Journal cover Journal topic
Atmospheric Chemistry and Physics An interactive open-access journal of the European Geosciences Union
Atmos. Chem. Phys., 16, 4511-4527, 2016
https://doi.org/10.5194/acp-16-4511-2016
© Author(s) 2016. This work is distributed under
the Creative Commons Attribution 3.0 License.
Research article
13 Apr 2016
Molecular transformations of phenolic SOA during photochemical aging in the aqueous phase: competition among oligomerization, functionalization, and fragmentation
Lu Yu1, Jeremy Smith2, Alexander Laskin3, Katheryn M. George4, Cort Anastasio2, Julia Laskin5, Ann M. Dillner4, and Qi Zhang1 1Department of Environmental Toxicology, University of California, 1 Shields Ave., Davis, CA 95616, USA
2Department of Land, Air and Water Resources, University of California, 1 Shields Ave., Davis, CA 95616, USA
3Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory, Richland, WA 99352, USA
4Crocker Nuclear Laboratory, University of California, 1 Shields Ave., Davis, CA 95616, USA
5Physical Sciences Division, Pacific Northwest National Laboratory, Richland, WA 99352, USA
Abstract. Organic aerosol is formed and transformed in atmospheric aqueous phases (e.g., cloud and fog droplets and deliquesced airborne particles containing small amounts of water) through a multitude of chemical reactions. Understanding these reactions is important for a predictive understanding of atmospheric aging of aerosols and their impacts on climate, air quality, and human health. In this study, we investigate the chemical evolution of aqueous secondary organic aerosol (aqSOA) formed during reactions of phenolic compounds with two oxidants – the triplet excited state of an aromatic carbonyl (3C) and hydroxyl radical (OH). Changes in the molecular composition of aqSOA as a function of aging time are characterized using an offline nanospray desorption electrospray ionization mass spectrometer (nano-DESI MS) whereas the real-time evolution of SOA mass, elemental ratios, and average carbon oxidation state (OSC) are monitored using an online aerosol mass spectrometer (AMS). Our results indicate that oligomerization is an important aqueous reaction pathway for phenols, especially during the initial stage of photooxidation equivalent to  ∼  2 h irradiation under midday winter solstice sunlight in Northern California. At later reaction times functionalization (i.e., adding polar oxygenated functional groups to the molecule) and fragmentation (i.e., breaking of covalent bonds) become more important processes, forming a large variety of functionalized aromatic and open-ring products with higher OSC values. Fragmentation reactions eventually dominate the photochemical evolution of phenolic aqSOA, forming a large number of highly oxygenated ring-opening molecules with carbon numbers (nC) below 6. The average nC of phenolic aqSOA decreases while average OSC increases over the course of photochemical aging. In addition, the saturation vapor pressures (C) of dozens of the most abundant phenolic aqSOA molecules are estimated. A wide range of C values is observed, varying from < 10−20 µg m−3 for functionalized phenolic oligomers to > 10 µg m−3 for small open-ring species. The detection of abundant extremely low-volatile organic compounds (ELVOC) indicates that aqueous reactions of phenolic compounds are likely an important source of ELVOC in the atmosphere.

Citation: Yu, L., Smith, J., Laskin, A., George, K. M., Anastasio, C., Laskin, J., Dillner, A. M., and Zhang, Q.: Molecular transformations of phenolic SOA during photochemical aging in the aqueous phase: competition among oligomerization, functionalization, and fragmentation, Atmos. Chem. Phys., 16, 4511-4527, https://doi.org/10.5194/acp-16-4511-2016, 2016.
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Short summary
The chemical evolution of SOA formed during aqueous reactions of phenolic compounds is studied via combined bulk and molecular analysis. Phenolic SOA evolve dynamically during photochemical aging, with different reaction mechanisms (oligomerization, fragmentation, and functionalization) leading to different generations of products that span an enormous range in volatilities and a large range in oxidation state and composition. Aqueous reactions of phenols are likely an important source of ELVOC.
The chemical evolution of SOA formed during aqueous reactions of phenolic compounds is studied...
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