Atmospheric Degradation and Climate and Air-Quality Impact of Furan-based Biomass Burning Emission Products: A Kinetic and Mechanistic study

Biomass burning emissions, domestic- and wild-fires, agricultural burning, and fuel use, emit a blend of gases and particles with adverse effects on humans-health, climate and air quality. Furans are heterocyclic organic compounds (OVOC) that have been recently identified as important biomass burning emission-products. It is estimated that furan (C₄H₄O), 2-methylfuran (C₅H₆O), 2-furaldehyde (C₅H₄O₂) and benzofuran (C₈H₆O) emission levels are 70 to 120 times higher compared to CO. Once furans are emitted in the atmosphere, they will undergo gas phase chemistry and, to an extent, they will be photolyzed at actinic wavelengths. OH and NO₃ radicals, Cl atoms and O₃ chemistry might result in tropospheric O₃ and in secondary organic aerosols (SOA) formation, which might be enhanced due to their potent low volatility. Therefore, it is essential to investigate the kinetics and the mechanism of all the photochemically induced degradation pathways and identify and quantify SOA precursors, so as to evaluate their impact on Air-Quality and Climate.

 

Within this framework, a thorough laboratory study, using two complementary techniques has been carried out. First, major atmospheric oxidants reaction rate coefficients with furans were determined. Secondly, the degradation mechanisms were investigated from both kinetic and conversion-yields perspectives. A Teflon atmospheric simulation chamber, named THALAMOS (THermALly regulated AtMOSpheric simulation chamber), was used to study the reactions at atmospheric pressure. State-of-the-art in-line instrumentation, e.g., FTIR spectroscopy and Chemical ionization mass spectrometry, were used for the real-time monitoring of reactants and products. To further our understanding, the reactions rate coefficients were also measured at 2 mTorr, between 253 and 363 K, with the continuous flow technique of the Very Low Pressure Reactor, in which an effusive molecular beam is analyzed with Quadrupole Mass Spectrometry (VLPR/QMS). Intercomparing the results from the two techniques reactions mechanistic-scheme was mapped-out and their impact was evaluated.

 

OH and NO₃ radicals and Cl atoms reactions with all the furans were measured to be in the order of 10^-11, 10^-10 and 10^-12 cm³ molecule^-1 s^-1, respectively, leading to atmospheric-lifetimes between 2 and 10 hours. Temperature and pressure dependent kinetic measurements revealed association as the dominant reaction channel. However, experiments at very-low-pressure regime showed that HCl elimination cannot be excluded, especially when the furan-ring aromaticity is not breaking.

 

Finally, it is evident that furans degradation will occur at low altitudes and SOA precursors, i.e., end-oxidation products will be formed nearby their emission locations. Further, kinetics studies were used to study the structure-reactivity trend of furans and to estimate their Photochemical Ozone Creation Potential (POCP). Results from this study are expected to significantly improve our insight on furans tropospheric photochemistry and via identifying and quantifying end-products and SOA formation, to assess their indirect and direct impact, on Climate, Air-Quality and humans-health.