Emerging investigator series: heterogeneous OH oxidation of primary brown carbon aerosol: effects of relative humidity and volatility

Brown Carbon from Photo-Oxidation of Glyoxal and SO2 in Aqueous Aerosol

Aqueous-phase dark reactions during the co-oxidation of glyoxal and S(IV) were recently identified as a potential source of brown carbon (BrC). Here, we explore the effects of sunlight and oxidants on aqueous solutions of glyoxal and S(IV), and on aqueous aerosol exposed to glyoxal and SO₂. We find that BrC is able to form in sunlit, bulk-phase, sulfite-containing solutions, albeit more slowly than in the dark. In more atmospherically relevant chamber experiments where suspended aqueous aerosol particles are exposed to gas-phase glyoxal and SO₂, the formation of detectable amounts of BrC requires an OH radical source and occurs most rapidly after a cloud event. From these observations we infer that this photobrowning is caused by radical-initiated reactions as evaporation concentrates aqueous-phase reactants and aerosol viscosity increases. Positive-mode electrospray ionization mass spectrometric analysis of aerosol-phase products reveals a large number of C_xH_yO_z oligomers that are reduced rather than oxidized (relative to glyoxal), with the degree of reduction increasing in the presence of OH radicals. This again suggests a radical-initiated redox mechanism where photolytically produced aqueous radical species trigger S(IV)-O₂ auto-oxidation chain reactions, and glyoxal-S(IV) redox reactions especially if aerosol-phase O₂ is depleted. This process may contribute to daytime BrC production and aqueous-phase sulfur oxidation in the atmosphere. The BrC produced, however, is about an order of magnitude less light-absorbing than wood smoke BrC at 365 nm.

Dynamic Wood Smoke Aerosol Toxicity during Oxidative Atmospheric Aging

Wildfires are a major source of biomass burning aerosol to the atmosphere, with their incidence and intensity expected to increase in a warmer future climate. However, the toxicity evolution of biomass burning organic aerosol (BBOA) during atmospheric aging remains poorly understood. In this study, we report a unique set of chemical and toxicological metrics of BBOA from pine wood smoldering during multiphase aging by gas-phase hydroxyl radicals (OH). Both the fresh and OH-aged BBOA show activity relevant to adverse health outcomes. The results from two acellular assays (DTT and DCFH) show significant oxidative potential (OP) and reactive oxygen species (ROS) formation in OH-aged BBOA. Also, radical concentrations in the aerosol assessed by electron paramagnetic resonance (EPR) spectroscopy increased by 50% following heterogeneous aging. This enhancement was accompanied by a transition from predominantly carbon-centered radicals (85%) in the fresh aerosol to predominantly oxygen-centered radicals (76%) following aging. Both the fresh and aged biomass burning aerosols trigger prominent antioxidant defense during the in vitro exposure, indicating the induction of oxidative stress by BBOA in the atmosphere. By connecting chemical composition and toxicity using an integrated approach, we show that short-term aging initiated by OH radicals can produce biomass burning particles with a higher particle-bound ROS generation capacity, which are therefore a more relevant exposure hazard for residents in large population centers close to wildfire regions than previously studied fresh biomass burning emissions.

Connecting the Light Absorption of Atmospheric Organic Aerosols with Oxidation State and Polarity

The light-absorbing organic aerosol (OA) constitutes an important fraction of absorbing components, counteracting major cooling effect of aerosols to climate. The mechanisms in linking the complex and changeable chemistry of OA with its absorbing properties remain to be elucidated. Here, by using solvent extraction, ambient OA from an urban environment was fractionated according to polarity, which was further nebulized and online characterized with compositions and absorbing properties. Water extracted high-polar compounds with a significantly higher oxygen to carbon ratio (O/C) than methanol extracts. A transition O/C of about 0.6 was found, below and above which the enhancement and reduction of OA absorptivity were observed with increasing O/C, occurring on the less polar and high polar compounds, respectively. In particular, the co-increase of nitrogen and oxygen elements suggests the important role of nitrogen-containing functional groups in enhancing the absorptivity of the less polar compounds (e.g., forming nitrogen-containing aromatics), while further oxidation (O/C > 0.6) on high-polar compounds likely led to fragmentation and bleaching chromophores. The results here may reconcile the previous observations about darkening or whitening chromophores of brown carbon, and the parametrization of O/C has the potential to link the changing chemistry of OA with its polarity and absorbing properties.

Brown Carbon in Primary and Aged Coal Combustion Emission

Smog chamber experiments were conducted to characterize the light absorption of brown carbon (BrC) from primary and photochemically aged coal combustion emissions. Light absorption was measured by the UV-visible spectrophotometric analysis of water and methanol extracts of filter samples. The single-scattering albedo at 450 nm was 0.73 ± 0.10 for primary emissions and 0.75 ± 0.13 for aged emissions. The light absorption coefficient at 365 nm of methanol extracts was higher than that of water extracts by a factor of 10 for primary emissions and a factor of 7 for aged emissions. This suggests that the majority of BrC is water-insoluble even after aging. The mass absorption efficiency of this BrC (MAE₃₆₅) for primary OA (POA) was dependent on combustion conditions, with an average of 0.84 ± 0.54 m² g^-1, which was significantly higher than that for aged OA (0.24 ± 0.18 m² g^-1). Secondary OA (SOA) dominated aged OA and the decreased MAE₃₆₅ after aging indicates that SOA is less light absorbing than POA and/or that BrC is bleached (oxidized) with aging. The estimated MAE₃₆₅ of SOA (0.14 ± 0.08 m² g^-1) was much lower than that of POA. A comparison of MAE₃₆₅ of residential coal combustion with other anthropogenic sources suggests that residential coal combustion emissions are among the strongest absorbing BrC organics.