Production of the Hydroxyl Radical in Indoor Air

Abstract

Certain alkenes known to have significant indoor concentrations react with O 3 at rates that are comparable to or faster than typical air exchange rates in commercial and residential buildings; these reac tions produce the hydroxyl radical (OH) in meaningful yields. The present study uses a one-compartment mass balance model to investigate the indoor production of OH as a consequence of such chemistry. Important sources for indoor OH include the reactions of O 3 with d -limonene, α-terpinene, 2-methyl-2-butene, and α-pinene, while important sinks include the reaction of indoor OH with d -limonene, nitrogen dioxide, ethanol, formaldehyde, carbon monoxide, and isoprene. The modeling indicates that the OH sources and sinks equilibrate in tens of mil liseconds, an interval much shorter than the residence time for a parcel of indoor air. Indoor O 3 drives the production of OH, which scales nonlinearly with the concentration of O 3 . At 20 ppb indoor O 3, using average indoor concentrations for key reactants, the model predicts a steady-state indoor OH concentra tion of 6.7 × 10 - 6 ppb (1.7 × 10 5 molecules/cm 3 ). This is smaller than a typical midday outdoor OH concentration of 2 × 10 -4 ppb (5 × 10 6 molecules/cm 3 ) but larger than measured nighttime outdoor levels. The OH radical at 6.7 × 10 - 6 ppb will oxidize saturated organics 2−5 orders of magnitude faster than O 3 at 20 ppb. In many cases, the expected oxidation products are more irritating and corrosive than their precursors.