Effect of smoke and clouds on the transmissivity of photosynthetically active radiation inside the canopy

[摘要] Biomass burning activities emit high concentrations of aerosol particles tothe atmosphere. Such particles can interact with solar radiation, decreasingthe amount of light reaching the surface and increasing the fraction ofdiffuse radiation through scattering processes, and thus has implicationsfor photosynthesis within plant canopies. This work reports results fromphotosynthetically active radiation (PAR) and aerosol optical depth (AOD)measurements conducted simultaneously at Reserva Biológica do Jaru(Rondonia State, Brazil) during LBA/SMOCC (Large-Scale Biosphere-AtmosphereExperiment in Amazonia/ Smoke, Aerosols, Clouds, Rainfall, and Climate) andRaCCI (Radiation, Cloud, and Climate Interactions in the Amazon during theDry-to-Wet Transition Season) field experiments from 15 September to 15November 2002. AOD values were retrieved from an AERONET (Aerosol RoboticNetwork) radiometer, MODIS (Moderate Resolution Spectroradiometer) and aportable sunphotometer from the United States Department of Agriculture –Forest Service. Significant reduction of PAR irradiance at the top of thecanopy was observed due to the smoke aerosol particles layer. This radiationreduction affected turbulent fluxes of sensible and latent heats. Theincrease of AOD also enhanced the transmission of PAR inside the canopy. Asa consequence, the availability of diffuse radiation was enhanced due tolight scattering by the aerosol particles. A complex relationship wasidentified between light availability inside the canopy and net ecosystemexchange (NEE). The results showed that the increase of aerosol opticaldepth corresponded to an increase of CO₂ uptake by the vegetation.However, for even higher AOD values, the corresponding NEE was lower thanfor intermediate values. As expected, water vapor pressure deficit (VPD),retrieved at 28m height inside the canopy, can also affect photosynthesis.A decrease in NEE was observed as VPD increased. Further studies are neededto better understand these findings, which were reported for the first timefor the Amazon region under smoky conditions.

[学科分类] 大气科学