Atmospheric particulate matter influences the Earth’s energy balance directly, by altering or absorbing solar radiation, and indirectly by influencing cloud formation1. Whether organic particulate matter exists in a liquid, semi-solid, or solid state can affect particle growth and reactivity2,3, and hence particle number, size and composition. The properties and abundance of particles, in turn, influence their direct and indirect effects on energy balance4. Non-liquid particulate matter was identified over a boreal forest of Northern Europe5, but laboratory studies suggest that, at higher relative humidity levels, particles can be liquid6,7. Here we measure the physical state of particulate matter with diameters smaller than 1 μm over the tropical rainforest of central Amazonia in 2013. A real-time particle rebound technique shows that the particulate matter was liquid for relative humidity greater than 80% for temperatures between 296 and 300 K during both the wet and dry seasons. Combining these findings with the distributions of relative humidity and temperature in Amazonia, we conclude that near-surface sub-micrometre particulate matter in Amazonia is liquid most of the time during both the wet and the dry seasons.

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Institutional support was provided by the Central Office of the Brazil Large-Scale Biosphere Atmosphere Experiment in Amazonia (LBA) and the Brazil National Institute of Amazonian Research (INPA). We acknowledge the Atmospheric Radiation Measurement (ARM) Climate Research Facility, a user facility of the United States Department of Energy, Office of Science, sponsored by the Office of Biological and Environmental Research, and support from the Atmospheric System Research (ASR) Program of that office. Funding was obtained from the United States Department of Energy (DOE), the São Paulo Research Foundation (FAPESP), the Amazonas State Research Foundation (FAPEAM), and the Brazil Scientific Mobility Program (CsF/CAPES). The authors acknowledge the Air Resources Laboratory of the United States National Oceanic and Atmospheric Administration (NOAA) for the provision of the website http://www.ready.noaa.gov for use of HYSPLIT. The research described herein was conducted under scientific licence 001030/2012-4 of the Brazilian National Council for Scientific and Technological Development (CNPq). Y. Ishida and B. Takeshi are gratefully acknowledged for logistical support of the measurements.

Author information


  1. School of Engineering and Applied Sciences, Harvard University, Cambridge, Massachusetts 02138, USA

    • Adam P. Bateman
    • , Zhaoheng Gong
    • , Pengfei Liu
    • , Yue Zhang
    •  & Scot T. Martin
  2. Federal University of São Carlos, São Paulo 13565-905, Brazil

    • Bruno Sato
  3. National Institute of Amazonian Research, Manaus, Amazonas 60060-000, Brazil

    • Glauber Cirino
    •  & Antonio O. Manzi
  4. University of São Paulo, São Paulo 05508-900, Brazil

    • Paulo Artaxo
  5. Department of Chemistry, University of British Columbia, Vancouver, British Columbia V6T 1Z1, Canada

    • Allan K. Bertram
  6. Department of Earth and Exact Sciences, Federal University of São Paulo, São Paulo 09972-270, Brazil

    • Luciana V. Rizzo
  7. Amazonas State University, Amazonas 69050-020, Brazil

    • Rodrigo A. F. Souza
  8. Pacific Northwest National Laboratory, Richland, Washington 99352, USA

    • Rahul A. Zaveri
  9. Department of Earth and Planetary Sciences, Harvard University, Cambridge, Massachusetts 02138, USA

    • Scot T. Martin


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A.P.B. carried out measurements, processed and analysed data, and contributed to the writing. P.L., Y.Z., B.S. and G.C. aided in the measurements. Z.G. participated in data analysis. A.O.M., P.A. and R.A.F.S. served in leadership roles for the infrastructure of the measurements. P.A., A.K.B., L.V.R. and R.A.Z. contributed to interpretation and writing. S.T.M. served as principal investigator of the project and contributed to data analysis, interpretation, and writing.

Competing interests

The authors declare no competing financial interests.

Corresponding author

Correspondence to Scot T. Martin.

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