Major secondary aerosol formation in southern African open biomass burning plumes

Open biomass burning contributes significantly to air quality degradation and associated human health impacts over large areas. It is one of the largest sources of reactive trace gases and fine particles to Earth's atmosphere and consequently a major source of cloud condensation nuclei on a global scale. However, there is a large uncertainty in the climate effect of open biomass burning aerosols due to the complexity of their constituents. Here, we present an exceptionally large dataset on southern African savannah and grassland fire plumes and their atmospheric evolution, based on 5.5 years of continuous measurements from 2010 to 2015. We find that the mass of submicrometre aerosols more than doubles on average, in only three hours of daytime ageing. We also evaluate biomass burning aerosol particle size distributions and find a large discrepancy between the observations and current model parameterizations, especially in the 30-100 nm range. We conclude that accounting for near-source secondary organic aerosol formation and using measurement-based size distribution parameterizations in smoke plumes is essential to better constrain the climate and air quality effects of savannah and grassland fires.