Preprints
https://doi.org/10.5194/acp-2021-765
https://doi.org/10.5194/acp-2021-765

  09 Sep 2021

09 Sep 2021

Review status: this preprint is currently under review for the journal ACP.

Occurrence and growth of sub-50 nm aerosol particles in the Amazonian boundary layer

Marco A. Franco1,2, Florian Ditas2,a, Leslie Ann Kremper2, Luiz A. T. Machado1,2, Meinrat O. Andreae2,3, Alessandro Araújo4, Henrique M. J. Barbosa1, Joel F. de Brito5, Samara Carbone6, Bruna A. Holanda2, Fernando G. Morais1, Janaína P. Nascimento7,b, Mira L. Pöhlker2, Luciana V. Rizzo8, Marta Sá7, Jorge Saturno2,c, David Walter2,9,d, Stefan Wolff2, Ulrich Pöschl2, Paulo Artaxo1, and Christopher Pöhlker2 Marco A. Franco et al.
  • 1Institute of Physics, University of São Paulo, São Paulo 05508-900, Brazil
  • 2Multiphase Chemistry Department, Max Planck Institute for Chemistry, 55128 Mainz, Germany
  • 3Scripps Institution of Oceanography, University of California San Diego, La Jolla, CA 92037, USA
  • 4Empresa Brasileira de Pesquisa Agropecuária (Embrapa) Amazonia Oriental, CEP 66095-100, Belém, Brazil
  • 5IMT Lille Douai, Institut Mines-Télécom, Université de Lille, Centre for Energy and Environment, F-59000 Lille, France
  • 6Federal University of Uberlândia, Uberlândia-MG, 38408-100, Brazil
  • 7National Institute for Amazonian Research, Manaus, AM, 69.060-000, Brazil
  • 8Federal University of Sao Paulo, Department of Environmental Sciences, Diadema, Brazil
  • 9Department of Biogeochemical Systems, Max Planck Institute for Biogeochemistry, 07701 Jena, Germany
  • anow at: Hessian Agency for Nature Conservation, Environment and Geology, 65203 Wiesbaden, Germany
  • bnow at: NOAA Global Systems Laboratory, Boulder, CO, 80305, US
  • cnow at: Physikalisch-Technische Bundesanstalt, 38116 Braunschweig, Germany
  • dnow at: Climate Geochemistry Department, Max Planck Institute for Chemistry, 55128 Mainz, Germany

Abstract. New particle formation (NPF), referring to the nucleation of molecular clusters and their subsequent growth into the cloud condensation nuclei (CCN) size range, is a globally significant and climate-relevant source of atmospheric aerosols. Classical NPF exhibiting continuous growth from a few nanometers to the Aitken mode around 60–70 nm is widely observed in the planetary boundary layer (PBL) around the world, but not in central Amazonia. Here, classical NPF events are rarely observed in the PBL, but instead, NPF begins in the upper troposphere (UT), followed by downdraft injection of sub-50 nm (CN< 50) particles into the PBL and their subsequent growth. Central aspects of our understanding of these processes in the Amazon have remained enigmatic, however. Based on more than six years of aerosol and meteorological data from the Amazon Tall Tower Observatory (ATTO, Feb 2014 to Sep 2020), we analyzed the diurnal and seasonal patterns as well as meteorological conditions during 254 of such Amazonian growth events on 217 event days, which show a sudden occurrence of particles between 10 and 50 nm in the PBL, followed by their growth to CCN sizes. The occurrence of events was significantly higher during the wet season, with 88 % of all events from January to June, than during the dry season, with 12 % from July to December, probably due to differences in the condensation sink (CS), atmospheric aerosol load, and meteorological conditions. Across all events, a median growth rate (GR) of 5.2 nm h−1 and a median CS of 0.0011 s−1 were observed. The growth events were more frequent during the daytime (74 %) and showed higher GR (5.9 nm h−1) compared to nighttime events (4.0 nm h−1), emphasizing the role of photochemistry and PBL evolution in particle growth. About 70 % of the events showed a negative anomaly of the equivalent potential temperature (∆θ'e) – as a marker for downdrafts – and a low satellite brightness temperature (Tir) – as a marker for deep convective clouds – in good agreement with particle injection from the UT in the course of strong convective activity. About 30 % of the events, however, occurred in the absence of deep convection, partly under clear sky conditions, and with a positive ∆θ'e anomaly. Therefore, these events do not appear to be related to downdraft injection and suggest the existence of other currently unknown sources of the sub-50 nm particles.

Marco A. Franco et al.

Status: open (until 21 Oct 2021)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse

Marco A. Franco et al.

Marco A. Franco et al.

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Short summary
In Central Amazonia, new particle formation in the planetary boundary layer is rare. Instead, there is the appearance of sub-50 nm aerosols with diameters larger than about 20 nm, that eventually grow to cloud condensation nuclei size range. Here, 254 growth events were characterized, which have higher predominance in the wet season. About 70 % of them showed direct relation to convective downdrafts while 30 % occurred partly under clear sky conditions, evidencing still unknown particle sources.
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