tropical forest near Manaus, Brazil using a tethered balloon system. The measurements were made up to a maximum height of 300 m on ten consecutive nights in November 1995. Simulta neous surface flux and in-canopy concentration measurements were made at th e surface close to the site. The observation period included several differ ent types of conditions. Generally strong windshear and relatively weak tem perature gradients prevented the formation of a strong capping inversion to the nocturnal boundary layer. On some nights, however, the inversion was s ufficiently strong that the CO2 concentration at 100 m above the surface ex ceeded 400 ppm. The concentration within the canopy was largely controlled by the presence of an inversion very close to the canopy surface. The tempe rature and wind profiles are contrasted with conditions in Rondonia, Brazil , where the windshear was found to be ural;er and higher carbon dioxide con centrations were observed in the early morning. The difference in carbon di oxide concentrations in the nocturnal boundary layer between dusk and dawn is used to estimate the regional nighttime flux of carbon dioxide. The valu e obtained generally exceeds the measured surface flux and sometimes exceed s the sum of the surface flux and the in-canopy storage made at the tower s ite. The reasons for the discrepency are not clear; either one of the metho ds is in error or the regional carbon dioxide budget differs significantly from the local budget measured at the tower site.