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The influence of the OH radical precursor on brown carbon formation in the aqueous-phase OH radical photooxidation of phenolic compounds

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Light absorbing organic aerosols (“brown carbon”) can have significant climate impacts. Phenolic molecules, which are commonly associated with biomass burning, are known precursors of brown carbon. Previous studies have shown that the aqueous-phase OH radical photooxidation of phenolic compounds leads to an initial enhancement of light absorption due to the formation of brown carbon chromophores, followed by photobleaching due to fragmentation of the brown carbon chromophores. Majority of these studies use hydrogen peroxide photolysis to generate OH radicals. Nitrate, which is present in substantial concentrations in aerosols, is known to also photolytically generate OH radicals. In this work, we investigated how OH radical precursors impact the composition and light absorption properties of brown carbon formed in the aqueous-phase OH radical photooxidation of phenolic compounds. We report results from experiments of the photooxidation of guaiacol and 4-nitroguaiacol by OH radicals generated from the photolysis of either hydrogen peroxide or sodium nitrate. The composition and light absorption properties of products formed were compared. In the OH radical photooxidation of guaiacol, UV/Vis absorption spectra obtained using sodium nitrate as the OH radical precursor were significantly different from spectra obtained using hydrogen peroxide as the OH radical precursor. Mass spectral analysis revealed that the difference was due to the formation of nitrated compounds when sodium nitrate was used. In contrast, UV/Vis absorption spectra obtained from the OH radical photooxidation of 4-nitroguaiacol using sodium nitrate as the OH radical precursor were pretty similar to spectra obtained using hydrogen peroxide as the OH radical precursor. The composition of products formed were also found to be mostly similar. These results indicated that the presence of inorganic nitrate will impact the composition and light absorption properties of brown carbon formed in the aqueous-phase OH radical photooxidation of some phenolic compounds.

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TitleAGU Fall Meeting 2020
Date1/12/2017/12/20
LocationOnline
PlaceUnited States
Degree of recognitionInternational event

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