Strong influence of Black Carbon on aerosol optical properties in Central Amazonia during the fire season

Abstract. During the dry season, the Amazonian atmosphere is strongly impacted by fires, even in remote areas. However, there are still knowledge gaps regarding how each aerosol type affects the aerosol radiative forcing. This work characterizes the chemical composition of submicrometer aerosols and source apportionment of Organic Aerosols (OA) and Equivalent Black Carbon (eBC) to study their influence on light scattering and absorption at a remote site in central Amazonia during the dry season (August–December 2013). We applied Positive Matrix Factorization (PMF) and multi-linear regression models to estimate chemical-dependent mass scattering (MSE) and extinction (MEE) efficiencies. Mean PM1 aerosol mass loading was 6.3±3.3 μg m^-3, with 77 % of organics, grouped into 3 factors: Biomass Burning OA (BBOA), Isoprene derived Epoxydiol-Secondary OA (IEPOX-SOA) and Oxygenated OA (OOA). The bulk scattering and absorption coefficients at 637 nm were 17±10 Mm^-1 and 3±2 Mm^-1, yielding a single scattering albedo of 0.87±0.03. Although eBC represented only 6 % of the PM1 mass loading, MSE was highest for the eBC (13.58–7.62 m² g^-1 at 450–700 nm), followed by BBOA (7.96–3.10 m² g^-1) and ammonium sulfate (AS, 4.79–4.58 m² g^-1). MEE was dominated by eBC (30.8 %), followed by the OOA (19.9 %) and AS (17.6 %). The dominance of eBC over light scattering, in addition to absorption, depicts a surprisingly high role of this important climate agent, indicating the need to further investigate the chemical processing and interaction between natural and anthropogenic aerosol sources over remote tropical forested areas.