Chlorine-Initiated Oxidation of Limonene under Simulated Indoor and Outdoor Lighting Conditions

Limonene is the fourth most emitted biogenic volatile organic compound and is often used as a fragrance and emitted from personal care products, cleaning products, and others. Chlorine gas (Cl₂), a precursor for Cl atoms, is emitted from anthropogenic activities, including cleaning, disinfection, and industrial activities, and it also forms from heterogeneous reactions involving sea salt. Thus, limonene and Cl radical precursors can both be present in indoor and outdoor environments. We studied the chlorine-initiated oxidation of limonene under indoor lighting (LED lights) and simulated outdoor lighting (a combination of UVA and LED lights) using an environmental chamber and a suite of instruments measuring gas and particle phase products. OH radicals formed and dominated the oxidation of limonene in the presence of NO_x, Cl₂, and LED lights, highlighting that Cl-initiated chemistry can generate OH chemistry in indoor environments, even in the absence of sunlight. Measurements from an iodide chemical ionization mass spectrometer showed gas phase reaction products from both Cl and OH addition to limonene, including nitrated species LIMANO₃ (C₁₀H₁₇NO₄) and LIMALNO₃ (C₁₀H₁₇NO₆). Secondary organic aerosol (SOA) yields were high, exceeding 1.1 in experiments with low NO_x and high limonene concentrations, and showed strong dependence on temperature, NO_x, and the VOC/Cl₂ ratio. These findings suggest that Cl₂ can contribute to the indoor and outdoor SOA formation from limonene oxidation through direct oxidation and secondary OH chemistry.