Spatial variability of the direct radiative forcing of biomass burning aerosols and the effects of land use change in Amazonia

摘要

This paper addresses the Amazonian shortwave radiative budget overcloud-free conditions after considering three aspects of deforestation:(i) the emission of aerosols from biomass burning due to forest fires;(ii) changes in surface albedo after deforestation; and (iii) modifications in thecolumn water vapour amount over deforested areas. Simultaneous Clouds andthe Earth's Radiant Energy System (CERES) shortwave fluxes and aerosoloptical depth (AOD) retrievals from the Moderate Resolution ImagingSpectroRadiometer (MODIS) were analysed during the peak of the biomassburning seasons (August and September) from 2000 to 2009. Adiscrete-ordinate radiative transfer (DISORT) code was used to extendinstantaneous remote sensing radiative forcing assessments into 24-haverages.The mean direct radiative forcing of aerosols at the top of the atmosphere(TOA) during the biomass burning season for the 10-yr studied period was?5.6 ± 1.7 W m?2. Furthermore, the spatial distributionof the direct radiative forcing of aerosols over Amazonia was obtained for thebiomass burning season of each year. It was observed that for high AOD(larger than 1 at 550 nm) the maximum daily direct aerosol radiative forcingat the TOA may be as high as ?20 W m?2 locally. The surface reflectanceplays a major role in the aerosol direct radiative effect. The study of theeffects of biomass burning aerosols over different surface types shows thatthe direct radiative forcing is systematically more negative over forestthan over savannah-like covered areas. Values of ?15.7 ± 2.4 W m−2τ_{550 nm}and ?9.3 ± 1.7 W m−2τ_{550 nm}were calculated for the mean daily aerosol forcing efficienciesover forest and savannah-like vegetation respectively. The overall meanannual land use change radiative forcing due to deforestation over the stateof Rond?nia, Brazil, was determined as ?7.3 ± 0.9 W m?2.Biomass burning aerosols impact the radiative budget forapproximately two months per year, whereas the surface albedo impact isobserved throughout the year. Because of this difference, the estimatedimpact in the Amazonian annual radiative budget due to surface albedo-changeis approximately 6 times higher than the impact due to aerosol emissions.The influence of atmospheric water vapour content in the radiative budgetwas also studied using AERONET column water vapour. It was observed thatcolumn water vapour is on average smaller by about 0.35 cm (around 10% ofthe total column water vapour) over deforested areas compared to forestedareas. Our results indicate that this drying contributes to an increase inthe shortwave radiative forcing, which varies from 0.4 W m?2 to 1.2 W m?2depending on the column water vapour content beforedeforestation.The large radiative forcing values presented in this study point out thatdeforestation could have strong implications in convection, clouddevelopment and the ratio of direct to diffuse radiation, which impactscarbon uptake by the forest.