Preprints
https://doi.org/10.5194/acp-2021-277
https://doi.org/10.5194/acp-2021-277

  01 Jun 2021

01 Jun 2021

Review status: a revised version of this preprint is currently under review for the journal ACP.

Origin of water-soluble organic aerosols at the Maïdo high-altitude observatory, Réunion Island in the tropical Indian Ocean

Sharmine Akter Simu1,2, Yuzo Miyazaki2, Eri Tachibana2, Henning Finkenzeller3,4, Jérôme Brioude5, Aurélie Colomb6, Olivier Magand7, Bert Verreyken5,8,9, Stephanie Evan5, Rainer Volkamer3,4, and Trissevgeni Stavrakou8 Sharmine Akter Simu et al.
  • 1Graduate School of Environmental Science, Hokkaido University, Sapporo, 0600810, Japan
  • 2Institute of Low Temperature Science, Hokkaido University, Sapporo, 0600819, Japan
  • 3Department of Chemistry, University of Colorado Boulder, Boulder, CO 80309-0215, USA
  • 4Cooperative Institute for Research in Environmental Sciences (CIRES), University of Colorado Boulder, Boulder, CO
  • 5Laboratoire de l’Atmosphère et des Cyclones, UMR8105, CNRS, Université de La Réunion, 97744 Saint-Denis, France
  • 6Laboratoire de Météorologie Physique, UMR6016, CNRS, Université Clermont Auvergne, 63178 Aubière, France
  • 7Univ. Grenoble Alpes, CNRS, IRD, Grenoble INP, IGE, 38000 Grenoble, France
  • 8Royal Belgian Institute for Space Aeronomy, 1180 Brussels, Belgium
  • 9Department of Chemistry, Ghent University, 9000, Ghent, Belgium

Abstract. The tropical and subtropical Indian Ocean (IO) is expected to be a significant source of water-soluble organic aerosols (WSOAs), which are important factors relevant to cloud condensation nuclei and ice nuclei of aerosol particles. Current atmospheric numerical models significantly underestimate the budget of organic aerosols and their precursors, especially over tropical oceans. This is primarily due to poor knowledge of sources and the paucity of observations of these parameters considering spatial and temporal variation over the tropical open ocean. To evaluate the contribution of sources to WSOA as well as their formation processes, submicrometer aerosol sampling was conducted at the high-altitude Maïdo observatory (21.1° S, 55.4° E, 2,160 m a.s.l), located on the remote island of La Réunion in the southwest IO. The aerosol samples were continuously collected during local daytime and nighttime, which corresponded to the ambient conditions of the marine boundary layer (MBL) and free troposphere (FT), respectively, from March 15 to May 24, 2018. Chemical analysis showed that organic matter was the dominant component of submicrometer water-soluble aerosol (~45 ± 17 %) during the wet season (March 15–April 23), whereas sulfate dominated (~77 ± 17 %) during the dry season (April 24–May 24). Measurements of the stable carbon isotope ratio of water-soluble organic carbon (WSOC) suggested that marine sources contributed significantly to the observed WSOC mass in both the MBL and the FT in the wet season, whereas a mixture of marine and terrestrial sources contributed to WSOC in the dry season. The distinct seasonal changes in the dominant source of WSOC were also supported by Lagrangian trajectory analysis. Positive matrix factorization analysis suggested that marine secondary OA dominantly contributed to the observed WSOC mass (~70 %) during the wet season, whereas mixtures of marine and terrestrial sources contributed during the dry season in both MBL and FT. Overall, this study demonstrates that the effect of marine secondary sources is likely important up to the FT in the wet season, which may be responsible for cloud formation as well as direct radiative forcing over oceanic regions.

Sharmine Akter Simu et al.

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on acp-2021-277', Anonymous Referee #1, 24 Jun 2021
  • RC2: 'Comment on acp-2021-277', Anonymous Referee #2, 18 Jul 2021

Sharmine Akter Simu et al.

Sharmine Akter Simu et al.

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Short summary
The tropical Indian Ocean (IO) is expected to be a significant source of water-soluble organic carbon (WSOC), which is relevant to cloud formation. Our study showed that marine secondary organic formation dominantly contributed to the aerosol WSOC mass at the high-altitude observatory in the southwest IO in the wet season in both marine boundary layer and free troposphere (FT). This suggests that the effect of marine secondary sources is important up to FT, a process missing in climate models.
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