Real-time monitoring of the gas phase chemistry of key atmospheric VOCs using atmospheric simulation chambers to evaluate their SOA forming potential

The oxidation of a range of Volatile Organic Compounds (VOCs) has been studied, from small alkenes (e.g. ethene C2) to larger sesquiterpene species (e.g. β-caryophyllene C15). The gas-phase reactions of these VOCs, largely emitted from biogenic sources, can form oxidation products with high mass and low volatility to contribute to aerosol formation, namely for monoterpene and sesquiterpene species. These organic aerosols formed from chemical reactions in the atmosphere are secondary organic aerosols (SOA). Aerosols can have a profound impact on both climate and health issues at regional and global scales. Processes that govern these gas-to-particle phase reactions are still not fully understood. This thesis presents detailed gas-phase composition data from the various VOCs examined, and tries to highlight important gas-phase species involved in the processes for SOA formation in the atmosphere. The gas-phase composition was measured in real-time utilising the University of Leicester Chemical Ionisation Reaction-Time of Flight-Mass Spectrometer (CIR-ToF-MS). Experiments were conducted under two different environments, “dark” ozonolysis experiments were studied at the EUropean PHOtoREactor (EUPHORE) atmospheric simulation chamber (Valencia, Spain) whilst “light” photooxidation experiments were conducted at the Manchester Aerosol Chamber (MAC) facility (Manchester, UK). The ozonolysis experiments focused around small alkene species (ethene, isobutene, and trans-2-butene), isoprene and monoterpenes (myrcene, α-pinene and limonene) in the absence of NOx and investigated with and without radical scavengers in order to suppress side reactions. Under dry conditions the primary oxidation products for smaller alkene ozonolysis averaged yields for formaldehyde (HCHO) as 1.56 ± 0.09, 1.21 ± 0.03 and 0.15 ± 0.01 for ethene, isobutene and trans-2-butene respectively. Other major gas phase product yields were recorded. Under wet conditions HCHO yields increased dramatically for ethene ozonolysis, to 3.09 ± 0.12 and 1.94 ± 0.31 for isobutene, but no substantial difference was observed for trans-2-butene with an average yield of 0.19 ± 0.04. Observations on gas-phase composition varied little based on the latter and model comparisons were made using the Master Chemical Mechanism (MCMv3.1). Photolysis experiments were conducted for isoprene, monoterpenes (limonene, α-pinene and myrcene) and a sesquiterpene, β-caryophyllene. This led to a direct comparison of composition and yields were obtained for certain oxygenated VOCs (oVOCs). The major gas phase products of isoprene ozonolysis, methacrolein (MACR) recorded average yields of 0.24 ± 0.16 and methyl-vinyl ketone (MVK) at 0.15 ± 0.01 for dry conditions, whilst yields of 0.36 ± 0.04 and 0.17 ± 0.02 were observed for wet conditions respectively. Similar yields were observed for photolysis conditions. The highest average yields in the gas phase for all monoterpene species were the primary aldehyde species formed (e.g. pinonaldehyde for α-pinene), ranging averaged yields from 0.115 to 0.583 for ozonolysis reactions and 0.119 to 0.270 under photolysis conditions. Where applicable, SOA yields were determined using a Differential Mobility Particle Sizer (DMPS) and composition of the particle phase made off-line using Liquid chromatography-ion trap mass spectrometry (LC-MSn). A unique method of organic seed formation was also constructed for photolysis experiments for isoprene and limonene using β-caryophyllene as a precursor for the organic seed. Finally mesocosm experiments of direct emissions from tree species Ficus cyathistipula, Ficus benjamina and Caryota millis (to simulate tropical Asian conditions) and Betula Pendula (to encompass European environments). The tropical monoterpene producing species formed SOA, whereas the European isoprene dominant species did not. Implications of this are further discussed along with the difference observed in gas-phase composition and yields of oxidation products produced from all experiments. An Am241 source and a newly developed hollow cathode source was utilised in both campaigns so instrumental sensitivity, in particular for lower mass species is also discussed. Evidence from the experiments shows that SOA formation is only observed from monoterpene and sesquiterpene compounds. Here isoprene did not form any substantial SOA and we argue it can inhibit SOA formation. Important gas phase species for SOA contribution were those of C10 or higher, in particular the primary aldehyde oxidation products of monoterpenes that were observed in both gas and particulate phase.