Compositional changes of biomass burning aerosols during ageing in the Photochemical Large Aerosol Chamber (PHOTO-LAC).

Abstract

Wildfire events release vast amounts of gases and particulate matter (PM) containing polyaromatic compounds (PACs) including polycyclic aromatic hydrocarbons (PAHs) and their oxygenated derivatives (Oxy-PAHs). During the atmospheric transport of dense biomass burning (BB) plumes, photochemical oxidation and dark ageing alter the fresh plume composition significantly. In the Photochemical Large Aerosol Chamber (PHOTO-LAC), Siberian biomass-burning emissions from flaming and smouldering combustion were aged under alternating photochemical and dark conditions at concentrations representative of dense plumes. Particulate carbon and polycyclic aromatic compounds (PACs) were chemically speciated using targeted and untargeted analyses. With 27 g kg-1, smouldering BB generated higher fresh emissions of organic carbon (OC) than flaming (2.8 g kg-1), but less elemental carbon (EC; smouldering: 7.8 g kg-1; flaming: 21 g kg-1). During alternating photochemical and dark ageing, formation of secondary OC was observed for flaming, but decreasing OC for smouldering. PAHs and Oxy-PAHs in fresh emissions were substantially higher for smouldering and comparable within uncertainty for PAHs and Oxy-PAHs under each combustion condition (flaming: 21 mg kg-1 PAH and 17 mg kg-1 Oxy-PAH; smouldering: 430 mg kg-1 PAH and 230 mg kg-1 Oxy-PAH). Both PAHs and Oxy-PAHs declined predominantly during the first photochemical ageing, associated with changes in their molecular pattern and PAH diagnostic ratios, but remained constant during the following dark and second photochemical ageing. Finally, untargeted analysis of PACs revealed a large contribution of resin diterpenoids to BB emissions, particularly for smouldering. A new approach to determine the total resin content using thermal-optical carbon analysis photoionisation time-of-flight mass spectrometry (TOCA-PIMS) analysis of abietic acid was introduced, providing a marker for fresh BB emissions. Our exceptional laboratory-ageing experiments in the PHOTO-LAC provide insights into the changing composition of BB emissions at concentrations of dense plumes and alternating day- and nighttime chemistry with implications for the exposure of the population to transported wildfire emissions.

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