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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.

Secondary organic aerosols (SOA) contribute significantly to the total organic aerosol mass in the atmosphere. Understanding the SOA formation is crucial in order to estimate its impact on the air quality, climate and human health. SOA is typically produced from the oxidation of anthropogenic volatile organic compounds (AVOCs) or biogenic volatile organic compounds (BVOCs) by the oxidants such as OH radical, O3, or NO3 radical. SOA formation from single BVOCs or AVOCs has been widely investigated, while there are limited studies of the SOA formation from VOCs mixtures. There are many uncertainties in the physicochemical properties of SOA formed in the mixed VOC precursors. This work designed a series of experiments to explore SOA formation from the mixed AVOC (o-cresol) and BVOCs (a-pinene and isoprene) under the presence of NOx in the Manchester Aerosol Chamber. The online and offline analytical techniques were applied to investigate the chemical and physical properties of SOA. The gas- and particle-phase oxidation products were monitored by the near real-time online iodide chemical ionization mass spectrometry (CIMS) coupled with Filter Inlet for Gases and AEROsols (FIGAERO). The collected filter at the end of each experiment was characterised by the offline liquid chromatography orbitrap mass spectrometry (LC-Orbitrap MS) to investigate particle-phase chemical components. This study found that the chemical components of SOA formed from various VOCs systems showed big differences. In the a-pinene and a-pinene/isoprene mixture, the products with carbon, hydrogen and oxygen (CHO group) dominated the signals in the particle phase, broadly consistent with the LC-Orbitrap MS negative mode analysis which was able to better identify the sulphur-containing fraction. By contrast, o-cresol containing systems were dominated by the CHON signal fraction (>60%) from offline negative mode analysis. More compounds with high carbon numbers (nC>=16) were detected by the LC-Orbitrap MS positive ionisation mode, which indicated a fraction missed by the negative mode and CIMS measurements. Additionally, unique-to-the-mixture products were observed in the mixture, suggesting molecular interactions in the mixture systems. This work brought an insight into the SOA chemical composition from the mixed volatile precursors by the combination of online FIGAERO-CIMS and offline LC-Orbitrap MS analytical techniques and highlighted the importance of SOA studies in the mixed volatile precursors.

Aerosol particles are ubiquitous in the atmosphere, impacting air quality, global climate and threatening public health. These aerosol-induced environmental impacts are tightly bounded by their water uptake. However, it is not yet well understood how complex chemical composition influences water uptake and the related impacts. In addition, direct observations (especially the long-term) of aerosol water uptake worldwide are still lacking due to expensive expenses. The aim of the presented thesis is to demonstrate the chemical controls of aerosol water uptake, its impacts on aerosol phase state and air pollution, and to retrieve hygroscopicity parameter from open-access data using a combination of laboratory chamber experiments, field observational data and theoretical calculations. Chamber experiments on the secondary organic aerosol (SOA) from mixed biogenic and anthropogenic precursors, showed that the water saturation ratio and organic mass fraction are the key factors of water uptake whereas the SOA composition plays a second-order role. For the reconciliation of sub- and super-saturated water uptake, the hygroscopicity parameter ksub/ksuper ratio increased with organic mass fraction. The factors which influence the aerosol water uptake, control the aerosol phase transition between the liquid and non-liquid. In addition, aerosol water uptake was found to play a key role in Beijing winter haze development via a positive feedback between aerosol liquid water and particulate nitrate formation, where the aerosol liquid water can contribute up to 24 % to visibility reduction. To resolve the shortage of hygroscopicity observations worldwide, a novel method was developed, based on k-Kohler and Mie theory, to retrieve aerosol hygroscopicity from open access PM2.5 mass concentration, ambient RH and visibility data. The method was applied in Delhi, India and reported the high aerosol hygroscopicity for the first time. To sum up, the presented thesis improved our understanding on water uptake of multi-component aerosol particles and the relevant impacts. More interestingly, the developed method is applicable in wider regions worldwide and make the aerosol hygroscopicity knowledge achievable. Furthermore, this method shed lights on the quantification of co-condensation of condensable vapours on aerosol hygroscoscopicity in future after its successful case in Delhi.

