Properties of Surface-Active Organics in Aerosol Particles Produced from Combustion of Biomass Fuels under Simulated Prescribed-Fire and Wildfire Conditions

Surface-active organics (surfactants) have previously been measured in atmospheric aerosol particles to quantify their ability to reduce particle surface tension and influence the indirect effects of aerosol particles on the climate. However, surfactants have not been extensively studied in biomass burning aerosols (BBA), which could result in an underestimation when quantifying the contribution of surface tension on aerosol-cloud interactions. The interfacial properties of the surface-active organic fraction of BBA, such as the critical micelle concentration (CMC) and surfactant composition, may vary based on the origin and moisture content of the fuel and the resulting combustion conditions. Surfactant concentration and chemical composition, the contribution of surfactants to total particle mass, bulk surface tension minimums, and CMC values of surface-active organics extracted from fresh and aged BBA produced using fuel beds from three unique Georgia ecoregions (Piedmont, Coastal Plain, and Blue Ridge) and with fuel moisture contents representative of prescribed fires or drought-induced wildfires were measured using high resolution mass spectrometry, UV–vis spectroscopy, and pendant drop tensiometry. Aerosol particles were collected onto filters during the Georgia Wildland Fire Simulation Experiment (GWISE) at the U.S. Forest Service Southern Research Combustion Facility in Athens, GA. Surface tension minimums of surface-active organics extracted from all BBA were low (<45 mN m^–1), and surfactants were ∼2% of the total particle mass. The total mass of the surfactant fraction showed a strong dependence on combustion conditions, with the highest fractions present in BBA produced from the most efficient (highest temperature) combustion. Additionally, aging of BBA using a potential aerosol mass oxidative flow reactor resulted in an increase in the surfactant fractions of total BBA mass. The dependence of the surfactant mass fraction on combustion conditions may have implications for the microphysics of BBA from wildland fires. Additionally, the chemical composition of BBA was measured online during GWISE-2 under additional conditions to further investigate potential changes in combustion conditions and BBA emissions. Preliminary analysis shows that the chemical composition of BBA is dependent on certain experimental conditions such as mass loading of the fuel, wind speed, compactness of duff, and inclusion of structural material, highlighting the importance of combustion conditions on the chemical properties of BBA.