Co-formation of hydroperoxides and ultra-fine particles during the reactions of ozone with a complex VOC mixture under simulated indoor conditions

Abstract

In this study we examined the co-formation of hydrogen peroxide and other hydroperoxides (collectively presented as H2O2*) as well as submicron particles, including ultra-fine particles (UFP), resulting from the reactions of ozone (O3) with a complex mixture of volatile organic compounds (VOCs) under simulated indoor conditions. The VOC mixture contained 23 compounds, including two terpenes (d-limonene and α-pinene), two unsaturated alkenes (1-decene and 1-octene), and 19 other saturated organic compounds. These compounds are commonly found in indoor air but their concentrations were higher than typical indoor levels. When O3 was added to a 25-m3 controlled environmental facility (CEF) containing the 23 VOC mixture, both H2O2* and submicron particles were formed. The 2-h average concentration of H2O2* was 1.89±0.30 ppb, and the average total particle number concentration was 46,000±12,000 particles cm−3. A small increase of UFP (0.02–0.1 μm) occurred 5 min after the O3 addition (17 min after the VOC addition) and a sharp increase of UFP occurred 13 min after the O3 addition, suggesting homogeneous nucleation. The delayed onset of this event might reflect the time required to achieve saturated concentrations of the condensable organics. When the 2 terpenes were removed from the O3/23 VOCs mixture, no H2O2* or particles were formed, indicating that the reactions of O3 with the two terpenes were the key processes contributing to the formation of H2O2* and submicron particles in the O3/23 VOCs system. The present study confirmed the findings of a previous study carried out in a real-world office and generated new findings regarding co-formation of UFP. Through a comparative analysis of H2O2* yields under different reaction conditions, this study demonstrates that VOCs co-present with the terpenes and O3 may play a role in producing H2O2*.