Aerosol particles originate from both primary and secondary sources with implications on climate, air quality and human health. Secondary organic aerosols (SOA) comprise a large fraction of the total aerosol mass and are formed from the oxidation of anthropogenic and biogenic volatile organic compounds (AVOC and BVOC, respectively). The VOC emission rates and sources are diverse, leading to a highly complex SOA composition that can substantially vary temporarily and spatially. Until recently, the SOA mass was thought to be independently formed from the oxidation of each VOC precursor, however recent evidence demonstrated that the molecular interactions of the oxidised products may alter the SOA formation potential. This could partly explain our inability to predict the SOA loadings and impacts that are still highly uncertain. This thesis explores the photochemical SOA formation, composition and volatility derived from the mixing of various key AVOC (o-cresol) and BVOC (a-pinene and isoprene) by conducting experiments in an atmospheric simulation chamber. In order to assess the capabilities and limitation of the facilities used in this thesis, a detailed characterisation of the Manchester Aerosol Chamber (MAC) was conducted. This revealed the importance of regularly characterising such facilities for losses of particles and gases and highlighted the need for the development of a unified framework to characterise the atmospheric simulation chamber facilities. The concept of initial iso-reactivity was conceived that enabled the preparation of VOC mixtures that had comparable reactivity towards the assumed dominat oxidant (OH) in all the systems examined. The SOA formation potential was found to be enhanced, supressed or unaffected by the mixing of the precursors, suggesting that the effect of mixing is system-dependent and not straightforward. Similarly, certain mixed systems showed to have higher SOA particle volatility than that observed in single precursor experiments (e.g., o-cresol/isoprene), others showed lower (e.g., a-pinene/o-cresol and a-pinene/o-cresol/isoprene), while others appeared to be unaffected (e.g., a-pinene/isoprene). All the mixed systems however showed clear differences in the SOA chemical composition. Two main processes were observed in all the mixed VOC systems; the suppression in the formation of products that formed in single precursor systems and the formation of unique products in each mixture. The trade-off between these two processes are likely, at least partly, defining the SOA composition in mixed VOC systems that in turn may affect the SOA formation and volatility.

Organic aerosols (OA) are an important component of the atmosphere with regards to resolving the impact aerosols have on both climate and air quality. To predict how OA will behave in the atmosphere requires knowledge of their physicochemical properties. A key property for predicting what fraction of a compound will partition into the aerosol phase and what fraction will partition into the gas phase is the saturation vapour pressure (Psat) of the compound. It has been estimated that the number of organic compounds in the atmosphere is in excess of 100,000; therefore it is not feasible to measure the Psat of each compound experimentally. Instead group contribution methods (GCMs) are used to predict Psat. Many GCMs were originally designed for use in chemical engineering and were developed for use with monofunctional compounds and hydrocarbons. This means that they often lack parameters to account for various steric effects and intramolecular interactions that can occur in multifunctional compounds and the impact these interactions have on Psat. As the vast majority of OA consist of multifunctional compounds this leads to GCMs performing poorly when predicting Psat for OA. As well as not properly accounting for intramolecular interactions between functional groups, some functional groups are underrepresented in the data sets that are used to fit GCMs or can be missing entirely. If a functionality is poorly represented this can lead to a GCM overfitting to a limited amount of data, and if a functionality is not represented at all, the effects of that functionality can be misrepresented or ignored entirely. In order to more accurately predict P sat of OA more experimental data is needed, especially for multifunctional compounds that contain functionalities that are poorly represented in GCM fitting data sets. In this project experimental Psat are measured for a range of nitroaromatic compounds and a range of benzaldehydes using Knudsen Effusion Mass Spectrometry (KEMS). These measured values are then compared to each other and chemical explanations are given for the observed trends. The experimental Psat are then compared to predicted Psat using GCMs and potential causes for the observed differences are discussed. Following this multivariate regression techniques are used to calculate feature importance for several GCMs to determine which functionalities give rise to the largest sources of error when predicting Psat.