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https://doi.org/10.5194/acp-2020-637
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/acp-2020-637
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

  10 Aug 2020

10 Aug 2020

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This preprint is currently under review for the journal ACP.

Characterisation of African biomass burning plumes and impacts on the atmospheric composition over the South-West Indian Ocean

Bert Verreyken1,2,3, Crist Amelynck1,2, Jérôme Brioude3, Jean-François Müller1, Niels Schoon1, Nicolas Kumps1, Aurélie Colomb4, Jean-Marc Metzger5, Christopher F. Lee6,7, Theodore K. Koenig6,7, Rainer Volkamer6,7, and Trissevgeni Stavrakou1 Bert Verreyken et al.
  • 1Royal Belgian Institute for Space Aeronomy, B-1180 Brussels, Belgium
  • 2Department of Chemistry, Ghent University, B-9000 Ghent, Belgium
  • 3Laboratoire de l'Atmosphère et des Cyclones, UMR 8105, CNRS, Université de La Réunion, 97744 Saint-Denis, France
  • 4Laboratoire de Météorologie Physique, UMR6016, CNRS, Université Clermont Auvergne, 63178 Aubière, France
  • 5Observatoire des Science de l'Univers de La Réunion, UMS3365, 97744 Saint-Denis, France
  • 6Cooperative Institute for Research in Environmental Sciences (CIRES), University of Colorado, Boulder, CO, USA
  • 7Department of Chemistry, University of Colorado, Boulder, CO, USA

Abstract. We present an investigation of biomass burning (BB) plumes originating from Africa and Madagascar based on measurements of carbon monoxide (CO), ozone (O3), nitrogen dioxide (NO2) and a suite of volatile organic compounds (VOCs) obtained during the dry season of 2018 and 2019 at the high altitude Maïdo observatory (21.1° S, 55.4° E, 2250 m above sea level), located on the remote island of La Réunion in the South-West Indian Ocean (SWIO). Biomass burning plume episodes were identified from increased acetonitrile (CH3CN) mixing ratios. Enhancement ratios (EnRs) – relative to CO – were calculated from in situ measurements for CH3CN, acetone (CH3COCH3), formic acid (HCOOH), acetic acid (CH3COOH), benzene (C6H6), methanol (CH3OH) and O3. We compared the EnRs to emission ratios (ERs) – relative to CO – reported in literature in order to estimate loss/production of these compounds during transport. For CH3CN and CH3COOH, the calculated EnRs are similar to the ERs. For C6H6 and CH3OH, the EnR is lower than the ER, indicating a significant net sinks of these compounds. For CH3COCH3 and HCOOH, the calculated EnRs are larger than the ERs. The discrepancy reaches an order of magnitude for HCOOH (18–34 pptv ppbv−1 compared to 1.8–4.5 pptv ppbv−1). This points to significant secondary production of HCOOH during transport. The Copernicus Atmospheric Monitoring Service (CAMS) global model simulations reproduces well the temporal variation of CO mixing ratios at the observatory but underestimates O3 and NO2 mixing ratios in the plumes on average by 16 ppbv and 60 pptv respectively. This discrepancy between modelled and measured O3 mixing ratios was attributed to (i) large uncertainties in VOC and NOx (NO + NO2) emissions due to BB in CAMS and (ii) misrepresentation of NOx recycling in the model during transport. Finally, transport of pyrogenically emitted CO is calculated with FLEXPART in order to (i) determine the mean plume age during the intrusions at the observatory and (ii) estimate the impact of BB on the pristine marine boundary layer (MBL). By multiplying the excess CO in the MBL with inferred EnRs at the observatory, we calculated the expected impact of BB on CH3CN, CH3COCH3, CH3OH and C6H6 concentrations in the MBL. These excesses constitute increases of ~ 20 %–150 % compared to background measurements in the SWIO MBL reported in literature.

Bert Verreyken et al.

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Short summary
Biomass burning (BB) plumes arriving at the Maïdo observatory located in the South-West Indian Ocean during August 2018 and August 2019 are studied using trace gas measurements, Lagrangian transport models and the CAMS near-real-time atmospheric composition service. We investigate (i) secondary production of volatile organic compounds during transport, (ii) efficacy of the CAMS model to reproduce the chemical makeup of BB plumes and (iii) the impact of BB on the remote marine boundary layer.
Biomass burning (BB) plumes arriving at the Maïdo observatory located in the South-West Indian...
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