We present a model for the growth of organic films on impermeable indoor surfaces. The model couples transport through a gas-side boundary layer adjacent to the surface with equilibrium partitioning of semivolatile organic compounds (SVOCs) between the gas phase and the surface film. Model predictions indicate that film growth would primarily be influenced by the gas-phase concentration of SVOCs with octanol-air partitioning (Koa ) values in the approximate range 10≤log Koa ≤13. Within the relevant range, SVOCs with lower values will equilibrate with the surface film more rapidly. Over time, the film becomes relatively enriched in species with higher log Koa values, while the proportion of gas-phase SVOCs not in equilibrium with the film decreases. Given stable airborne SVOC concentrations, films grow at faster rates initially and then subsequently diminish to an almost steady growth rate. Once an SVOC is equilibrated with the film, its mass per unit film volume remains constant, while its mass per unit area increases in proportion to overall film thickness. The predictions of the conceptual model and its mathematical embodiment are generally consistent with results reported in the peer-reviewed literature

Nazaroff, W. W.

Weschler, Charles J.

Indoor Air

Weschler, CJ

Nazaroff, WW

eScholarship - University of California

UC BerkeleyUC Berkeley Previously Published WorksTitleOrganic Films on indoor surfacesPermalinkhttps://escholarship.org/uc/item/08t6r0w9ISBN9788379472604AuthorsWeschler, CJNazaroff, WWPublication Date2017 Peer reviewedeScholarship.org Powered by the California Digital LibraryUniversity of California  Healthy Buildings 2017 Europe July 2-5, 2017, Lublin, Poland  Paper ID to be filled in by Organizers ISBN:  978-83-7947-232-1   Organic Films on indoor surfaces  Charles J. Weschler1,2* and William W Nazaroff3  1Rutgers University, Piscataway, NJ 08854, USA 2Technical University of Denmark, DK-2800 Lyngby, Denmark 3University of California, Berkeley, CA 94720-1710, USA *Corresponding email: weschlch@rwjms.rutgers.edu     Keywords: absorption, partitioning, semivolatile organic compounds, surface chemistry, window films   1 Introduction When new and clean, surfaces exposed to indoor environments can possess distinctive chemical properties. Over time, organic films form on indoor surfaces via sorption of gas-phase organics and deposition of airborne particles. On vertical surfaces in typical indoor environments, calculations suggest that the former process occurs at a much faster rate than the latter. In what follows, we illuminate the dynamics of organic film formation, exploring the important emergent properties associated with the growth of such films on indoor surfaces. This study has several specific aims: (1) to apply knowledge of the physics and chemistry of semivolatile organic compounds (SVOCs) so as to construct a conceptual model of the dynamics of surface film growth focusing on uptake of gas-phase SVOCs with varying Koa values; (2) to reconcile and interpret what we know from experiments with knowledge of physical and chemical behavior of SVOCs; and (3) to add new insights to the collective scientific understanding of indoor surface film occurrence and growth.  2 Methods In an indoor setting, one might anticipate that gas-phase SVOCs initially adsorb to impermeable indoor surfaces. This process is envisioned to occur quickly, but to only form a thin layer, perhaps of the order of a monolayer thick. Soon SVOCs will begin to absorb into the organics previously accumulated. As the sorption process proceeds, the thickness of this nascent surface film grows. One can describe the process governing the absorption and growth of this organic film in terms of partitioning of SVOC species between the gas-phase and the surface film. Gas-phase SVOCs possessing lower values of octanol-air partition coefficients (Koa) equilibrate (by partitioning into these films) faster than those with larger Koa values. Consequently, the fraction of SVOCs in surface films possessing larger Koa values is expected to increase over time. We have formulated a mathematical model of the growth and evolution of organic films on impermeable surfaces. The formulation is based on a blended treatment of SVOCs in which species can be modeled as individual SVOCs or clustered into aggregate groupings. The groupings are organized around a basis set of log Koa values; this treatment has parallels to the volatility basis set concept that has been developed for understanding the partitioning of SVOCs between the gas and particle phase (Donahue et al., 2006). The model accounts for the coupled dynamics of film growth and changes in the surface abundance (mass/film area) and volumetric concentration (mass/film volume) for SVOCs sorbed into the film.  3 Results and Discussion The mathematical model predicts several features of organic film growth on impervious indoor surfaces that conform to published experimental observations. Predictions indicate that under typical indoor conditions, the organic constituents would be dominated by species with intermediate log Koa values (~ 9.5-12.5).   