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. The Amazon rainforest is a sensitive ecosystem experiencing the combinedpressures of progressing deforestation and climate change. Its atmosphericconditions oscillate between biogenic and biomass burning (BB) dominatedstates. The Amazon further represents one of the few remaining continentalplaces where the atmosphere approaches pristine conditions during occasionalwet season episodes. The Amazon Tall Tower Observatory (ATTO) has beenestablished in central Amazonia to investigate the complex interactionsbetween the rainforest ecosystem and the atmosphere. Physical and chemicalaerosol properties have been analyzed continuously since 2012. This paperprovides an in-depth analysis of the aerosol's optical properties at ATTObased on data from 2012 to 2017. The following key results have beenobtained. The aerosol scattering and absorption coefficients at 637 nm,σsp,637 and σap,637, show a pronouncedseasonality with lowest values in the clean wet season (mean ± SD:σsp,637=7.5±9.3 M m−1; σap,637=0.68±0.91 M m−1) and highest values in the BB-polluted dry season(σsp,637=33±25 M m−1; σap,637=4.0±2.2 M m−1). The single scattering albedo at 637 nm,ω0, is lowest during the dry season (ω0=0.87±0.03) andhighest during the wet season (ω0=0.93±0.04). The retrieved BC mass absorption cross sections, αabs, aresubstantially higher than values widely used in the literature (i.e.,6.6 m2 g−1 at 637 nm wavelength), likely related to thick organicor inorganic coatings on the BC cores. Wet season values ofαabs=11.4±1.2 m2 g−1 (637 nm) and dry seasonvalues of αabs=12.3±1.3 m2 g−1 (637 nm) wereobtained. The BB aerosol during the dry season is a mixture of rather fresh smoke fromlocal fires, somewhat aged smoke from regional fires, and strongly aged smokefrom African fires. The African influence appears to be substantial, with itsmaximum from August to October. The interplay of African vs. South AmericanBB emissions determines the aerosol optical properties (e.g., the fractionsof black vs. brown carbon, BC vs. BrC). By analyzing the diel cycles, it was found that particles from elevatedaerosol-rich layers are mixed down to the canopy level in the early morningand particle number concentrations decrease towards the end of the day. Browncarbon absorption at 370 nm, σap,BrC,370, was found todecrease earlier in the day, likely due to photo-oxidative processes. BC-to-CO enhancement ratios, ERBC, reflect the variability of burntfuels, combustion phases, and atmospheric removal processes. A wide range ofERBC between 4 and 15 ng m−3 ppb−1 was observed withhigher values during the dry season, corresponding to the lowest ω0levels (0.86–0.93). The influence of the 2009/2010 and 2015/2016 El Niño periods and theassociated increased fire activity on aerosol optical properties was analyzedby means of 9-year σsp and σap time series(combination of ATTO and ZF2 data). Significant El Niño-relatedenhancements were observed: in the dry season, σsp,637increased from 24±18 to 48±33 M m−1 and σap,637 from 3.8±2.8 to 5.3±2.5 M m−1. The absorption Ångström exponent, åabs,representing the aerosol absorption wavelength dependence, was mostly&lt;1.0 with episodic increases upon smoke advection. Aparameterization of åabs as a function of the BC-to-OAmass ratio for Amazonian aerosol ambient measurements is presented. The browncarbon (BrC) contribution to σap at 370 nm was obtained bycalculating the theoretical BC åabs, resulting in BrCcontributions of 17 %–29 % (25th and 75th percentiles) toσap 370 for the entire measurement period. The BrCcontribution increased to 27 %–47 % during fire events under ElNiño-related drought conditions from September to November2015. The results presented here may serve as a basis to understand Amazonianatmospheric aerosols in terms of their interactions with solar radiation andthe physical and chemical-aging processes that they undergo during transport.Additionally, the analyzed aerosol properties during the last two El Niñoperiods in 2009/2010 and 2015/2016 offer insights that could help to assessthe climate change-related potential for forest-dieback feedbacks underwarmer and drier conditions.

Black and brown carbon over central Amazonia: long-term aerosol measurements at the ATTO site