<p>We present an investigation of biomass burning (BB) plumes originating from Africa and Madagascar based on measurements of carbon monoxide (CO), ozone (O<sub>3</sub>), nitrogen dioxide (NO<sub>2</sub>) 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 (CH<sub>3</sub>CN) mixing ratios. Enhancement ratios (EnRs) – relative to CO – were calculated from in situ measurements for CH<sub>3</sub>CN, acetone (CH<sub>3</sub>COCH<sub>3</sub>), formic acid (HCOOH), acetic acid (CH<sub>3</sub>COOH), benzene (C<sub>6</sub>H<sub>6</sub>), methanol (CH<sub>3</sub>OH) and O<sub>3</sub>. 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 CH<sub>3</sub>CN and CH<sub>3</sub>COOH, the calculated EnRs are similar to the ERs. For C<sub>6</sub>H<sub>6</sub> and CH<sub>3</sub>OH, the EnR is lower than the ER, indicating a significant net sinks of these compounds. For CH<sub>3</sub>COCH<sub>3</sub> and HCOOH, the calculated EnRs are larger than the ERs. The discrepancy reaches an order of magnitude for HCOOH (18–34 pptv ppbv<sup>−1</sup> compared to 1.8–4.5 pptv ppbv<sup>−1</sup>). 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 O<sub>3</sub> and NO<sub>2</sub> mixing ratios in the plumes on average by 16 ppbv and 60 pptv respectively. This discrepancy between modelled and measured O<sub>3</sub> mixing ratios was attributed to (i) large uncertainties in VOC and NO<sub>x</sub> (NO + NO<sub>2</sub>) emissions due to BB in CAMS and (ii) misrepresentation of NO<sub>x</sub> 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 CH<sub>3</sub>CN, CH<sub>3</sub>COCH<sub>3</sub>, CH<sub>3</sub>OH and C<sub>6</sub>H<sub>6</sub> concentrations in the MBL. These excesses constitute increases of ~ 20 %–150 % compared to background measurements in the SWIO MBL reported in literature.</p>