Insights into secondary organic aerosol formed via aqueous-phase reactions of phenolic compounds based on high resolution mass spectrometry

摘要

Recent work has shown that aqueous-phase reactions of phenolic compounds – phenol (CHO), guaiacol (CHO), and syringol(CHO) – can form secondary organic aerosol (SOA) at highyields. Here we examine the chemical characteristics of this SOA and itsformation mechanisms using a High-Resolution Time-of-Flight Aerosol MassSpectrometer (HR-AMS), an Ion Chromatography system (IC), and a TotalOrganic Carbon (TOC) analyzer. The phenolic SOA are highly oxygenated withoxygen-to-carbon (O/C) ratios in the range of 0.80–1.06 and carbonoxidation states (=2×O/C-H/C) between −0.14 and +0.47. Theorganic mass-to-carbon (OM/OC) ratios determined by the HR-AMS (=2.21–2.55) agree well with values determined based on the SOA mass measuredgravimetrically and the OC mass from the TOC analyzer. Both the O/C andOM/OC ratios of the phenolic SOA are similar to the values observed forambient low-volatility oxygenated/secondary OA (LV-OOA). Oxalate is a minor,but ubiquitous, component of the SOA formed from all three phenolicprecursors, accounting for 1.4−5.2% of the SOA mass, with generallyhigher yields in experiments with HO added as an OH sourcecompared to without. The AMS spectra show evidence for the formation ofsyringol and guaiacol dimers and higher oligomers via C-C and C-O couplingof phenoxyl radicals, which are formed through oxidation pathways such asabstraction of the phenolic hydrogen atom or OH addition to the aromaticring. This latter pathway leads to hydroxylation of the aromatic ring, whichis one mechanism that increases the degree of oxidation of the SOA products.Compared to direct photochemical reactions of the phenols, OH-initiatedreactions favor the formation of smaller oxidation products but less dimersor higher oligomers. Two unique and prominent ions in the syringol andguaiacol SOA spectra, 306 (CHO) and 246(CHO), respectively, are observed in ambientaerosols significantly influenced by wood combustion and fog processing. Ourresults indicate that cloud and fog processing of phenolic compounds,especially in areas with active biomass burning, might represent animportant pathway for the formation of low-volatility and highly oxygenatedorganic species, which would remain in the particle phase after fog/cloudevaporation and affect the chemical and optical properties of atmosphericparticles.