Characterising the biomass burning emissions using Sentinel-5P and reanalysis data: Towards the development of a novel biomass burning air quality index for South Africa

Biomass burning is a major source of atmospheric pollutants and a critical driver of air quality degradation, particularly in fire-prone regions with limited ground-based monitoring infrastructure. This study characterises biomass burning emissions across the Cape Peninsula, South Africa, during the December 2023 wildfire event, using data from the Sentinel-5P TROPOMI instrument and the MERRA-2 reanalysis dataset. Five pollutants were analysed across three defined periods: pre-fire (1–18 December), active fire (19–22 December), and post-fire (23–31 December). These include carbon monoxide (CO), sulphur dioxide (SO₂), organic carbon (OC), and particulate matter (PM_2.5) before, during, and after a fire event in December 2023. The findings revealed that pollutant levels increased during the fire, with very high concentrations of CO (exceeding 4000 µg/m³), PM_2.5 reaching 18.75 µg/m³, SO₂ peaking at 94.13 µg/m³, OC at 8.14 µg/m³, and BC at 0.659 µg/m³. The composite biomass burning Air Quality Index (AQI) deteriorated to ‘Very Unhealthy’ categories across large portions of the Cape Peninsula during the active fire period, before partially recovering to ‘Good’ in the post-fire period, though it did not return to pre-fire baseline levels. Meteorological analysis showed that reduced precipitation (0.33–0.66 mm/day), elevated temperatures (19.35–19.88 °C), and shifting wind vectors during the active fire period created conditions conducive to fire ignition, smoke persistence, and lateral plume dispersion. Post-fire increases in precipitation and humidity contributed to partial but incomplete recovery of air quality. These findings demonstrate the value of integrating multi-source satellite and reanalysis data for comprehensive wildfire emission characterisation, and support the development of a spatially explicit, multi-pollutant biomass burning AQI framework applicable to fire-prone regions of South Africa.