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Abstract. Brown carbon (BrC) is an important component of biomass-burning (BB) emissions that impacts Earth's radiation budget. BB directly emits primary BrC as well as gaseous phenolic compounds (ArOH), which react in the gas and aqueous phases with oxidants – such as hydroxyl radical (OH) and organic triplet excited states (3C∗) – to form light-absorbing secondary organic aerosol (SOA). These reactions in atmospheric aqueous phases, such as cloud/fog drops and aerosol liquid water (ALW), form aqueous SOA (aqSOA), i.e., low-volatility, high-molecular-weight products. While these are important routes of aqSOA formation, the light absorption and lifetimes of the BrC formed are poorly characterized. To study these aspects, we monitored the formation and loss of light absorption by aqSOA produced by reactions of six highly substituted phenols with OH and 3C∗. While the parent phenols absorb very little tropospheric sunlight, they are oxidized to aqSOA that can absorb significant amounts of sunlight. The extent of light absorption by the aqSOA depends on both the ArOH precursor and oxidant: more light-absorbing aqSOA is formed from more highly substituted phenols and from triplet reactions rather than OH. Under laboratory conditions, extended reaction times in OH experiments diminish sunlight absorption by aqSOA on timescales of hours, while extended reaction times in 3C∗ experiments reduce light absorption much more slowly. Estimated lifetimes of light-absorbing phenolic aqSOA range from 3 to 17 h in cloud/fog drops, where OH is the major sink, and from 0.7 to 8 h in ALW, where triplet excited states are the major sink.
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Photoaging of phenolic secondary organic aerosol in the aqueous phase: evolution of chemical and optical properties and effects of oxidants
Abstract. While gas-phase reactions are well established to have significant impacts on the mass concentration, chemical composition, and optical properties of secondary organic aerosol (SOA), the aqueous-phase aging of SOA remains poorly understood. In this study, we performed a series of long-duration photochemical aging experiments to investigate the evolution of the composition and light absorption of the aqueous SOA (aqSOA) from guaiacyl acetone (GA), a semivolatile phenolic carbonyl that is common in biomass burning smoke. The aqSOA was produced from reactions of GA with hydroxyl radical (•OH-aqSOA) or a triplet excited state of organic carbon (3C∗-aqSOA) and was then photoaged in water under conditions that simulate sunlight exposure in northern California for up to 48 h. The effects of increasing aqueous-phase •OH or 3C∗concentration on the photoaging of the aqSOA were also studied. High-resolution aerosol mass spectrometry (HR-AMS) and UV–Vis spectroscopy were utilized to characterize the composition and the light absorptivity of the aqSOA and to track their changes during aging. Compared to •OH-aqSOA, the 3C∗-aqSOA is produced more rapidly and shows less oxidation, a greater abundance of oligomers, and higher light absorption. Prolonged photoaging promotes fragmentation and the formation of more volatile and less light-absorbing products. More than half of the initial aqSOA mass is lost, and substantial photobleaching occurs after 10.5 h of prolonged aging under simulated sunlight illumination for 3C∗-aqSOA and 48 h for •OH-aqSOA. By performing positive matrix factorization (PMF) analysis of the combined HR-AMS and UV–Vis spectral data, we resolved three generations of aqSOA with distinctly different chemical and optical properties. The first-generation aqSOA shows significant oligomer formation and enhanced light absorption at 340–400 nm.The second-generation aqSOA is enriched in functionalized GA species and has the highest mass absorption coefficients in 300–500 nm, while the third-generation aqSOA contains more fragmented products and is the least light absorbing. These results suggest that intermediately aged phenolic aqSOA is more light absorbing than other generations, and that the light absorptivity of phenolic aqSOA results from a competition between brown carbon (BrC) formation and photobleaching, which is dependent on aging time. Although photoaging generally increases the oxidation of aqSOA, a slightly decreased O/C of the •OH-aqSOA is observed after 48 h of prolonged photoaging with additional •OH exposure. This is likely due to greater fragmentation and evaporation of highly oxidized compounds.Increased oxidant concentration accelerates the transformation of aqSOA and promotes the decay of BrC chromophores, leading to faster mass reduction and photobleaching. In addition, compared with •OH, photoaging by3C∗ produces more low-volatility functionalized products, which counterbalances part of the aqSOA mass loss due to fragmentation and evaporation.
