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Volume 18, issue 17
Atmos. Chem. Phys., 18, 12817-12843, 2018
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Special issue: Amazon Tall Tower Observatory (ATTO) Special Issue

Special issue: Observations and Modeling of the Green Ocean Amazon (GoAmazon2014/5)...

Atmos. Chem. Phys., 18, 12817-12843, 2018
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 06 Sep 2018

Research article | 06 Sep 2018

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

Jorge Saturno1,a, Bruna A. Holanda1, Christopher Pöhlker1, Florian Ditas1, Qiaoqiao Wang1,2, Daniel Moran-Zuloaga1, Joel Brito3,4, Samara Carbone3,5, Yafang Cheng1, Xuguang Chi6, Jeannine Ditas1,2, Thorsten Hoffmann7, Isabella Hrabe de Angelis1, Tobias Könemann1, Jošt V. Lavrič8, Nan Ma1,2, Jing Ming1, Hauke Paulsen9, Mira L. Pöhlker1, Luciana V. Rizzo10, Patrick Schlag3, Hang Su1, David Walter1, Stefan Wolff1, Yuxuan Zhang1, Paulo Artaxo3, Ulrich Pöschl1, and Meinrat O. Andreae1,11 Jorge Saturno et al.
  • 1Multiphase Chemistry & Biogeochemistry Departments, Max Planck Institute for Chemistry, 55128 Mainz, Germany
  • 2Jinan University Institute for Environmental and Climate Research, Guangzhou, 510630, China
  • 3Institute of Physics, University of São Paulo, São Paulo, 05508-900, Brazil
  • 4Laboratory for Meteorological Physics, Université Clermont Auvergne, 63000 Clermont-Ferrand, France
  • 5Institute of Agrarian Sciences, Federal University of Uberlândia, Uberlândia, Minas Gerais, 38408-100, Brazil
  • 6Institute for Climate and Global Change Research & School of Atmospheric Sciences, Nanjing University, Nanjing, 210093, China
  • 7Department of Chemistry, Johannes Gutenberg University, 55128 Mainz, Germany
  • 8Biogeochemical Systems & Biogeochemical Processes Departments, Max Planck Institute for Biogeochemistry, 07701 Jena, Germany
  • 9Institute of General Botany, Johannes Gutenberg University, 55128 Mainz, Germany
  • 10Departamento de Ciencias Ambientais, Universidade Federal de Sao Paulo, Diadema, SP, Brazil
  • 11Scripps Institution of Oceanography, University of California San Diego, La Jolla, CA 92098, USA
  • anow at: Physikalisch-Technische Bundesanstalt, Bundesallee 100, 38116 Braunschweig, Germany

Abstract. The Amazon rainforest is a sensitive ecosystem experiencing the combined pressures of progressing deforestation and climate change. Its atmospheric conditions oscillate between biogenic and biomass burning (BB) dominated states. The Amazon further represents one of the few remaining continental places where the atmosphere approaches pristine conditions during occasional wet season episodes. The Amazon Tall Tower Observatory (ATTO) has been established in central Amazonia to investigate the complex interactions between the rainforest ecosystem and the atmosphere. Physical and chemical aerosol properties have been analyzed continuously since 2012. This paper provides an in-depth analysis of the aerosol's optical properties at ATTO based on data from 2012 to 2017. The following key results have been obtained.

  • The aerosol scattering and absorption coefficients at 637nm, σsp,637 and σap,637, show a pronounced seasonality with lowest values in the clean wet season (mean±SD: σsp,637 = 7.5±9.3Mm−1; σap,637 = 0.68±0.91Mm−1) and highest values in the BB-polluted dry season (σsp,637 = 33±25Mm−1; σap,637 = 4.0±2.2Mm−1). The single scattering albedo at 637nm, ω0, is lowest during the dry season (ω0 = 0.87±0.03) and highest during the wet season (ω0 = 0.93±0.04).

  • The retrieved BC mass absorption cross sections, αabs, are substantially higher than values widely used in the literature (i.e., 6.6m2g−1 at 637nm wavelength), likely related to thick organic or inorganic coatings on the BC cores. Wet season values of αabs = 11.4±1.2m2g−1 (637nm) and dry season values of αabs = 12.3±1.3m2g−1 (637nm) were obtained.

  • The BB aerosol during the dry season is a mixture of rather fresh smoke from local fires, somewhat aged smoke from regional fires, and strongly aged smoke from African fires. The African influence appears to be substantial, with its maximum from August to October. The interplay of African vs. South American BB emissions determines the aerosol optical properties (e.g., the fractions of black vs. brown carbon, BC vs. BrC).

  • By analyzing the diel cycles, it was found that particles from elevated aerosol-rich layers are mixed down to the canopy level in the early morning and particle number concentrations decrease towards the end of the day. Brown carbon absorption at 370nm, σap,BrC,370, was found to decrease earlier in the day, likely due to photo-oxidative processes.

  • BC-to-CO enhancement ratios, ERBC, reflect the variability of burnt fuels, combustion phases, and atmospheric removal processes. A wide range of ERBC between 4 and 15ngm−3ppb−1 was observed with higher values during the dry season, corresponding to the lowest ω0 levels (0.86–0.93).

  • The influence of the 2009/2010 and 2015/2016 El Niño periods and the associated increased fire activity on aerosol optical properties was analyzed by means of 9-year σsp and σap time series (combination of ATTO and ZF2 data). Significant El Niño-related enhancements were observed: in the dry season, σsp,637 increased from 24±18 to 48±33Mm−1 and σap, 637 from 3.8±2.8 to 5.3±2.5Mm−1.

  • The absorption Ångström exponent, åabs, representing the aerosol absorption wavelength dependence, was mostly <1.0 with episodic increases upon smoke advection. A parameterization of åabs as a function of the BC-to-OA mass ratio for Amazonian aerosol ambient measurements is presented. The brown carbon (BrC) contribution to σap at 370nm was obtained by calculating the theoretical BC åabs, resulting in BrC contributions of 17%–29% (25th and 75th percentiles) to σap 370 for the entire measurement period. The BrC contribution increased to 27%–47% during fire events under El Niño-related drought conditions from September to November 2015.

The results presented here may serve as a basis to understand Amazonian atmospheric aerosols in terms of their interactions with solar radiation and the physical and chemical-aging processes that they undergo during transport. Additionally, the analyzed aerosol properties during the last two El Niño periods in 2009/2010 and 2015/2016 offer insights that could help to assess the climate change-related potential for forest-dieback feedbacks under warmer and drier conditions.

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Short summary
Biomass burning emits light-absorbing aerosol particles that warm the atmosphere. One of them is the primarily emitted black carbon, which strongly absorbs radiation in the visible and UV spectral regions. Another one is the so-called brown carbon, a fraction of organic aerosol particles that are able to absorb radiation, especially in the UV spectral region. The contribution of both kinds of aerosol particles to light absorption over the Amazon rainforest is studied in this paper.
Biomass burning emits light-absorbing aerosol particles that warm the atmosphere. One of them is...