Heterogeneous oxidation of indoor surfaces by gas-phase hydroxyl radicals

Abstract

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.

Indoor boundary layer chemistry modeling

“…For these conditions, the FMBL is approximately 10 mm deep but OH remains at the bulk room air concentration down to about 1 mm or less. The inflection occurs at approximately 1 mm, which is consistent with the prediction by Alwarda et al that the diffusion boundary layer for OH under indoor conditions would range from 0.5 to 1.6 mm. We predict that the concentration where y = mfp is 22 molec/cm 3 .…”

“…We predict that the concentration where y = mfp is 22 molec/cm 3 . Based on the gradient, Equation (18), the flux is 4.3 × 10 5 molec/(cm 2 s), which is at the low end of the range (5 × 10 5 to 200 × 10 5 molec/(cm 2 s)) predicted by Alwarda et al…”

“…Alwarda et al observed substantial heterogeneous oxidation of phthalate esters, saturated monocarboxylic acids and “genuine indoor surface samples” by OH in laboratory flow‐tube experiments. They hypothesized that OH flux to indoor surfaces may be much higher than otherwise predicted, recognizing that OH can be generated within the boundary layer, that is, the OH radicals that collide with a surface do not have to diffuse through the full boundary layer from the well‐mixed bulk of the room.…”

“…In this work, we evaluate the Alwarda et al hypothesis in a more rigorous manner by combining the transport characteristics of indoor boundary layers with ozone‐alkene chemistry to derive concentration profiles and deposition rates of reactive intermediates. The overall goal is to assess the importance of boundary layer chemistry toward the flux of reactive molecules to indoor surfaces.…”

Impact of material emissions and sorption of volatile organic compounds on indoor air quality in a low energy building: Field measurements and modeling

“…or gaseous oxidants such as ozone and free radicals. 53 Moreover, the variability between the campaigns, called temporal variability, was assessed by comparing these results with those obtained during MERMAID 1. No significant differences between the periods were noticed as the variability between the two campaigns is within the variability of the location (see in the supporting information, Table S9).…”