Highly oxygenated organic molecules produced by the oxidation of benzene and toluene in a wide range of OH exposure and NO_<i>x</i> conditions

Oxidation of aromatic volatile organic compounds (VOCs) leads to the formation of tropospheric ozone and secondary organic aerosol, for which gaseous oxygenated products are important intermediates. We show, herein, the experimental results of highly oxygenated organic molecules (HOMs) produced by the oxidation of benzene and toluene in a wide range of OH exposure and NOx conditions. The results suggest that multigeneration OH oxidation plays an important role in the product distribution, which likely proceeds more preferably via H subtraction than OH addition for early generation products from light aromatics. More oxygenated products present in our study than in previous flow tube studies, highlighting the impact of experimental conditions on product distributions. The formation of dimeric products, however, was suppressed and might be unfavorable under conditions of high OH exposure and low NOx in toluene oxidation. Under high-NOx conditions, nitrogen-containing multifunctional products are formed, while the formation of other HOMs is suppressed. Products containing two nitrogen atoms become more important as the NOx level increases, and the concentrations of these compounds depend significantly on NO2. The highly oxygenated nitrogen-containing products might be peroxyacyl nitrates, implying a prolonged effective lifetime of RO2 that facilitates regional pollution. Our results call for further investigation on the roles of high-NO2 conditions in the oxidation of aromatic VOCs.

Photochemical degradation of benzene, toluene, ethylbenzene, and xylenes (BTEX) using UV/H₂0₂

The oxidation of benzene, toluene, ethylbenzene, and xylenes (BTEX) by advanced oxidation processes in water was investigated. The degradation of BTEX by UV-185 and UV-254 nm in conjunction with H₂O₂ was studied. It was observed that the recommended H₂0₂ concentration to degrade 100 mgTOC/L of BTEX was 250 mg/L and 300 mg/L for UV-185 and UV-254 nm, respectively. In addition, it was observed that using the lamps in series did not have any advantages in the TOC removal of BTEX. Under acidic condition, pH 3, UV-185/H₂O₂ removed 10% more than UV-254/H₂O₂. At the recommended H₂O₂ concentration, 90% of BTEX mineralization was occurred with UV-185 nm/HO₂ under acidic condition of pH 3. It was observed that 21-32% BOD/TOC ratio of BTEX was decreased with an increase in residence time (within 140 min) in the photoreactor.