Long Term Analysis of Optical and Radiative Properties of Aerosols in the Amazon Basin

The Amazon region is an excellent laboratory for analysis of natural aerosols in the wet season; however, in the dry season the biomass burning emissions highly influence it, which considerably alters the physical and chemical properties of the atmosphere. We analyzed long-term time series (2000-2017) of optical and radiative properties of aerosols during dry and rain seasons from nine AERONET stations located in the Amazon Basin (Western Brazil). Aerosols have been classified into two groups: organic carbon (OC) and elemental carbon (EC), which allowed quantification of their effects on the radiative forcing for these sites. It was possible to conclude that the optical depth values of aerosols (AOD 500 nm), which remained in a downward inclination beginning in 2010 to 2012, returned to rise since 2013. The analysis showed that the fraction of biogenic particles varied from approximately 38% to 67% at the site of Manaus EMBRAPA. However, for ATTO this fraction remained practically constant throughout the year, around 68% as the fraction of OC-EC went from approximately 16 to 27%. In that way, the study found that in the central Amazon region, in the dry season, the OC fraction generates instantaneous effects of up to -100 W m^(-2) in the radiative surface forcing (FR-Surface), while in the Cerrado region (Cuiabá) the instantaneous values reached up to -350 W m^(-2). The impact of the primary biogenic particles on the radiative flux resulted in a mean, approximately -20 W m^(-2) FR-Surface, representing 45% of the total effect caused by aerosols in the Amazon.