Changes in solar radiation partitioning reaching the surface due to biomass burning aerosol particles in the Amazon Basin

In this paper, results from the partition of downward surface solar radiation between direct and diffuse components under distinct aerosol loadings will be presented. The field experiment was conducted during 2007 dry season in the Reserva Biologica do Jaru, located in the Southwestern portion of the Amazon Basin (-10.145 deg, -61.908 deg ), in a region with tropical rainforest vegetation. A Multi-Filter Rotating Shadow-band Radiometer (MFRSR) was used to measure global and diffuse components of solar radiation in narrow bands (centered at 415, 670, 870 and 1036 nm) as well as broadband (from about 350 to 1100 nm) spectral regions. Due to biomass burning emissions, high values of aerosol optical depth were observed (accompanying paper by Rosario et al., 2008). Consequently, a significant increase in diffuse solar irradiance was also observed. For example, at channel around 670 nm, the diffuse fraction corresponded to about 80%, even at noon, for aerosol optical depth of about 2 at the same wavelength, which is expected to be about 20% at this time of the day in clean conditions. For clean days, diffuse fraction can reach maximum about 35% during early morning or late afternoon. For broadband irradiance, 70% of radiation is diffuse when AOD is about 2 around noon and would be about 15-30% for low AOD values.