Effect of combustion conditions on aerosol particle emissions from savanna and grassland fires

Fire is an integral part of savanna and grassland biomes and globally approximately half of landscape fire emissions originate from savannas and grasslands. Emissions of trace gases and aerosol particles from landscape fires are characterised by emission factors (EFs), which denote the amount of emitted substance per mass of combusted biomass. EFs vary depending on both the biomass that is consumed in the fire and the combustion characteristics of the fire, i.e. the ratio of flaming to smouldering combustion. However, emission inventories tend to use only one average EF for each biome. Here, we use a set of 27 laboratory experiments to characterise the effect of combustion characteristics on submicron aerosol EFs from savanna and grassland biomass acquired from South Africa as well as boreal forest floor samples from Finland. Combustion experiments were carried out at the ILMARI facility in Kuopio, Finland from May to June 2022 under an open stack mimicking natural burning and dilution. Sample was injected into a 29 m3 environmental chamber for ageing studies. Chemical and physical properties of both fresh and aged smoke were observed with a host of instruments including e.g. AMS, FIGAERO-CIMS, VOCUS, SP2 and SMPS. The ratio of flaming to smouldering combustion was characterised by modified combustion efficiency (MCE), i.e. CO2/(CO2+CO). The increase of organic aerosol EF with increasing smouldering fraction (i.e. decreasing MCE) was very similar for both the grassland and savanna combustion experiments. Surprisingly, also the boreal forest floor EFs closely follow the same trend, where smouldering-dominated combustion EFs are more than 10 times higher than EFs for flaming combustion. We observed also that the submicron aerosol particle size distribution shifts towards larger sized particles with increasing smouldering fraction. Furthermore, both the number and the mass of the size distribution cannot be fully characterised with a single log-normal size distribution, which needs to be considered when converting mass emissions into number size distribution in simulations.