Estimation of Regional Net Surface Fluxes of CO2 and O3 over Amazonia from Aircraft Data (ABLE 2B)

Evidence suggests the existence of a 1-2 Gt so-called “missing” sink of atmospheric
CO2 in the terrestrial biosphere, but the distribution of this sink among the world’s
ecosystems is still unknown. Tropical forests, of which the Amazon Basin represents
the single largest area, account for 30-50% of terrestrial productivity and are
potentially significant sinks in the global carbon cycle. Simultaneous measurements
of CO2, O3, and CO concentrations from 14 flight missions in the lower troposphere
over the Brazilian Amazon during the wet season (ABLE 2B Experiment, April-May
1987) were analyzed in order to estimate regional net surface fluxes of CO2 and O3.
We used a “steady-state” approximation in which flux due to net biotic exchange was
balanced by subsidence. Determination of the subsidence flux was accomplished by
calculating the concentration gradient between the PBL and free troposphere and by
using a mean subsidence rate of –600 m/day from NCEP analyzed fields for the
experimental period. A linear decrease in CO2 column amount over the day was
observed and associated with a net CO2 flux of –4.8x1012 molecules/cm2
-s,
representing a carbon sink of -0.84 kg C/ha-day. The CO2 flux represents a weak net
exchange corresponding to only -0.15 Gt C/y if projected to the whole basin. The
results for O3 showed a flux of –6.9x1010 molecules/cm2
-s, consistent with other
measurements from ABLE 2B. Our study indicates that regional net fluxes can be
accurately determined using intensive airborne measurements and suggests attention
to two issues of experimental design: (1) removal of anthropogenic combustion
effects via a tracer (CO) can significantly affect the CO2 flux estimate, and (2) flight
data should be coupled with surface measurements between 0-150 m to fully account
for behavior below the minimum altitude of the aircraft.