African biomass burning plumes detected at La Réunion

Chemical composition and climate
Biomass burning is a major source of volatile organic compounds (VOCs), key ingredients in the formation of surface level ozone and secondary organic aerosols. Characterisation of sources and sinks of these VOCs in the atmosphere supports research on both climate change and air quality. Researchers at BIRA-IASB used VOC measurements at the high-altitude Maïdo observatory at La Réunion, a remote island in the Indian Ocean, to search for clues on missing sources of VOCs in biomass burning plumes from Africa.

In the framework of the OCTAVE project, a near-continuous high time-resolution dataset of VOCs was realised between October 2017 and November 2019 at the high-altitude Maïdo observatory on Réunion Island. This dataset is used to better understand the sources and sinks of VOCs in the tropical marine atmosphere.

Biomass burning plumes reach the remote site

African and Madagascan burning sources can impact the local atmospheric composition at La Réunion. At times, these plumes are visible to the human eye as a brown haze around the island and they are reported in local media:

  • Un voile brumeux plane sur La Réunion - Imaz Press Réunion (8 August 2019)
  • Incendies en Afrique : les Hauts de La Réunion particulièrement impactés - Imaz Press Réunion (9 August 2019)

Researchers at BIRA-IASB have selected six such events taking place in August 2018 and August 2019 to characterise biomass burning plumes and their impact on the atmosphere over the South-West Indian Ocean.

Identifying gaps in current knowledge

VOC data were combined with routine measurements of carbon monoxide (CO), nitrogen oxides (NOx = NO + NO2), and tropospheric ozone (O3) to look for gaps in current knowledge. We were able to identify two clues on missing sources:

  1. Recorded concentrations of formic acid were about 5 to 10 times higher compared to what was expected from the literature. This indicates that formic acid is produced inside the biomass burning plumes during transport, but the formation mechanism is yet unknown.
  2. The Copernicus near-real-time atmospheric composition service (CAMS) underestimates the concentrations of O3, especially in young plumes. This is linked to an underestimation of NOx concentrations, the second key ingredient in O3 formation near the surface, for which the reason is not yet identified.

Transport of biomass burning plumes

Employing atmospheric transport models, we were able to show that African or Madagascan biomass burning plumes are transported at high altitudes in the atmosphere above the South-West Indian Ocean, reaching Maïdo observatory in about 7 to 14 days.  

The concentrations of acetonitrile, acetone, benzene and methanol are estimated to increase by a factor of 1.25 to 2 within these biomass burning plumes compared to the otherwise pristine atmosphere over the ocean surface. This shows the potential of using simultaneous measurements over the ocean (using ship-borne instruments) and at the Maïdo observatory to better estimate ocean-atmosphere interactions. This is especially of interest for species such as acetone and methanol for which the role of the ocean is still uncertain.

 

References

Verreyken, B., Amelynck, C., Brioude, J., Müller, J.-F., Schoon, N., Kumps, N., Colomb, A., Metzger, J.-M., Lee, C.F., Koenig, T.K., Volkamer, R., and Stavrakou, T. (2020). Characterisation of African biomass burning plumes and impacts on the atmospheric composition over the south-west Indian Ocean, Atmospheric Chemistry and Physics, 20(23), 14821-14845. https://doi.org/10.5194/acp-20-14821-2020 Open Access Logo

Comparison between measured and modelled CO, O3 and NO2
Comparison between measured (black dots) and modelled (colour lines) CO (a), O3 (b), and NO2 (c). The red hatched area corresponds to the periods of biomass burning plumes reaching the observatory. The coloured lines relate to different vertical levels from the model, where the green line can be used as reference for the altitude of the Maïdo observatory (800 mbar). It is clear that the model reproduces CO concentrations well. However, the model systematically underestimates both NO2 and O3, especially in the first two (younger) plumes. Figure from Verreyken et al. (2020).
Biomass burning plumes recorded at the Maïdo observatory
From the biomass burning plumes recorded at the Maïdo observatory (black cross on the figure), we found that CO concentrations are directly related (linear relation) to the concentrations of acetonitrile (CH3CN), methanol (CH3OH), acetone (CH3COCH3), and benzene (C6H6). Hence, if we can calculate the concentration and excess of CO due to biomass burning in Africa and Madagascar, we can estimate the concentration and excess of these other compounds. Figure from Verreyken et al. (2020).