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Abstract. We present an investigation of biomass burning (BB) plumes originating fromAfrica and Madagascar based on measurements of a suite of volatile organiccompounds (VOCs), carbon monoxide (CO), ozone ( O3 ) and nitrogendioxide ( NO2 ) obtained during the dry season of 2018 and 2019 at thehigh-altitude Maido observatory (21.1 ∘  S, 55.4 ∘  E,2160  m a . s . l . ), located on the remote island of La Reunion inthe south-west Indian Ocean (SWIO). Biomass burning plume episodes wereidentified from increased acetonitrile ( CH3CN ) mixingratios. Enhancement ratios (EnRs) – relative to CO – were calculated from insitu 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 the literature in order to estimateloss or 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 net sink ofthese compounds which was found to be in line with the expected atmosphericlifetime. For CH3COCH3 and HCOOH, the calculated EnRs are larger thanthe ERs. The discrepancy reaches an order of magnitude for HCOOH(18–34  pptv ppbv−1 compared to1.8–4.5  pptv ppbv−1 ). This points to significant secondaryproduction of HCOOH during transport. The Copernicus Atmospheric MonitoringService (CAMS) global model simulations reproduce the temporal variationof CO mixing ratios well at the observatory but underestimate O3 and NO2 mixing ratios in the plumes by on average 16  ppbv and60  pptv respectively. This discrepancy between modelled and measured O3 mixing ratios was attributed to (i) large uncertainties in VOC andNO x ( NO+NO2 ) emissions due to BB in CAMS and (ii)misrepresentation of NO x recycling in the model duringtransport. Finally, transport of pyrogenically emitted CO is calculated withFLEXPART in order to (i) determine the mean plume age during the intrusions atthe observatory and (ii) estimate the impact of BB on the pristine marineboundary layer (MBL). By multiplying the excess CO in the MBL with inferredEnRs 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 SWIOMBL reported in the literature.

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