Laboratory Studies to Investigate the Role of Volatile and Semi-Volatile Organic Compounds in the Atmospheric Aging of Biomass Devolatilization Products

Although biomass burning organic aerosol (BBOA) contributes a significant fraction of atmospheric organic aerosol (OA), the temporal variation in chemical composition due to oxidation has been difficult to characterize. Volatile, intermediately-volatile, and semi-volatile organic compounds (VOCs, IVOCs, and SVOCs, respectively) are co-emitted with primary OA (POA) during biomass combustion and participate in the formation of secondary organic aerosol (SOA) as biomass burning plumes age in the atmosphere. However, the chemical reaction pathways and products associated with S/I/VOC oxidation and subsequent SOA formation remain poorly understood. In this study, the role of S/I/VOCs in the atmospheric photochemical aging of biomass emissions was investigated using a Potential Aerosol Mass (PAM) flow reactor in conjunction with a Thermal desorption Aerosol Gas chromatograph (TAG), a Proton Transfer Reaction Mass Spectrometer (PTR-MS), and a high resolution time-of-flight Aerosol Mass Spectrometer (AMS).

Primary emission products were created by devolatilizing oak wood and oak leaf samples in a custom-built emissions chamber. The biomass samples were resistively heated in the chamber to 200°C, the consistent heat pulse providing emissions that are more reproducible at the molecular level as compared with direct biomass combustion techniques. The primary material was subsequently oxidized in the PAM reactor to simulate different levels of atmospheric aging. PAM oxidation experiments were conducted on primary particles alone, with the vapor fraction of the emission product removed with a denuder prior to entering the PAM, and separately on the gas-phase material following the removal of particles with a Teflon filter. The TAG provided chemical speciation data for the oxidized aerosol, allowing for molecular level identification at each level of oxidation. Additionally, the TAG measured the thermal decomposition fragments of the most thermally labile aerosol components. The AMS was used to obtain high-resolution mass spectra and oxidation state information for the bulk aerosol, and the PTR-MS was used to measure concentrations of different gas-phase species. A comparison of the chemical properties of filtered, denuded, and unfiltered/undenuded biomass emission oxidation products will be presented in order to demonstrate the impact of S/I/VOCs on the chemical evolution of BBOA in the atmosphere. Specifically, the oxidative evolution of key molecular marker compounds will be shown in order to elucidate the chemical reaction pathways associated with SOA formation and aging of BBOA.