Investigation of high resolution mass spectrometry to study the formation and evolution of secondary organic aerosol

Secondary organic aerosol (SOA) is well known to have adverse effects on climate, air quality and human health. However, the dynamic mechanisms occurring during SOA formation and ageing are poorly understood. In this study, a number of new instrumental methods based on liquid chromatography coupled to mass spectrometry were developed and validated to study the complex mechanisms controlling SOA formation and ageing. The method was tested on methyl chavicol, an oxygenated biogenic volatile organic compound that was recently identified as the main floral emission from an oil palm plantation in Malaysian Borneo. The photo-oxidation of methyl chavicol was investigated at the European Photoreactor. The SOA yield was determined to range between 19 - 31% depending on initial precursor (VOC:NOx) mixing ratios. In total, 79 SOA compounds were observed and the structures of 10 compounds have been identified. The use of a time resolved aerosol collection method followed by offline mass spectrometry allowed the temporal evolution of individual compounds in the particulate phase to be observed. The majority of the observed initial SOA composition consisted of substituted nitrophenols. The photo-oxidation of two other aromatic systems (toluene and 4-methyl catechol) were also investigated. The photo-oxidation of toluene also resulted in the formation of substituted nitrophenols during initial aerosol growth. However, based on the volatility of these species and the amount of absorptive mass present, these compounds would not be expected to be in the aerosol phase. One possible explanation for this observation, is the formation of gas-phase clusters; suggesting that the presence of the nitro-group on the aromatic ring may be responsible for new particle formation. However, the structure of the nitrophenols were found to be critically important; highlighting the importance of studying SOA formation and evolution at a molecular level.