Hydroxyl radical as an indoor oxidant

The hydroxyl radical (OH) is the dominant oxidant in outdoor air. Indoors, its role is secondary to ozone (O₃). However, growing evidence indicates that OH contributes more significantly to indoor oxidation chemistry than previously recognized. This review critically examines the state of knowledge regarding OH as an indoor oxidant, synthesizing findings from modeling studies, laboratory experiments, and field measurements. Indoor OH concentrations range from 1 to 100 ppq under common conditions, with a central tendency of ∼5 ppq-one to two orders of magnitude below urban daytime outdoor levels and comparable to urban nighttime values. The primary sources of OH in occupied environments are gas-phase reactions between ozone and monoterpenes or alkenes, particularly limonene, 6-methyl-5-hepten-2-one, and geranyl acetone. These compounds are also important sinks for OH, and these dual roles tend to buffer indoor OH concentrations. Photolysis of nitrous acid can be an important indoor OH source under some conditions. The dominant sink for indoor OH is homogeneous gas-phase reactions with volatile organic compounds (VOCs). Total OH reactivity is commonly on the order of 50 s^-1 for occupied residences and can be much higher during events that release VOCs, such as cooking and cleaning. Radical cycling via the NO + HO₂ reaction can significantly amplify indoor OH concentrations, depending on indoor NO levels. OH-initiated autoxidation chemistry contributes to the long-term oxidation of semivolatile organic compounds on indoor surfaces. OH chemistry, spatially variable and strongly influenced by occupant behavior, warrants continued investigation given its relevance to indoor air quality and human health.