Species with smaller log Koa values do not partition strongly to surface films from the gas phase; species with larger log Koa values approach equilibrium only slowly.    Figure 1: Evolution of the thickness of an organic surface film predicted using a multicomponent gas/surface film-partitioning model with initial film thickness of 2 nm.  Using estimates of typical indoor concentrations for SVOCs with log Koa values between 8 and 13, Figure 1 shows modeled organic film thickness versus time over a span of 500 days, at which time the thickness has grown from an initial 2 nm to 14 nm. The model predicts progressive enrichment of the film toward species with higher log Koa values. In the early stages of film growth (~ 10 h), the dominant SVOCs in surface films have log Koa values between 9 and 11; at 100 hours, between 10 and 11, and beyond 1000 hours, between 11 and 12.   Although the specifics will vary, the model indicates that organic surface films grow at a faster rate in the initial days and subsequently grow at a diminished and almost constant rate over time scales on the order of a hundred days. For common indoor conditions, the estimated growth rates are roughly 0.05 – 0.1 nm day-1 at 100 days and 0.03 – 0.06 nm day-1 at 200 days.   The above predictions are consistent with results reported for the organic surface films based on sampling from windows in actual indoor environments (Bennett et al. 2015; Huo et al. 2016; Li et al. 2010; Liu et al. 2003; Melymuk et al. 2016; Pan et al. 2012; Venier et al. 2016), from windows, plates and mirrors in a test house (Bi et al. 2015), and from Petri dishes and aluminum foils exposed to indoor air for different time intervals (Wallace et al. 2017).   4 Conclusions Growing knowledge on this topic will contribute to improved understanding of multiple indoor processes – from surface-mediated indoor chemistry to dermal uptake following contact with organic-covered surfaces.  5 References  Bennett DH, Moran RE, Wu X, et al. 2015. Polybrominated diphenyl ether (PBDE) concentrations and resulting exposure in homes in California: relationships among passive air, surface wipe and dust concentrations, and temporal variability. Indoor Air. 25: 220-229. Bi CY, Liang YR, Xu Y. 2015. Fate and transport of phthalates in indoor environments and the influence of temperature: A case study in a test house. Environ Sci Technol. 49:9674-9681. Donahue NM, Robinson AL, Stanier CO, Pandis SN. 2006. Coupled partitioning, dilution, and chemical aging of semivolatile organics. Environ Sci Technol. 40:2635-2643. Huo CY, Liu LY, Zhang ZF, et al. 2016. Phthalate esters in indoor window films in a northeastern Chinese urban center: Film growth and implications for human exposure. Environ Sci Technol. 50:7743-7751.  Li J, Lin T, Pan SH, et al. 2010. Carbonaceous matter and PBDEs on indoor/outdoor glass window surfaces in Guangzhou and Hong Kong, South China. Atmos Environ. 44:3254-3260. Liu QT, Chen R, McCarry BE, et al. 2003. Characterization of polar organic compounds in the organic film on indoor and outdoor glass windows. Environ Sci Technol. 37:2340-2349. Melymuk L, Bohlin-Nizzetto P, Vojta S, et al. 2016. Distribution of legacy and emerging semivolatile organic compounds in five indoor matrices in a residential environment. Chemosphere 153:179-186. Pan SH, Li J, Lin T, et al. 2012. Polycyclic aromatic hydrocarbons on indoor/outdoor glass window surfaces in Guangzhou and Hong Kong, south China. Environ Pollut. 169:190-195.  Venier M, Audy O, Vojta Š, et al. 2016. Brominated flame retardants in the indoor environment — Comparative study of indoor contamination from three countries. Environ Int. 94:150-160.  Wallace LA, Ott WR, Weschler CJ, Lai ACK. 2017. Desorption of SVOCs from heated surfaces in the form of ultrafine particles.  Environ Sci Technol. 51:1140-1146.  

English

Organic Films on indoor surfaces

We present a model for the growth of organic films on impermeable indoor surfaces. The model couples transport through a gas-side boundary layer adjacent to the surface with equilibrium partitioning of semivolatile organic compounds (SVOCs) between the gas phase and the surface film. Model predictions indicate that film growth would primarily be influenced by the gas-phase concentration of SVOCs with octanol-air partitioning (Koa) values in the approximate range 10≤log Koa≤13. Within the relevant range, SVOCs with lower values will equilibrate with the surface film more rapidly. Over time, the film becomes relatively enriched in species with higher log Koa values, while the proportion of gas-phase SVOCs not in equilibrium with the film decreases. Given stable airborne SVOC concentrations, films grow at faster rates initially and then subsequently diminish to an almost steady growth rate. Once an SVOC is equilibrated with the film, its mass per unit film volume remains constant, while its mass per unit area increases in proportion to overall film thickness. The predictions of the conceptual model and its mathematical embodiment are generally consistent with results reported in the peer-reviewed literature

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