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- Award ID(s):
- 2220307
- PAR ID:
- 10513031
- Publisher / Repository:
- Copernicus Publications
- Date Published:
- Journal Name:
- Atmospheric Chemistry and Physics
- Volume:
- 23
- Issue:
- 12
- ISSN:
- 1680-7324
- Page Range / eLocation ID:
- 7103 to 7120
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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null (Ed.)Guaiacyl acetone (GA) is a phenolic carbonyl emitted in significant quantities by wood combustion that undergoes rapid aqueous-phase oxidation to produce aqueous secondary organic aerosol (aqSOA). We investigate the photosensitized oxidation of GA by an organic triplet excited state (3C*) and the formation and aging of the resulting aqSOA in wood smoke-influenced fog/cloud water. The chemical transformations of the aqSOA were characterized in situ using a high-resolution time-of-flight aerosol mass spectrometer. Additionally, aqSOA samples collected over different time periods were analyzed using high-performance liquid chromatography coupled with a photodiode array detector and a high-resolution Orbitrap mass spectrometer (HPLC-PDA-HRMS) to provide details on the molecular composition and optical properties of brown carbon (BrC) chromophores. Our results show efficient formation of aqSOA from GA, with an average mass yield around 80%. The composition and BrC properties of the aqSOA changed significantly over the course of reaction. Three generations of aqSOA products were identified via Positive Matrix Factorization analysis of the AMS data. Oligomerization and functionalization dominated the production of the first-generation aqSOA, whereas fragmentation and ring-opening reactions controlled the formation of more oxidized second- and third-generation products. Significant formation of BrC was observed in the early stages of the photoreaction, while organic acids were produced throughout the experiment. High-molecular-weight molecules (m/z > 180) with high aromaticity were identified via HPLC-PDA-HRMS and were found to account for a majority of the UV-vis absorption of the aqSOA.more » « less
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Pyrrole is a heterocyclic volatile organic compound (VOC) emitted into the atmosphere through processes like biomass burning and microbial decomposition. Pyrrole can readily react with atmospheric nitrate radicals (NO3) during nighttime to form secondary brown carbon (BrC) aerosols. The evolution of BrC from the NO3 oxidation of heterocyclic VOCs during chemical aging has not been extensively studied. In this work, we carried out experiments in a 10 m3 smog chamber to investigate the evolution of chemical composition and optical properties resulting from NO3- and OH-induced aging of BrC from the NO3 oxidation of pyrrole. The chemical composition and optical properties of the aged BrC were characterized using a suite of online and offline instrumentation. Our results show that compared to the SOA from initial NO3 oxidation, the bulk mass absorption coefficient () of aged SOA shows an 8% decrease from OH aging and a 21% decrease from NO3 aging on average. A number of monomeric and dimeric products were measured by mass spectrometry which are likely comprised of light-absorbing species. Analysis of the gas- and particle-phase chemical composition before and after aging suggests that strongly absorbing chromophores are consumed to produce less absorbing dimer products, likely phthalimides. This dimerization process is suggested to occur to a larger extent in the NO3-induced aging, responsible for the larger decrease in light absorption. Overall, this study expands the understanding of chromophores from the NO3 oxidation of pyrroles and helps to elucidate how these chromophores evolve during daytime and nighttime aging.more » « less
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- Free Publicly Accessible Full Text
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Accepted Manuscript
- Journal Article:
- https://doi.org/10.5194/acp-23-7103-2023
