Effects of Atmospheric Processing on the Oxidative Potential of Biomass Burning Organic Aerosols

Oxidative potential (OP), which is the ability of certain components in atmospheric particles to generate reactive oxidative species (ROS) and deplete antioxidants in vivo, is a prevailing toxicological mechanism underlying the adverse health effects associated with exposure to ambient aerosols. While previous studies have identified the high OP of fresh biomass burning organic aerosols (BBOA), it remains unclear how it evolves throughout atmospheric transport. Using the dithiothreitol (DTT) assay as a measure of OP, a combination of field observations and laboratory experiments is used to determine how atmospheric aging transforms the intrinsic OP (OPmassDTT) of BBOA. For ambient BBOA collected during the fire seasons in Greece, OPmassDTT was observed to increase by a factor of 2.1 ± 0.9 for samples of atmospheric ages up to 68 h. Laboratory experiments indicate that aqueous photochemical aging (aging by UVB and UVA photolysis; as well as OH oxidation), as well as aging by ozone and atmospheric dilution can transform the OPmassDTT of the water-soluble fraction of wood smoke within 2 days of atmospheric transport. The results from this work suggest that the air quality impacts of biomass burning emissions can extend beyond regions near fire sites and should be accounted for.

Athanasios Nenes, Kalliopi Violaki

Oxidative potential (OP), which is the ability of certain components in atmospheric particles to generate reactive oxidative species (ROS) and deplete antioxidants in vivo, is a prevailing toxicological mechanism underlying the adverse health effects associated with exposure to ambient aerosols. While previous studies have identified the high OP of fresh biomass burning organic aerosols (BBOA), it remains unclear how it evolves throughout atmospheric transport. Using the dithiothreitol (DTT) assay as a measure of OP, a combination of field observations and laboratory experiments is used to determine how atmospheric aging transforms the intrinsic OP (OPmassDTT) of BBOA. For ambient BBOA collected during the fire seasons in Greece, OPmassDTT was observed to increase by a factor of 2.1 +/- 0.9 for samples of atmospheric ages up to 68 h. Laboratory experiments indicate that aqueous photochemical aging (aging by UVB and UVA photolysis; as well as 01-1 oxidation), as well as aging by ozone and atmospheric dilution can transform the OPmassDTT of the water-soluble fraction of wood smoke within 2 days of atmospheric transport. The results from this work suggest that the air quality impacts of biomass burning emissions can extend beyond regions near fire sites and should be accounted for.

AMER CHEMICAL SOC2019

Atmospheric evolution of molecular-weight-separated brown carbon from biomass burning

Athanasios Nenes, Kalliopi Violaki

Biomass burning is a major source of atmospheric brown carbon (BrC), and through its absorption of UV/VIS radiation, it can play an important role in the planetary radiative balance and atmospheric photochemistry. The considerable uncertainty of BrC impacts is associated with its poorly constrained sources, transformations, and atmospheric lifetime. Here we report laboratory experiments that examined changes in the optical properties of the water-soluble (WS) BrC fraction of laboratory-generated biomass burning particles from hardwood pyrolysis. Effects of direct UVB photolysis and OH oxidation in the aqueous phase on molecular-weight-separated BrC were studied. Results indicated that the majority of low-molecular-weight (MW) BrC ( < 400 Da) was rapidly photobleached by both direct photolysis and OH oxidation on an atmospheric timescale of approximately 1 h. High MW BrC ( >= 400 Da) underwent initial photoenhancement up to similar to 15 h, followed by slow photobleaching over similar to 10 h. The laboratory experiments were supported by observations from ambient BrC samples that were collected during the fire seasons in Greece. These samples, containing freshly emitted to aged biomass burning aerosol, were analyzed for both water- and methanol-soluble BrC. Consistent with the laboratory experiments, high-MW BrC dominated the total light absorption at 365 nm for both methanol and water-soluble fractions of ambient samples with atmospheric transport times of 1 to 68 h. These ambient observations indicate that overall, biomass burning BrC across all molecular weights has an atmospheric lifetime of 15 to 28 h, consistent with estimates from previous field studies - although the BrC associated with the high-MW fraction remains relatively stable and is responsible for light absorption properties of BrC throughout most of its atmospheric lifetime. For ambient samples of aged (> 10 h) biomass burning emissions, poor linear correlations were found between light absorptivity and levoglucosan, consistent with other studies suggesting a short atmospheric lifetime for levoglucosan. However, a much stronger correlation between light absorptivity and total hydrous sugars was observed, suggesting that they may serve as more robust tracers for aged biomass burning emissions. Overall, the results from this study suggest that robust model estimates of BrC radiative impacts require consideration of the atmospheric aging of BrC and the stability of high-MW BrC.

COPERNICUS GESELLSCHAFT MBH2019

Atmospheric evolution of molecular-weight-separated brown carbon from biomass burning

Athanasios Nenes, Kalliopi Violaki

Atmospheric evolution of molecular-weight-separated brown carbon from biomass burning

Athanasios Nenes, Kalliopi Violaki

COPERNICUS GESELLSCHAFT MBH2019