Combustion Conditions are the Major Drivers of the Variability in Organic Aerosol Chemical Composition and Secondary Organic Aerosol Formation in Emissions from Simulated Wildland Fires

Wildland-fire emissions exhibit large variability in aerosol physicochemical properties and potential for forming secondary organic aerosol (SOA). Whether this variability is primarily driven by differences in fuel type or combustion conditions remains unresolved. To address this gap, we performed simulated wildland-fire experiments using fuel beds that contained either surface fuels only or a duff layer underneath the surface fuels. We varied the dry mass loading and moisture content of the surface fuels within ranges encountered in prescribed fires in the Southeastern U.S. Less efficient combustion, which resulted from increased dry mass loading, increased moisture content, or including duff in the fuel bed, produced less light-absorbing organic aerosol (OA) and formed more SOA. Including duff also produced OA with higher nitrogen content. A proof-of-concept experiment using surface fuels conditioned to reproduce inefficient combustion conditions similar to those of duff emitted OA with nitrogen content, light-absorption properties, and SOA formation potential comparable to those of duff OA emissions. Given the profound differences in molecular composition between surface fuels and duff, these results provide strong evidence that the difference in combustion conditions between duff and surface fuels was the major driver of the observed differences in OA physicochemical properties and SOA formation potential.