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 (OAs) 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 multilinear regression (MLR) models to estimate chemical-dependent mass scattering efficiency (MSE) and extinction efficiency (MEE). 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-epoxydiol-derived secondary OA (IEPOX-SOA) and oxygenated OA (OOA). The bulk scattering and absorption coefficients at 637 nm were 17 ± 10 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). The MEE was dominated by eBC (30.8 %), followed by OOA (19.9 %) and AS (17.6 %). The dominance of eBC over light scattering, in addition to absorption, plays a remarkably important role for this important climate agent, with potentially broad implications for more precise radiative forcing quantification, increasing climate modeling precision and representing deep contributions to Earth's climate system comprehension.