Browsing by Author "Garmash, Olga"

A myriad of different volatile organic compounds is emitted to the atmosphere through biogenic and anthropogenic pathways. In the atmosphere, they get oxidised forming products of lower volatility, which may then condense and contribute to the formation and growth of aerosol particles. Extremely low-volatility organic compounds (ELVOC) is a group of highly oxidised molecules that were recently observed in oxidation of biogenically emitted molecules, monoterpenes. It has been shown that ELVOCs from monoterpene oxidation in the atmosphere explain most of the aerosol growth rates in the boreal forest. In this work, I investigate the ELVOC formation from aromatic compounds, which are primarily emitted from anthropogenic sources. This thesis focuses on oxidation of benzene, a simplest aromatic molecule, although ELVOCs forming from toluene, naphthalene and phenol are also presented. The experimental work with all compounds, except phenol, was conducted in a flow tube at the Universtity of Helsinki, Finland, while in-depth study of benzene was performed in the chamber at Forschungszentrum Jülich GmbH, Germany. The oxidation products were detected using nitrate-based scheme chemical ionisation atmospheric pressure interface time-of-flight mass spectrometer (CI-APi-TOF). It was found that all four molecules in the reaction with a hydroxyl radical produced ELVOC. In case of benzene, the detected ELVOC monomers had maximum of 11 oxygen atoms, while dimers had up to 18. Toluene and naphthalene oxidation gained ELVOC monomers with maximum 10 oxygen atoms. The possibility of multiple reactions with the hydroxyl radical, however, could not be eliminated as the products of aromatic oxidation usually have higher reaction rate coefficients than their parent molecule. The average ELVOC molar yield in benzene oxidation was 3.7%, while in phenol case it was 1.5%, which suggested that primary OH attack and further autoxidation is an important pathway for ELVOC formation in oxidation of benzene. The study of the ELVOC – aerosol particle interactions in the chamber revealed that ELVOCs from aromatic precursors behave similarly to the ELVOCs formed from monoterpenes, rapidly condensing on the introduced aerosol particles. The organic fraction of the aerosols had similar O:C ratio to the total gas-phase ELVOC, indicating that ELVOCs were primary condensing species. The decreasing O:C ratio of aerosol phase with higher aerosol loading confirmed that ELVOCs are dominating aerosol growth at low aerosol loading. In this thesis, also a review of known chemical pathways of benzene oxidation is presented and some further steps towards ELVOC formation are suggested. Benzene oxidation pathways may serve as a model for studying the oxidation of other aromatic molecules. Substituted aromatic molecules are more reactive with the hydroxyl radical and are likely to yield more ELVOCs, which may be dominant factor in the aerosol growth in urban and industrialised areas.