INVESTIGATIONS OF POTENTIALLY HARMFUL LIMONENE PHOTOOXIDATION PRODUCTS UNDER NOX AND NOX-FREE ATMOSPHERIC SIMULATED CONDITIONS

Abstract

Given the ubiquity of limonene in cleaning and cosmetic products, investigating its gasphase oxidation products from reactions with OH radicals and ozone has become important for human health. The present study investigates limonene gas-phase reaction with OH radicals under controlled NOx or NOx-free conditions using facilities from the 760 L Environmental Simulation Chamber made of Quartz (ESC-Q-UAIC) together with state-of-the-art instruments such as proton-transfer-reaction time-of-flight mass spectrometer (PTR-ToF-MS, model 6000 X2, IONICON). The obtained kinetic results (klimonene+OH = (16.42 � 2.20) ? 10-11 cm3?molecule-1?s-1), based on the relative rate techniques with 1,3,5-trimethylbenzene and cyclohexene as reference compounds, are in good agreement with the previous kinetic studies. The PTR-ToF-MS mass spectra acquired within the NOx-free experiments revealed signals at mass-to-charge ratios of 101.060, 139.112, and 169.122, that may be assigned to potentially harmful oxidation products, namely 4-oxopentanal (C5H8O2), 4-acetyl-1-methylcyclohexene (C9H14O), and 3-isopropenyl-6-oxo-heptanal (C10H16O2), respectively. Additionally, observed differences between NOx and NOx-free experiments highlight that NOx strongly suppresses the formation of secondary organic aerosols during limonene oxidation and favours the formation of 4-acetyl-1-methylcyclohexene.