Abstract
Abstract. Phenolic compounds, which are emitted in significant amounts from biomass burning, can undergo fast reactions in atmospheric aqueous phases to form secondary organic aerosol (aqSOA). In this study, we investigate the reactions of phenol (compound with formula C6H5OH)), guaiacol (2-methoxyphenol), and syringol (2,6-dimethoxyphenol) with two major aqueous-phase oxidants – the triplet excited states of an aromatic carbonyl (3C*) and hydroxyl radical (· OH). We thoroughly characterize the low-volatility species produced from these reactions and interpret their formation mechanisms using aerosol mass spectrometry (AMS), nanospray desorption electrospray ionization mass spectrometry (nano-DESI MS), and ion chromatography (IC). A large number of oxygenated molecules are identified, including oligomers containing up to six monomer units, functionalized monomer and oligomers with carbonyl, carboxyl, and hydroxyl groups, and small organic acid anions (e.g., formate, acetate, oxalate, and malate). The average atomic oxygen-to-carbon (O / C) ratios of phenolic aqSOA are in the range of 0.85–1.23, similar to those of low-volatility oxygenated organic aerosol (LV-OOA) observed in ambient air. The aqSOA compositions are overall similar for the same precursor, but the reactions mediated by 3C* are faster than · OH-mediated reactions and produce more oligomers and hydroxylated species at the point when 50% of the phenolic compound has reacted. Profiles determined using a thermodenuder indicate that the volatility of phenolic aqSOA is influenced by both oligomer content and O / C ratio. In addition, the aqSOA shows enhanced light absorption in the UV–visible region, suggesting that aqueous-phase reactions of phenols may contribute to formation of secondary brown carbon in the atmosphere, especially in regions influenced by biomass burning.
Highlights
Secondary organic aerosol (SOA) is ubiquitous in the atmosphere (Murphy et al, 2006; Zhang et al, 2007; Jimenez et al, 2009) and plays an important role in climate, human health, and air quality
The aqueous-phase reactions formed from aqueous-phase reactions (aqSOA) samples of phenol, guaiacol, and syringol were prepared during simulated sunlight illumination under two oxidant conditions: (1) via reaction with 3C∗ formed from 5 μmol L−1 3,4-dimethoxybenzaldehyde (3,4-DMB) and (2) via reaction with · OH generated from 100 μmol L−1 hydrogen peroxide (HOOH; Table 1). 3,4-DMB was chosen as the photosensitizer in this study to represent non-phenolic aromatic carbonyls, which are emitted in large quantities from wood burning (Schauer et al, 2001), exist nearly exclusively in condensed phases in the atmosphere, and rapidly form 3C∗ that efficiently oxidizes phenols (Anastasio et al, 1997)
These results are consistent with a previous study by Sun et al (2010), where O / C ratios of phenolic aqSOA formed from direct photodegradation and · OH oxidation were in the range of 0.80–1.06
Summary
Secondary organic aerosol (SOA) is ubiquitous in the atmosphere (Murphy et al, 2006; Zhang et al, 2007; Jimenez et al, 2009) and plays an important role in climate, human health, and air quality. Understanding the impacts of SOA requires a thorough knowledge of the formation, evolution, and composition of SOA. This knowledge, is still limited because atmospheric organic chemistry is extremely complex. SOA formation can take place in both the gas and condensed phases. Much of the previous research on SOA has mainly focused on gas-phase reactions of volatile organic compounds (Hallquist et al, 2009).
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