Aerosol chamber study of secondary organic aerosol formation from mixtures of anthropogenic and biogenic precursors

This thesis reports the use of an aerosol chamber to investigate Secondary Organic Aerosol (SOA) transformation and formation in biogenic and anthropogenic VOC mixtures. Experiments conducted to comprehensively describe and characterise the Manchester aerosol chamber (MAC) are reported. Photo-oxidation chamber experiments were conducted using online and offline mass spectrometric instrumentation, including Gas Chromatography-Mass Spectrometer (GC-MS), Aerosol Mass Spectrometer (AMS), Chemical Ionisation Mass spectrometer (CIMS) and liquid chromatography ultra-high resolution mass spectrometry (UPLC-orbitrap-MS). These were used to explore SOA transformation and formation in anthropogenic (o-cresol) and biogenic (a-pinene and isoprene) mixed precursors in the presence of NOx and inorganic seed particles (ammonium sulphate) through investigation of the SOA particle mass yield and characterisation of SOA composition in mixed precursor systems. The MAC characterisation study highlights the importance of conducting regular characterisation experiments to track the performance of the simulation chamber and emphasises the need to develop a routine set of standardised experiments for chambers, elucidating the particular characteristics of each facility, thus enabling the interpretation of the results. The study of SOA formation from mixed precursors established a novel chamber experimental design associated with initial "iso-reactivity" of the systems towards the dominant oxidant (OH), achieved by adjustment of the initial concentration of precursors in the mixed system. The SOA particle mass yield exhibited suppression when compared with that of a-pinene when mixed with isoprene, but a minor yield enhancement was tentatively found while adding isoprene to o-cresol. The a-pinene/o-cresol system exhibited a higher measured SOA particle mass yield than that calculated based on additivity. The measured and predicted yields were comparable in ternary systems. The ambiguity in reference for yield evaluation present challenges to the evaluation of changes in SOA formation when more than one precursor contributed to the SOA mass, as do differences in the prevailing chemical regime. An automated non-targeted accurate mass analysis of LC-Orbitrap-MS data was used to characterise the particulate products from mixed precursor systems. a-pinene oxidation products were shown to dominate the binary mixed a-pinene/isoprene system in terms of fractional signal contribution and the number of particle components detected. o-cresol oxidation products (methyl-nitrocatechol and methy-nitrophenol) dominated the SOA particle composition in both o-cresol/isoprene and o-cresol/a-pinene binary systems in negative ionisation mode. This thesis further stressed the significance of unique-to-mixture products formation, which contributes to molecular composition and signal abundance in o-cresol-containing binary systems in positive ionisation mode. Compounds uniquely identified in each mixture were identified, with the potential to be used as tracers in SOA source attribution in future ambient studies. Finally this thesis reports the alteration of average carbon oxidation state of SOA particle components in all systems, by employing HR-TOF-AMS, FIGAERO-CIMS and LC-Orbitrap MS to characterise the SOA composition. The oxidation state of nitrogen (OSN) for compounds that composed of Carbon (C), Hydrogen (H), Oxygen (O) and Nitrogen (N) element significantly influenced average OSc in single a-pinene and o-cresol system in FIGAERO-CIMS and LC-Orbitrap MS measurement. A substantial discrepancy in the estimated OSc between the FIGAERO-CIMS and HR-ToF-AMS and LC-Orbitrap MS techniques was found, likely associated with the characteristic of each technique and their limitation. a-pinene driven SOA and o-cresol oxidation products were shown to dominate the average OSc of SOA in binary a-pinene/isoprene system and binary o-cresol /isoprene system. In contrast, both a-pinene and o-cresol oxidation products contributed to the alteration of the average OSc during SOA formation in the binary a-pinene/o-cresol system. In the ternary precursor system, the OSc of SOA is not dominantly controlled by any single precursors but is associated with molecular interaction of the product.