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Exit Seminar "Mechanistic Studies of Oxidative Processing of Phenolic Compounds at the Atmospheric Interfaces

Speaker(s) / Presenter(s):
Md Sohel Rana

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Abstract: Earth’s atmosphere is a multicomponent system comprising gases, aerosols, cloud, and their interaction with sunlight impacts radiative forcing. Biomass burning and anthropogenic emissions releases volatile phenolic compounds at the atmosphere as primary organic aerosol. These phenolic compounds can undergo atmospheric processing such as chemical oxidation, and photochemical aging providing precursors for the formation of secondary organic aerosol. This dissertation ensembles laboratory studies of oxidative processing of group of representative phenolic compounds by O3, HO•, and NO3 at the relevant environmental conditions. Aerosolized microdroplets of phenolic aldehydes undergoes electron transfer reaction generating HO• at the online electrospray mass spectrometry (OESI) reactor when exposed to 0.045 ppmv to 5 ppmv O3(g) at the microsecond contact time, generating hydroxy-substituents of the parent molecules. Ozonolysis of  phenolic compounds and ring-functionalized product phenols produces compounds containing carboxylic, and ester-like functionalities. These phenolic compounds were then deposited on ZnSe FTIR windows and exposed to 1 L min-1 of 0.20 ppmv to 800 ppmv O3(g) for a longer timescale in a flow through reactor and analyzed by FTIR spectroscopy, UV-visible spectroscopy, ultrahigh pressure liquid chromatography (UHPLC) with UV-visible and mass spectrometry (MS) detection, ion chromatography (IC) with conductivity and MS detection, and nuclear magnetic resonance (NMR) spectroscopy for 1H and 13C nuclei and tow-dimensional heteronuclear single quantum coherence (HSQC) experiments. The reaction products were dominated by functionalized and oligomeric compounds. Syringic acid was used as a common standard to compare the responses of UHPLC-MS, IC-MS and NMR analysis and quantify methoxy-aromatic product compounds and syringaldehyde was used to compare results from UHPLC-MS and NMR, and pseudo quantify aromatic aldehydic compounds. The uptake of O3(g) by the phenolic compounds increased with increasing relative humidity (RH). The decay kinetics showed non-linear dependence against increasing molar ratio of O3(g).  Phenolic compounds were also studied for oxidation with NO3. When aerosolized microdroplet of phenolic compounds such as catechols were exposed to NO3, produced from the mixing of NO2(g) and O3(g) at the OESI-MS reactor, nitroaromatic compounds (NAC) were produced. Under variable pH (4.05 to 8.07) conditions, all these compounds generated NAC. Catechol thin film was deposited over ZnSe windows and oxidized by 1 L min-1 of NO2(g) and O3(g) mix, in the flow through reactor for longer times scale. Formation of 4-nitrocatechol (4NC) was observed after exposure to oxidant mixture of 200 ppbv NO2(g) and 50 ppbv O3(g) at 0% RH. 4NC production was highest at 0% RH and at elevated RH the reaction was dominated by O3(g) as observed by the increased production of cis,cis-muconic acid. Exposure to high ppbv oxidant mix such as 10700 ppbv NO2(g) and 2500 ppbv O3(g) at 0% RH showed generation of NAC and oligomers. Decay of catechol against increasing oxidant molar ratio showed non-linear dependence at 0% RH. All these results show the crucial role of daytime oxidant O3(g), and HO•, and nighttime oxidant NO3 on the oxidative processing of phenolic compounds. Considering these reaction pathways and kinetics parameters in future climate modelling would reduce the gap between field observation and computer simulation predictions.