Comparison of Secondary Organic Aerosol Formation between Gas-phase and Aqueous-phase Reactions: A Case Study on Guaiacol
Abstract
Recent studies have shown that guaiacol (C7H8O2; 2-methoxyphenol), which is a volatile compound emitted in significant quantities from wood combustion, can undergo fast reactions in both the gas- and aqueous-phase to form secondary organic aerosol (SOA) with high mass yields. In this study, we investigate the compositional differences between the SOA formed from gaseous (gSOA) and aqueous (aqSOA) reactions of guaiacol using high resolution time-of-flight aerosol mass spectrometry (HR-ToF-AMS). Gas-phase experiments were conducted under low NOx (<10 ppb) and high NOx (hundreds of ppb) conditions using hydroxyl radical (•OH) as the oxidant. Aqueous-phase experiments were conducted using two oxidants - the triplet excited state of an aromatic carbonyl (3C*) and •OH. Significant chemical differences are observed between aqSOA and gSOA. For example, after 50% of the guaiacol has reacted (i.e., one half-life, t1/2), aqSOA shows lower atomic oxygen-to-carbon (O/C) ratios (0.62-0.83 vs. 0.84-0.90) and higher hydrogen-to-carbon (H/C) ratios (1.47-1.71 vs. 1.17-1.26) than gSOA. However, by the time when 75% of the guaiacol has reacted, the O/C ratios of aqSOA and gSOA become more similar (0.69-0.98 and 0.85-0.93). These results indicate that in comparison to aqueous reactions, gas-phase reactions of guaiacol produce more oxidized SOA initially, but prolonged reactions increase the oxidation degree of aqSOA more significantly. In addition, ions representative of oligomers (e.g., C14H14O4+, C14H14O5+ and C21H20O6+) are observed only in aqSOA. This observation, together with a higher mass fraction of large ions (m/z > 120) in aqSOA than in gSOA, indicates that aqueous reactions have a higher tendency to form oligomers and other high molecular weight products. Furthermore, we found that fHCO2+ (the ratio of HCO2+ signal to total organic signal) increases more significantly in aqSOA than in gSOA over the course of reaction, suggesting more carboxylic acid formation in aqueous reactions. These results demonstrate major differences in SOA formation pathways between the gas and aqueous phases for guaiacol, which can help distinguish between these two pathways in ambient particles and are important for properly representing SOA formation pathways in models.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2016
- Bibcode:
- 2016AGUFM.A33H0336J
- Keywords:
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- 0305 Aerosols and particles;
- ATMOSPHERIC COMPOSITION AND STRUCTUREDE: 0317 Chemical kinetic and photochemical properties;
- ATMOSPHERIC COMPOSITION AND STRUCTUREDE: 0322 Constituent sources and sinks;
- ATMOSPHERIC COMPOSITION AND STRUCTUREDE: 0365 Troposphere: composition and chemistry;
- ATMOSPHERIC COMPOSITION AND STRUCTURE