Limonene photocatalytic oxidation at ppb levels: Assessment of gas phase reaction intermediates and secondary organic aerosol heterogeneous formation

Abstract

This study investigates the photocatalytic oxidation of limonene, used as a model terpenoid, close to indoor air conditions. Besides the characterization of limonene removal kinetic on the ppb range, special attention has been paid to reaction intermediates and by-products: (i) in the gas phase, (ii) in the adsorbed phase and (iii) in the particulate phase. All along the oxidation reaction organic reaction intermediates have been monitored. Despite a high conversion rate of limonene after 10h of treatment, 20 primary and secondary reaction intermediates were detected and quantified in the gas phase with acetone and acetaldehyde as the most abundant ones. The characterization of limonene adsorbed on TiO2 surface under UV illumination using DRIFTS pointed out the fact that the photocatalyst surface acts as a pool of heavy reaction intermediates, close to terpenoid structure, which may be responsible for the release of some secondary reaction intermediates. The mineralization of limonene has been assessed during the whole oxidation process through CO and CO2 monitoring. CO2 formation is observed more than 12h beyond limonene complete removal, confirming the long term oxidation of the adsorbed phase. The photocatalytic heterogeneous formation of secondary organic aerosol is reported for the first time. A massive production of SOA since the first steps of limonene photocatalytic oxidation is evidenced. Based on the quantitative analyses performed, carbon mass balances have been calculated along the oxidation reaction advancement. The highest contributions of organic reaction intermediates and SOA in the carbon balance are respectively 12% and 1.6%. When limonene is removed from the gas phase, more than 60% of the carbon balance remains unidentified but this contribution can be mainly attributed to the adsorbed organic species. After 24h of treatment, almost 75% of the organics are mineralized into CO2.