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

• Published in: Atmospheric chemistry and physics 2018-09, Vol.18 (17), p.12817-12843
• Main Authors: Saturno, Jorge, Holanda, Bruna A, Pöhlker, Christopher, Ditas, Florian, Wang, Qiaoqiao, Moran-Zuloaga, Daniel, Brito, Joel, Carbone, Samara, Cheng, Yafang, Chi, Xuguang, Ditas, Jeannine, Hoffmann, Thorsten, Hrabe de Angelis, Isabella, Könemann, Tobias, Lavrič, JoÅ¡t V, Ma, Nan, Ming, Jing, Paulsen, Hauke, Pöhlker, Mira L, Rizzo, Luciana V, Schlag, Patrick, Su, Hang, Walter, David, Wolff, Stefan, Zhang, Yuxuan, Artaxo, Paulo, Pöschl, Ulrich, Andreae, Meinrat O
• Format: Article
• Language: English
• Subjects: Absorption; Absorption coefficient; Absorption cross sections; Absorptivity; Advection; Aerosol absorption; Aerosol measurements; Aerosol optical properties; Aerosol properties; Aerosols; Ageing; Aging; Albedo; Albedo (solar); Atmosphere; Atmospheric aerosols; Atmospheric conditions; Biomass burning; Burning; Carbon; Climate change; Coefficients; Combustion; Deforestation; Dependence; Dieback; Drought; Drought conditions; Dry season; Ecosystems; El Nino; El Nino phenomena; Environmental aspects; Environmental assessment; Environmental changes; Environmental Sciences; Fires; Inorganic coatings; Interactions; Measurement; Optical properties; Organic chemistry; Parameterization; Plant cover; Rainforests; Rainy season; Ratios; Removal; Scattering; Seasonal variations; Seasonality; Seasons; Smoke; Solar radiation; Wavelength; Wet season
• ISSN: 1680-7324; 1680-7316; 1680-7324
• DOI: 10.5194/acp-18-12817-2018

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 637 nm, σsp,637 and σap,637, show a pronounced seasonality 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) 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., 6.6 m2 g−1 at 637 nm wavelength), likely related to thick organic or inorganic coatings on the BC cores. Wet season values of αabs=11.4±1.2 m2 g−1 (637 nm) and dry season values of αabs=12.3±1.3 m2 g−1 (637 nm) 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 370 nm, 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 a