# Characterisation of African biomass burning plumes and impacts on the atmospheric composition over the south-west Indian Ocean

We present an investigation of biomass burning (BB) plumes originating from Africa and Madagascar based on measurements of a suite of volatile organic compounds (VOCs), carbon monoxide (CO), ozone (inline-formulaO3) and nitrogen dioxide (inline-formulaNO2) obtained during the dry season of 2018 and 2019 at the high-altitude Maïdo observatory (21.1inline-formula S, 55.4inline-formula E, 2160 inline-formula $M5inlinescrollmathmlunit\mathrm{normal m}\phantom{\rule{0ex}{0ex}}\mathrm{normal a}.\mathrm{normal s}.\mathrm{normal l}.$ 36pt10ptsvg-formulamathimg87ef89b07503433cab1be4ab41553010 acp-20-14821-2020-ie00001.svg36pt10ptacp-20-14821-2020-ie00001.png ), located on the remote island of La Réunion in the south-west Indian Ocean (SWIO). Biomass burning plume episodes were identified from increased acetonitrile (inline-formulaCH3CN) mixing ratios. Enhancement ratios (EnRs) – relative to CO – were calculated from in situ measurements for inline-formulaCH3CN, acetone (inline-formulaCH3COCH3), formic acid (HCOOH), acetic acid (inline-formulaCH3COOH), benzene (inline-formulaC6H6), methanol (inline-formulaCH3OH) and inline-formulaO3. We compared the EnRs to emission ratios (ERs) – relative to CO – reported in the literature in order to estimate loss or production of these compounds during transport. For inline-formulaCH3CN and inline-formulaCH3COOH, the calculated EnRs are similar to the ERs. For inline-formulaC6H6 and inline-formulaCH3OH, the EnR is lower than the ER, indicating a net sink of these compounds which was found to be in line with the expected atmospheric lifetime. For inline-formulaCH3COCH3 and HCOOH, the calculated EnRs are larger than the ERs. The discrepancy reaches an order of magnitude for HCOOH (18–34 inline-formulapptv ppbv−1 compared to 1.8–4.5 inline-formulapptv ppbv−1). This points to significant secondary production of HCOOH during transport. The Copernicus Atmospheric Monitoring Service (CAMS) global model simulations reproduce the temporal variation of CO mixing ratios well at the observatory but underestimate inline-formulaO3 and inline-formulaNO2 mixing ratios in the plumes by on average 16 inline-formulappbv and 60 inline-formulapptv respectively. This discrepancy between modelled and measured inline-formulaO3 mixing ratios was attributed to (i) large uncertainties in VOC and NOinline-formulax (inline-formulaNO+NO2) emissions due to BB in CAMS and (ii) misrepresentation of NOinline-formulax 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 inline-formulaCH3CN, inline-formulaCH3COCH3, inline-formulaCH3OH and inline-formulaC6H6 concentrations in the MBL. These excesses constitute increases of inline-formula∼20 %–150 % compared to background measurements in the SWIO MBL reported in the literature.

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Verreyken, Bert / Amelynck, Crist / Brioude, Jérôme / et al: Characterisation of African biomass burning plumes and impacts on the atmospheric composition over the south-west Indian Ocean. 2020. Copernicus Publications.

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