Author ORCID Identifier
Year of Publication
Doctor of Philosophy (PhD)
Arts and Sciences
Marcelo I. Guzman
The Earth’s atmosphere is a multicomponent system comprising gases, aerosols, and clouds, and their interaction with sunlight impacts radiative forcing. Biomass burning and anthropogenic emissions release volatile phenolic compounds to the atmosphere, where they can undergo chemical oxidation and photochemical aging, providing precursors for the formation of secondary organic aerosol. This dissertation ensembles laboratory studies of oxidative processing of a group of representative phenolic compounds by O3, hydroxyl radical (HO), and nitrate radical (NO3) at relevant environmental conditions.
Online electrospray mass spectrometry (OESI) revealed that aerosolized microdroplets of phenolic aldehydes undergo electron transfer reaction generating HO• when exposed to ≥ 45 ppbv O3(g) during microsecond contact times, generating hydroxy-substituents of the parent molecules. Ozonolysis of phenolic compounds and ring-functionalized phenols produce compounds containing carboxylic, and ester functionalities. The phenolic compounds were also deposited on ZnSe FTIR windows and exposed to ≥ 200 ppbv O3(g) for a longer timescale in a flow through reactor for analysis 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 studied for oxidation with NO3. When aerosolized microdroplet of 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 a mix of NO2(g) and O3(g) in the flow through reactor. 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 [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.
Digital Object Identifier (DOI)
The work in this dissertation was supported by the funding from the U.S.A. National Science Foundation (NSF) under award 1903744 and NSF CAREER award (CHE-1255290) to Dr. Marcelo I. Guzman.
Rana, Md Sohel, "MECHANISTIC STUDIES OF OXIDATIVE PROCESSING OF PHENOLIC COMPOUNDS AT ATMOSPHERIC INTERFACES" (2022). Theses and Dissertations--Chemistry. 163.
Available for download on Wednesday, July 24, 2024