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Volume 18, issue 14
Atmos. Chem. Phys., 18, 10289-10331, 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

Atmos. Chem. Phys., 18, 10289-10331, 2018
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 19 Jul 2018

Research article | 19 Jul 2018

Long-term observations of cloud condensation nuclei over the Amazon rain forest – Part 2: Variability and characteristics of biomass burning, long-range transport, and pristine rain forest aerosols

Mira L. Pöhlker1, Florian Ditas1, Jorge Saturno1,a, Thomas Klimach1, Isabella Hrabě de Angelis1, Alessandro C. Araùjo2, Joel Brito3,b, Samara Carbone3,c, Yafang Cheng1, Xuguang Chi1,d, Reiner Ditz1, Sachin S. Gunthe4, Bruna A. Holanda1, Konrad Kandler5, Jürgen Kesselmeier1, Tobias Könemann1, Ovid O. Krüger1, Jošt V. Lavrič6, Scot T. Martin7,8, Eugene Mikhailov9, Daniel Moran-Zuloaga1, Luciana V. Rizzo10, Diana Rose11,e, Hang Su1, Ryan Thalman12,f, David Walter1, Jian Wang12, Stefan Wolff1, Henrique M. J. Barbosa3, Paulo Artaxo2, Meinrat O. Andreae1,13, Ulrich Pöschl1, and Christopher Pöhlker1 Mira L. Pöhlker et al.
  • 1Multiphase Chemistry and Biogeochemistry Departments, Max Planck Institute for Chemistry, 55020 Mainz, Germany
  • 2Empresa Brasileira de Pesquisa Agropecuária (EMBRAPA), Trav. Dr. Enéas Pinheiro, Belém, PA, 66095-100, Brazil
  • 3Institute of Physics, University of São Paulo, São Paulo 05508-900, Brazil
  • 4EWRE Division, Department of Civil Engineering, Indian Institute of Technology Madras, Chennai 600036, India
  • 5Institut für Angewandte Geowissenschaften, Technische Universität Darmstadt, Darmstadt, Germany
  • 6Department of Biogeochemical Systems, Max Planck Institute for Biogeochemistry, 07701 Jena, Germany
  • 7John A. Paulson School of Engineering and Applied Sciences, Harvard University, Cambridge, MA, USA
  • 8Department of Earth and Planetary Sciences, Harvard University, Cambridge, MA, USA
  • 9St. Petersburg State University, 7/9 Universitetskaya nab, St. Petersburg, 199034, Russia
  • 10Instituto de Ciências Ambientais, Químicas e Farmacêuticas, Universidade Federal de São Paulo (UNIFESP), Diadema, SP, Brazil
  • 11Institut für Atmosphäre und Umwelt, Goethe Universität, 60438 Frankfurt, Germany
  • 12Biological, Environmental & Climate Sciences Department, Brookhaven National Laboratory, Upton, NY 11973-5000, USA
  • 13Scripps 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: Laboratoire de Météorologie Physique, Université Clermont Auvergne, Aubière, France
  • cnow at: Institute of Agrarian Sciences, Federal University of Uberlândia, Uberlândia, Minas Gerais, Brazil
  • dnow at: Institute for Climate and Global Change Research & School of Atmospheric Sciences, Nanjing University, Nanjing, 210093, China
  • enow at: Hessian Agency for Nature Conservation, Environment and Geology, Rheingaustr. 186, 65203 Wiesbaden, Germany
  • fnow at: Department of Chemistry, Snow College, Richfield, UT, USA

Abstract. Size-resolved measurements of atmospheric aerosol and cloud condensation nuclei (CCN) concentrations and hygroscopicity were conducted over a full seasonal cycle at the remote Amazon Tall Tower Observatory (ATTO, March 2014–February 2015). In a preceding companion paper, we presented annually and seasonally averaged data and parametrizations (Part 1; Pöhlker et al., 2016a). In the present study (Part 2), we analyze key features and implications of aerosol and CCN properties for the following characteristic atmospheric conditions:

  • Empirically pristine rain forest (PR) conditions, where no influence of pollution was detectable, as observed during parts of the wet season from March to May. The PR episodes are characterized by a bimodal aerosol size distribution (strong Aitken mode with DAit ≈ 70nm and NAit ≈ 160cm−3, weak accumulation mode with Dacc ≈ 160nm and Nacc ≈ 90cm−3), a chemical composition dominated by organic compounds, and relatively low particle hygroscopicity (κAit ≈ 0.12, κacc  ≈ 0.18).

  • Long-range-transport (LRT) events, which frequently bring Saharan dust, African biomass smoke, and sea spray aerosols into the Amazon Basin, mostly during February to April. The LRT episodes are characterized by a dominant accumulation mode (DAit ≈ 80nm, NAit ≈ 120cm−3 vs. Dacc ≈ 180nm, Nacc ≈ 310cm−3), an increased abundance of dust and salt, and relatively high hygroscopicity (κAit ≈ 0.18, κacc ≈ 0.35). The coarse mode is also significantly enhanced during these events.

  • Biomass burning (BB) conditions characteristic for the Amazonian dry season from August to November. The BB episodes show a very strong accumulation mode (DAit ≈ 70nm, NAit ≈ 140cm−3 vs. Dacc ≈ 170nm, Nacc ≈ 3400cm−3), very high organic mass fractions ( ∼ 90%), and correspondingly low hygroscopicity (κAit ≈ 0.14, κacc ≈ 0.17).

  • Mixed-pollution (MPOL) conditions with a superposition of African and Amazonian aerosol emissions during the dry season. During the MPOL episode presented here as a case study, we observed African aerosols with a broad monomodal distribution (D ≈ 130nm, NCN, 10 ≈ 1300cm−3), with high sulfate mass fractions (∼ 20%) from volcanic sources and correspondingly high hygroscopicity (κ <  100 nm  ≈ 0.14, κ >  100 nm ≈ 0.22), which were periodically mixed with fresh smoke from nearby fires (D ≈ 110nm, NCN, 10 ≈ 2800cm−3) with an organic-dominated composition and sharply decreased hygroscopicity (κ <  150 nm ≈ 0.10, κ >  150 nm ≈ 0.20).

Insights into the aerosol mixing state are provided by particle hygroscopicity (κ) distribution plots, which indicate largely internal mixing for the PR aerosols (narrow κ distribution) and more external mixing for the BB, LRT, and MPOL aerosols (broad κ distributions).

The CCN spectra (CCN concentration plotted against water vapor supersaturation) obtained for the different case studies indicate distinctly different regimes of cloud formation and microphysics depending on aerosol properties and meteorological conditions. The measurement results suggest that CCN activation and droplet formation in convective clouds are mostly aerosol-limited under PR and LRT conditions and updraft-limited under BB and MPOL conditions. Normalized CCN efficiency spectra (CCN divided by aerosol number concentration plotted against water vapor supersaturation) and corresponding parameterizations (Gaussian error function fits) provide a basis for further analysis and model studies of aerosol–cloud interactions in the Amazon.

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Short summary
This paper presents the aerosol and cloud condensation nuclei (CCN) variability for characteristic atmospheric states – such as biomass burning, long-range transport, and pristine rain forest conditions – in the vulnerable and climate-relevant Amazon Basin. It summarizes the key properties of aerosol and CCN and, thus, provides a basis for an in-depth analysis of aerosol–cloud interactions in the Amazon region.
This paper presents the aerosol and cloud condensation nuclei (CCN) variability for...