Carbon cycling in the Amazon is closely linked to atmospheric processes andclimate in the region as a consequence of the strong coupling between theatmosphere and biosphere. This work examines the effects of changes in netradiation due to atmospheric aerosol particles and clouds on the netecosystem exchange (NEE) of CO in the Amazon region. Some of the majorenvironmental factors affecting the photosynthetic activity of plants, suchas air temperature and relative humidity, were also examined. An algorithmfor clear-sky irradiance was developed and used to determine the relativeirradiance, , which quantifies the percentage of solar radiation absorbedand scattered due to atmospheric aerosol particles and clouds. Aerosoloptical depth (AOD) was calculated from irradiances measured with the MODIS(Moderate Resolution Imaging Spectroradiometer) sensor, onboard the Terra andAqua satellites, and was validated with ground-based AOD measurements fromAERONET (Aerosol Robotic Network) sun photometers. Carbon fluxes weremeasured using eddy covariance technique at the Large-ScaleBiosphere-Atmosphere Experiment in Amazonia (LBA) flux towers. Two sites werestudied: the Jaru Biological Reserve (RBJ), located in Rondonia, and theCuieiras Biological Reserve at the K34 LBA tower (located in a preservedregion in the central Amazon). Analysis was performed continuously from 1999to 2009 at K34 and from 1999 to 2002 at RBJ, and includes wet, dry andtransition seasons. In the Jaru Biological Reserve, a 29% increase incarbon uptake (NEE) was observed when the AOD ranged from 0.10 to 1.5 at550 nm. In the Cuieiras Biological Reserve, the aerosol effect on NEE wassmaller, accounting for an approximate 20% increase in NEE. High aerosolloading (AOD above 3 at 550 nm) or high cloud cover leads to reductions insolar flux and strong decreases in photosynthesis up to the point where NEEapproaches zero. The observed increase in NEE is attributed to an enhancement(~50%) in the diffuse fraction of photosynthetic active radiation(PAR). The enhancement in diffuse PAR can be done through increases inaerosols and/or clouds. In the present study, it was not possible to separatethese two components. Significant changes in air temperature and relativehumidity resulting from changes in solar radiation fluxes under high aerosolloading were also observed at both sites. Considering the long-rangetransport of aerosols in the Amazon, the observed changes in NEE for thesetwo sites may occur over large areas in the Amazon, significantly alteringthe carbon balance in the largest rainforest in the world.
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