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

Abstract

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 O 2 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 ( 1 O 2 ) scavenger, it was shown that 1 O 2 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 O 2 was present but not for deoxygenated irradiation. Overall, the dependence on O 2 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.

Photolytic Mass Loss of Humic Substances Measured with a Quartz Crystal Microbalance

“…Previous studies have demonstrated that natural chromophoric dissolved organic matter (CDOM) possesses photosensitizing properties, enabling it to be promoted to a triplet state ( 3 CDOM*) after exposure to solar radiation in the aqueous phase or in the particle . The 3 CDOM* generated can subsequently participate in various photochemical processes, such as enhancing secondary organic aerosol (SOA) production through the adsorption of volatile organic compounds (VOCs), , promoting reactive oxygen species (ROS) production, − reduction of NO 2 to HONO, and promoting photoenhancement of biomass burning brown carbon during aqueous and condensed-phase photolysis . We hypothesize that the photosensitizing capability of humic substances may play a role in the present work as they are known constituents of CDOM.…”

“…Several studies have examined the effect that light exposure has on the absorption spectra of particulate matter representing various types of organic aerosols. For example, both photobleaching and photoenhancement have been reported for water-soluble brown carbon from wood smoke, ,,, ambient biomass burning brown carbon, brown carbon from burning of urban construction material, and 4-nitrophenols during aqueous photolysis experiments. While the extent to which photolysis modifies these spectra varies, some of them demonstrate the same general pattern that we have observed with SRFA, namely photobleaching at shorter UV wavelengths and photoenhancement at longer visible wavelengths. ,− Thus, in this respect SRFA appears to be a good surrogate for organic aerosols.…”