ИСТИНА

Plumes from wildfires may be transported over large distances from remote to populated areas or threaten sensitive ecosystems. During atmospheric transport, plumes are processed by atmospheric oxidants and complex multiphase chemistry. At high plume concentrations, ageing of biomass burning (BB) aerosol differ from its atmospheric fate at typical ambient levels (Decker et al 2021). The Large Aerosol Chamber for photochemical ageing (PHOTO-LAC) with a volume of 1,800 m3 was used to investigate the chemical composition of fresh and aged dense biomass burning aerosols. The PHOTO-LAC contains two ovens enabling the generation of aerosols under flaming and smouldering conditions. (Popovicheva et al, 2016). Equivalent particulate matter (ePM) and reactive gases O3 and NOx, were monitored by a multiangle / multi-wavelength nephelometer and gas analysers, respectively. Filter samples were collected on quartz fibre filter and analysed by Fourier-transform Ion Cyclotron Resonance Mass Spectrometry (FT-ICR MS). In the following, we describe the effect of alternating photochemical and dark plume ageing of pine flaming and smouldering aerosol, resembling day- and night-time conditions over a total experimental duration of 30 h. After conversion of NO into NO2 by O3 addition, UV lights were turned for 7 h, turned off for 13 h and turned on again for 10 h. In both flaming and smouldering of 300 g pine mixed with 100 g forest debris, ePM decreased by ~15% during photochemical, but reincreased during dark ageing, exceeding initial concentrations. Eventually, consecutive photochemical ageing increased ePM further to a net increase in ePM of 15% for flaming and 35% for smouldering (Figure 1). The chemical composition of the flaming and smouldering aerosols changed during the entire ageing experiment by increasing elemental O:C and organic matter to organic carbon (OM/OC) ratio as well as decreasing aromaticity. Interestingly, OM/OC increased when ePM declined during first photochemical ageing, hence fragmentation reactions play a significant role. However, secondary PM formation and increasing Ocontent, both well-known for ageing of BB aerosol, are apparently reduced compared to smog chamber experiments at levels closer to ambient PM (Tiitta et al, 2016), emphasising the need of laboratory plume ageing studies for a better understanding of the atmospheric fate of wildfire plumes