Volatility Resolved Optical Properties of Biomass Burning Aerosols from Simulated Wildland Fires in the G-WISE-2 Campaign

RYAN POLAND, John Allen, Muhammad Abdurrahman, Mike Lesiak, Michael Caraway, Thomas Carroll, Ariana Deegan, Mac A. Callaham, Joseph O'Brien, Rawad Saleh, Geoffrey Smith, University of Georgia

     Abstract Number: 184
     Working Group: Carbonaceous Aerosols

Abstract
Wildland fires represent a significant source of refractive carbonaceous aerosols, emitted on a spectrum of external mixing between brown carbon (BrC) and black carbon (BC). Two variables have been shown to dictate the relevant contributions of BrC and BC for wildland fires: (1) The moisture content of the fuels, existing between dry (~1.5%) (wildfires) and wet (5-18%) (prescribed fires) and (2) the inclusion of organic-rich duff layers in the fuel bed (McQueen et al. 2024; Zhang et al. 2021). The Georgia WIldland-fire Simulation Experiment 2 (G-WISE-2) campaign expands upon these findings by probing the effects that the dry composition, moisture content, density, and compactness of constructed fuel beds have on the chemical and optical properties of emitted aerosols (Saleh and O’Brien 2025). In the work presented here, custom-built photoacoustic (PAS) and cavity ring-down (CRD) spectrometers were operated downstream of parallel thermodenuders at 100ºC and 300ºC to quantify the intensive optical properties (namely the absorption Angstrom exponent (AAE) and single-scatter albedo (SSA)) of biomass burning aerosols from G-WISE-2 experiments in different “volatility regimes”. For dry fuel beds which exclude the duff layer, we observe an AAE of ~1 and SSA of ~ 0.3 after thermodenuding at 300ºC, indicating a remaining population consisting predominantly of BC. However, for burns where duff is present in the fuel bed, AAE values remain as high as 3 following thermodenuding at 300ºC, suggesting the presence of extremely low volatility BrC and BC coatings. We investigate whether the emission of these low volatility absorbers is a stronger function of combustion efficiency or specific fuel bed composition.