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The Amazon rain forest is considered a very sensitive ecosystem that could be significantly affected by a changing climate. It is still one of the few places on Earth where the atmosphere in the continent approaches near-pristine conditions for some periods of the year. The Amazon Tall Tower Observatory (ATTO) has been built in central Amazonia to monitor the atmospheric and forest ecosystem conditions. The atmospheric conditions at the ATTO site oscillate between biogenic and biomass burning (BB) dominated states. By using a comprehensive ground-based aerosol measurement setup, we studied the physical and chemical properties of aerosol particles at the ATTO site. This paper presents results from 2012 to 2017, with special focus on light absorbing aerosol particles. The aerosol absorption wavelength dependence (expressed as the absorption Ångström exponent, åabs) was usually below 1.0 and increased during the presence of smoke transported from fires in the southern and eastern regions of the Amazon or advected from savanna fires in Africa. In this study, the brown carbon (BrC) contribution to light absorption at 370 nm was obtained by calculating the theoretical wavelength dependence of åabs (WDA). Our calculations resulted in BrC contributions of 17–29 % (25th and 75th percentiles) to total light absorption at 370 nm (σap 370) during the measurement period (2012–2017). The BrC contribution increased up to 27–47 % during fire events occurring under the influence of El Niño, between September and November 2015. An extended time series of ATTO and ZF2 (another Amazon rain forest sampling site) data showed enhanced light scattering and absorption coefficients during El Niño periods in 2009 and 2015. Long-range transport (LRT) aerosol particles that reached the central Amazon Basin from Africa or from southern Amazon exhibited a wide range of black carbon (BC) to carbon monoxide (CO) enhancement ratios (ERBC) (between 4 and 15 ng m−3 ppb−1) reflecting the variability of fuels, combustion phase, and removal processes in the atmosphere. Higher ERBC were measured during the dry season when we observed values up to 15 ng m−3 ppb−1, which were related to the lowest single scattering albedo (ω0) measured during the studied period, (0.86–0.93). A parameterization of åabs as a function of the BC to OA mass ratio was investigated and was found applicable to tropical forest emissions but further investigation is required, especially by segregating fuel types. Additionally, important enhancements of the BC mass absorption cross‑section (αabs) were found over the measurement period. This enhancement could be linked to heavy coating of the BC aerosol particles. In the future, the BC mixing state should be systematically investigated by using different instrumental approaches.
