Carbon dioxide in the atmosphere over the Amazon Basin

Abstract

The cycle of atmospheric CO 2 in the Amazon Basin was studied using measured vertical profiles of CO 2 concentrations in the canopy and aloft and direct measurements of CO 2 emissions from soils. The results provide detailed information on daily exchanges of air between the tropical forest (0–30 m) and the atmospheric boundary layer (30–2000 m). The forest was a net source of CO 2 at night and a sink during the day. High concentrations of CO 2 (380–400 ppm) accumulated in the forest at night and were transported into the nascent mixed layer in the morning. Between 1000 and 1200 LT, CO 2 in the mixed layer declined to concentrations 4–6 ppm lower than free tropospheric values and remained low for the rest of the afternoon. The afternoon mixed layer became isolated at night, and the low values of CO 2 were preserved until a new mixed layer developed the next morning. Entrainment of air into the mixed layer appeared to be a one‐way process during the early stages of mixed layer growth. The rate for uptake of atmospheric CO 2 by forests (soil and canopy) near midday was estimated to be 9(±4) kg C ha −1 hr −1 , and mean uptake over the sunlit period was about 2.8(±1.2) kg C ha −1 hr −1 . Forest soils emitted CO 2 at a mean rate of 1.8(±0.2) kg C ha −1 hr −1 . The atmospheric CO 2 cycle over wetlands was weaker than over forests, with daytime uptake rates of approximately 1.6 kg C ha −1 hr −1 . Rivers remained net sources of CO 2 throughout the day.