Indoor Hydrogen Peroxide Derived from Ozone/d-Limonene Reactions

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Abstract
In this pilot study, performed in an office manipulated to resemble an environment with a strong indoor ozone source or a significant influx of outdoor air during a smog event, reactions between ozone and d-limonene produced hydroperoxides. Hydrogen peroxide (H2O2) presumably constituted the majority of the measured hydroperoxides, although a small amount of organic hydroperoxides (ROOH) may have contributed to the signal. Total hydroperoxides were 1.0−1.5 ppb at low air exchange rates (0.5−4 h-1) and 0.6−0.8 ppb at high air exchange rates (12−18 h-1). The net estimated yield ranged from 1.5 to 3.2%, consistent with values reported in the literature. Based on these yields and typical indoor scenarios, peak indoor concentrations of H2O2 are projected to be comparable with, but not significantly larger than, peak outdoor concentrations. Hygroscopic secondary organic aerosols (SOA; 10−100 μg m-3) were simultaneously generated by the ozone/d-limonene reactions; their co-occurrence with H2O2 provides a mechanism whereby H2O2 can be transported into the lower respiratory tract. The results demonstrate that reduced air exchange rates lead to increased concentrations of H2O2 and SOA as well as a shift in the size-distribution toward larger particles (0.3−0.7 μm diameter), potentially increasing the amount of H2O2 delivered to the lower respiratory region. This study increases our understanding of H2O2 exposures, including exposures to H2O2 associated with co-occurring hygroscopic aerosols. It also re-emphasizes the potential of ozone-driven chemistry to alter indoor environments, often producing products more irritating than their precursors.