UHRMS has enormous potential to provide complete molecular characterisation of organic aerosol. However atmospheric aerosol often has complex, mixed organic and inorganic composition and soft ionisation techniques that have been developed to ionise organic aerosol specifically, are susceptible to salt suppression effects and competitive ionisation when applied to complex ambient aerosol.

In this thesis, characterisation experiments were conducted on a novel soft ionisation technique called Extractive Electrospray Ionisation (EESI) using mixtures of water-soluble organic compounds (WSOCs) with increasing ammonium sulphate solute concentrations. The WSOCs MS signals in the mixtures were not significantly affected by salt suppression over an atmospherically relevant range of 0-70% inorganic molar percentage relative to organic, recommending EESI-MS as a suitable technique for analysing ambient aerosol from a range of environments. EESI-MS was also used for the first time to detect Secondary Organic Aerosol (SOA) generated in the Cambridge Atmospheric Simulation Chamber (CASC) from the OH radical-initiated oxidation and ozonolysis of Anthropogenic Volatile Organic Compounds (AVOCs) and Biogenic Volatile Organic Compounds (BVOCs). Several oligomer products from complex BVOC and AVOC oxidation, previously observed in chamber studies using similar soft ionisation techniques, were detected using EESI-MS.

This thesis also includes the first study, to our knowledge, characterising the detailed molecular composition of organic compounds present in rainwater and aerosol collected during the same rain event, using chip-based direct infusion Nano-ESI and LC-ESI-MS. A Solid Phase Extraction (SPE) method was developed to extract the organic compounds from the rainwater and aerosol samples collected during the AERORAIN field campaign conducted in León, Spain, prior to MS analysis. More mono-aromatic compounds were observed in the aerosol samples whereas more poly-aromatic compounds were observed in the rainwater. HYSPLIT air mass backward trajectories revealed that both air mass origin and seasonality had strong influences on the sample composition. Samples collected during the summer and samples influenced by the Sahara desert contained the most compounds due to increased photochemical activity and extra industrial emissions from North Africa respectively. All samples contained semi-volatile oxygenated organic aerosol and multiple organosulphates were tentatively identified. Continental samples contained low-volatility oxidised organic aerosol and exhibited the most efficient aerosol scavenging and CHO and CHON compounds were preferentially scavenged over CHOS and CHONS compounds.

The combination of EESI-MS, Nano-ESI-MS and LC-ESI-MS analysis in this thesis, along with optimised analytical and data processing methods, has provided an in depth characterisation of the composition of complex aerosol, and highlights the importance of seasonality, long range transport and below-cloud scavenging on the compound distribution in ambient samples.