Investigation of the light absorption properties of wood combustion particles using the extended wavelength range of the new AE36s Aethalometer

Particulate matter from wood combustion has a significant impact on climate and human health. Black carbon (BC) and brown carbon (BrC) are strong light absorbers that reduce the transparency of the atmosphere in the long and short wavelength range. As the aerosol ages, some of the molecular chromophores break down and the particles become less absorbent (photobleaching). On the other hand, photochemical ageing can modify organic molecules, which become light-absorbing even though they were colorless after emission. In addition, the secondary formed particles increase the light absorption and scattering of the pollution emitted by wood burning. To better estimate the contribution of particles emitted from wood combustion to the global radiation balance, it is therefore necessary to understand the optical properties of secondary particles and the changes in optical properties with ageing of primary aerosols. Limited information is available, in particular on the light absorption in the deep UV region where BrC is expected to have significant absorption. To extend the spectral information on the aerosol light absorption, the aerosol sample from wood combustion was measured with the new Aethalometer model (AE36s, Aerosol Magee Scientific), which has a spectral resolution of nine wavelengths in the 340-950 nm interval. The measurements were performed in the 200 m3 simulation chamber of the CEAM-EUPHORE research center in Valencia, Spain. Flaming and smoldering burning conditions were separately investigated and compared to diesel emission. After emission, the combustion products were introduced into the chamber, where the particles were subjected to different types of ageing (photooxidation, dark ageing). In addition to the light absorption, light scattering of the particles was measured by a nephelometer (Aurora 3000, ACOEM). The OC/BC ratio was also measured using the real-time Carbonaceous Aerosol Speciation System (CASS, Aerosol Magee Scientific). Particle formation and growth dynamics were monitored by SMPS (TSI) measurement. The optical properties of the primary emitted particles can be related to the combustion mode. Diesel emission resulted in the most absorbing aerosol over the whole wavelength range, while particles emitted from smoldering wood burning were the least absorbing, except in the UV range where BrC has high absorbance. During the photooxidation period, significant changes in the optical properties of the aerosol were observed. The absorbance of particles from smoldering emission increased significantly after the light exposure in the 470-660 nm wavelength interval. Later the absorbance decreased due to the photobleaching effect. During the photooxidation period, secondary organic aerosol formation was observed. The increase in absorbance was found to be lower than the increase in mass of the newly formed particles, while the increase in light scattering exceeded the increase in particle mass. These results suggest that the secondary aerosol was mostly transparent or had lower mass absorption efficiency but a higher scattering cross section compared to the primary emission. Therefore, the consideration of secondary formation and aging is crucial for a better understanding of the climate impact of wood combustion aerosol.