Heterogeneous oxidation of indoor surfaces by gas-phase hydroxyl radicals

We investigate heterogeneous oxidation kinetics of monolayer-thick, surface-sorbed organics, namely di-n-octyl phthalate (DnOP) and palmitic acid (PA), with gas-phase OH. The pseudo-first order rate constants for organic loss at OH concentrations of 1.6 × 10⁸  molecules/cm³ are: (2.3 ± 0.1) × 10^-4 to (4.8 ± 0.8) × 10^-4  s^-1 , and (1.3 ± 0.5) × 10^-4  s^-1 for DnOP and PA, respectively. Films developed in indoor office environments over a few weeks are also oxidized using the same OH concentration. Heterogeneous decay rate constants of mass signals from these films, attributed to phthalates (MW = 390.6) and to PA, are similar to those for the single-component films, ie, (1.9 ± 0.4) × 10^-4 to (3.4 ± 0.5) × 10^-4  s^-1 , and (1.1 ± 0.4) × 10^-4  s^-1 , respectively. These results suggest that the lifetimes for OH heterogeneous oxidation of monolayer-thick indoor organic films will be on the timescale of weeks to months. To support this argument, we present the first analysis of the mass transfer processes that occur when short-lived gas-phase molecules, such as OH, are taken up by reactive indoor surfaces. Due to rapid chemical production, the diffusion limitation to mass transfer is less important for short-lived molecules than for molecules with little chemical production, such as ozone.