Light Exposure of Wood Smoke Aerosol: Connecting Optical Properties, Oxidation, Radical Formation, and Chemical Composition

Biomass burning brown carbon (BrC) aerosol affects the climate by absorbing light in the visible and UV wavelengths, with daytime photochemistry changing its optical properties. Here, aging by UVB irradiation of BrC in both the aqueous and particle states was conducted to evaluate the conditions and mechanism required to change the ability of BrC to absorb light with an emphasis on the role of radicals and oxidation. Photoreactions enhanced visible light absorption with O₂ present in both the aqueous phase and particulate states. Deoxygenated samples did not show this enhancement, indicating the importance of oxidation in this aging mechanism. Using furfuryl alcohol as a singlet oxygen (¹O₂) scavenger, it was shown that ¹O₂ contributes to roughly one third of the visible absorption increase in the aqueous phase. Conversely, hydroxyl radicals (OH) were demonstrated to have no impact. Carbon-centered radicals, as measured by electron paramagnetic resonance in the particle state, were formed when O₂ was present but not for deoxygenated irradiation. Overall, the dependence on O₂ indicates that highly viscous BrC particles, as will be present in the atmosphere at low temperatures and/or low relative humidity, may not experience the same degree of photochemical change as more liquid-like particles.