Ozone-Initiated Reactions with Mixtures of Volatile Organic Compounds under Simulated Indoor Conditions

This study examines the primary and secondary products resulting from reactions initiated by adding ozone to complex mixtures of volatile organic compounds (VOC). The mixtures were representative of organic species typically found indoors, but the concentrations tended to be higher than normal indoor levels. Each 4-h experiment was conducted in a controlled environmental facility (CEF, 25 m3) ventilated at ∼1.8 h-1. The mixture investigated included 23 VOC (no O3), O3/23 VOC, O3/21 VOC (no d-limonene or α-pinene), and O3/terpene only (d-limonene and α-pinene). The net O3 concentration was ∼40 ppb in each experiment, and the total organic concentration was 26 mg/m3 for the 23 VOC mixture, 25 mg/m3 for the 21 VOC mixture, and 1.7 mg/m3 for the d-limonene and α-pinene mixture. When the 23 VOC were added to the CEF containing no O3, no compounds other than those deliberately introduced were observed. When O3 was added to the CEF containing the 23 VOC mixture, both gas and condensed phase products were found, including aldehydes, organic acids, and submicron particles (140 μg/m3). When O3 was added to the CEF containing the 21 VOC without the two terpenes (O3/21 VOC condition), most of the products that were observed in the O3/23 VOC experiments were no longer present or present at much lower concentrations. Furthermore, the particle mass concentration was 2−7 μg/m3, indistinguishable from the background particle concentration level. When O3 was added to the CEF containing only two terpenes, the results were similar to those in the O3/23 VOC experiments, but the particle mass concentration (190 μg/m3) was higher. The results indicate that (i) O3 reacts with unsaturated alkenes under indoor conditions to generate submicron particles and other potentially irritating species, such as aldehydes and organic acids; (ii) the major chemical transformations that occurred under our experimental conditions were driven by O3/d-limonene and O3/α-pinene reactions; and (iii) the hydroxyl radicals (OH) that were generated from the O3/terpene reactions played an important role in the chemical transformations and were responsible for approximately 56−70% of the formaldehyde, almost all of the p-tolualdehyde, and 19−29% of the particle mass generated in these experiments.