Observed reductions of total solar irradiance by biomass-burning aerosols in the Brazilian Amazon and Zambian Savanna

Abstract

Several aerosol and solar flux monitoring sites were established in Brazil for the Large Scale Biosphere-Atmosphere Experiment in Amazônia project. CIMEL sunphotometers and collocated pyranometers were employed at two southern Amazonian sites in order to quantify instantaneous reductions of total irradiance due to high aerosol optical thickness (AOT) smoke events (relative to values modeled for background aerosol conditions). Results from the Brazilian sites are presented for 1999 and for comparison, a similar analysis is discussed for data from three south-central African sites during the burning season of 2000. The relative reductions in total irradiance at the surface resulting from biomass burning aerosol are observed to be substantial at all sites, ranging from 16% for an aerosol optical thickness (500 nm) of 1.0 for the Brazilian sites to an average rate of 22% for the African sites. For a solar zenith interval (25–35°), these rates equate to reductions of roughly 145 W/m² and 210 W/m² respectively, for an AOT = 1.0. Instantaneous reductions of 337 W/m² were observed for the heaviest smoke conditions (AOT: ~3.0) in Brazil.