Articles | Volume 18, issue 13
Atmos. Chem. Phys., 18, 10055–10088, 2018

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

Atmos. Chem. Phys., 18, 10055–10088, 2018

Research article 16 Jul 2018

Research article | 16 Jul 2018

Long-term study on coarse mode aerosols in the Amazon rain forest with the frequent intrusion of Saharan dust plumes

Daniel Moran-Zuloaga1, Florian Ditas1, David Walter1, Jorge Saturno1,a, Joel Brito2,b, Samara Carbone2,c, Xuguang Chi1,d, Isabella Hrabě de Angelis1, Holger Baars3, Ricardo H. M. Godoi4, Birgit Heese3, Bruna A. Holanda1, Jošt V. Lavrič5, Scot T. Martin6,7, Jing Ming1, Mira L. Pöhlker1, Nina Ruckteschler1, Hang Su1,8, Yaqiang Wang9, Qiaoqiao Wang1,8, Zhibin Wang1, Bettina Weber1, Stefan Wolff1,10, Paulo Artaxo2, Ulrich Pöschl1, Meinrat O. Andreae1,11, and Christopher Pöhlker1 Daniel Moran-Zuloaga et al.
  • 1Multiphase Chemistry & Biogeochemistry Departments, Max Planck Institute for Chemistry, 55020 Mainz, Germany
  • 2Institute of Physics, University of São Paulo, São Paulo 05508-900, Brazil
  • 3Leibniz Institute for Tropospheric Research, Permoserstraße 15, 04318 Leipzig, Germany
  • 4Environmental Engineering Department, Federal University of Parana, Curitiba PR, Brazil
  • 5Department of Biogeochemical Systems, Max Planck Institute for Biogeochemistry, 07701 Jena, Germany
  • 6John A. Paulson School of Engineering and Applied Sciences, Harvard University, Cambridge, MA 02138, USA
  • 7Department of Earth and Planetary Sciences, Harvard University, Cambridge, MA 02138, USA
  • 8Institute for Environmental and Climate Research, Jinan University, Guangzhou, 510630, China
  • 9State Key Laboratory of Severe Weather & Key Laboratory of Atmospheric Chemistry of CMA, Chinese Academy of Meteorological Sciences, Beijing, China
  • 10Instituto Nacional de Pesquisas da Amazonia, Manaus-AM, CEP 69083-000, Brazil
  • 11Scripps Institution of Oceanography, University of California San Diego, La Jolla, CA 92037, USA
  • anow at: Physikalisch-Technische Bundesanstalt, Bundesallee 100, 38116 Braunschweig, Germany
  • bnow at: Laboratory for Meteorological Physics, Université Clermont Auvergne, 63000 Clermont-Ferrand, France
  • cnow at: Federal University of Uberlândia, Uberlândia-MG, 38408-100, Brazil
  • dnow at: Institute for Climate and Global Change Research & School of Atmospheric Sciences, Nanjing University, Nanjing, 210093, China

Abstract. In the Amazonian atmosphere, the aerosol coarse mode comprises a complex, diverse, and variable mixture of bioaerosols emitted from the rain forest ecosystem, long-range transported Saharan dust (we use Sahara as shorthand for the dust source regions in Africa north of the Equator), marine aerosols from the Atlantic Ocean, and coarse smoke particles from deforestation fires. For the rain forest, the coarse mode particles are of significance with respect to biogeochemical and hydrological cycling, as well as ecology and biogeography. However, knowledge on the physicochemical and biological properties as well as the ecological role of the Amazonian coarse mode is still sparse. This study presents results from multi-year coarse mode measurements at the remote Amazon Tall Tower Observatory (ATTO) site. It combines online aerosol observations, selected remote sensing and modeling results, as well as dedicated coarse mode sampling and analysis. The focal points of this study are a systematic characterization of aerosol coarse mode abundance and properties in the Amazonian atmosphere as well as a detailed analysis of the frequent, pulse-wise intrusion of African long-range transport (LRT) aerosols (comprising Saharan dust and African biomass burning smoke) into the Amazon Basin.

We find that, on a multi-year time scale, the Amazonian coarse mode maintains remarkably constant concentration levels (with 0.4 cm−3 and 4.0 µg m−3 in the wet vs. 1.2 cm−3 and 6.5 µg m−3 in the dry season) with rather weak seasonality (in terms of abundance and size spectrum), which is in stark contrast to the pronounced biomass burning-driven seasonality of the submicron aerosol population and related parameters. For most of the time, bioaerosol particles from the forest biome account for a major fraction of the coarse mode background population. However, from December to April there are episodic intrusions of African LRT aerosols, comprising Saharan dust, sea salt particles from the transatlantic passage, and African biomass burning smoke. Remarkably, during the core period of this LRT season (i.e., February–March), the presence of LRT influence, occurring as a sequence of pulse-like plumes, appears to be the norm rather than an exception. The LRT pulses increase the coarse mode concentrations drastically (up to 100 µg m−3) and alter the coarse mode composition as well as its size spectrum. Efficient transport of the LRT plumes into the Amazon Basin takes place in response to specific mesoscale circulation patterns in combination with the episodic absence of rain-related aerosol scavenging en route. Based on a modeling study, we estimated a dust deposition flux of 5–10 kg ha−1 a−1 in the region of the ATTO site. Furthermore, a chemical analysis quantified the substantial increase of crustal and sea salt elements under LRT conditions in comparison to the background coarse mode composition. With these results, we estimated the deposition fluxes of various elements that are considered as nutrients for the rain forest ecosystem. These estimates range from few g ha−1 a−1 up to several hundreds of g ha−1 a−1 in the ATTO region.

The long-term data presented here provide a statistically solid basis for future studies of the manifold aspects of the dynamic coarse mode aerosol cycling in the Amazon. Thus, it may help to understand its biogeochemical relevance in this ecosystem as well as to evaluate to what extent anthropogenic influences have altered the coarse mode cycling already.

Short summary
This study presents multiple years of aerosol coarse mode observations at the remote ATTO site in the Amazon Basin. The results are discussed in light of the frequent and episodic long-range transport of Saharan dust plumes in the early wet season as well as the persistent background bioaerosol cycling in the rain forest ecosystem. This work provides a solid basis for future studies on the dynamic coarse mode aerosol cycling and its biogeochemical relevance in the Amazon.
Final-revised paper