Deposition velocities indoors

A number of models have been developed for particles indoors (e.g., Nazaroff and Cass, 1989a Sinclair et al., 1990b Nazaroff et al., 1990a Weschler et al., 1996 Wallace et al., 1996, and references therein). This is a complex problem, given the number of potential sources, different deposition velocities for particles of different sizes (e.g., see Chapter 9.A.3 and Nazaroff and Cass (1989b)), the different particle compositions, and the effects of outdoor concentrations and ventila-... 

A limited number of sink effect studies have been conducted in full-sized environments. Tichenor et al. [20] showed the effect of sinks on indoor concentrations of total VOCs in a test house from the use of a wood stain. Sparks et al. [50] reported on test house studies of several indoor VOC sources (i.e., p-dichlorobenzene moth cakes, clothes dry-cleaned with perchloroethylene, and aerosol perchloroethylene spot remover) and they were compared with computer model simulations. These test house studies indicated that small-chamber-derived sink parameters and kj) may not be applicable to full-scale, complex environments. The re-emission rate (kj) appeared to be much slower in the test house. This result was also reported by other investigators in a later study [51]. New estimates of and were provided,including estimates of fca (or deposition velocity) based on the diffusivity of the VOC molecule [50]. In a test house study reported by Guo et al. [52], ethylbenzene vapor was injected at a constant rate for 72 h to load the sinks. Re-emissions from the sinks were determined over a 50-day period using a mass-balance approach. When compared with concentrations that would have occurred by simple dilution without sinks, the indoor concentrations of ethylbenzene were almost 300 times higher after 2 days and 7 times higher after 50 days. Studies of building bake-out have also included sink evaluations. Offermann et al. [53] reported that formaldehyde and VOC levels were reduced only temporarily by bake-out. They hypothesized that the sinks were depleted by the bake-out and then returned to equilibrium after the post-bake-out ventilation period. Finally, a test house study of latex paint emissions and sink effects again showed that... 

The intent of this paper is to present a methodology for estimating, from available information on concentrations and deposition velocities, the potential effects of anthropogenically derived acidic substances on indoor surfaces. Surface accumulation rates are derived that are applicable to all types of indoor surfaces. The discussion of the possible effects of the accumulated substances will concentrate on zinc and aluminum surfaces because data exists on the behavior of these metals in indoor environments (0. Aluminum forms a passivating oxide which protects against corrosion in most environments, while zinc is expected to corrode at a roughly linear rate over its lifetime. 

For the purposes of this discussion, it is reasonable to assume that the outdoor environment is the source of most of the anthropogenically derived substances (4) that are present in the indoor environment. The accumulation rates of species on indoor surfaces are related to the outdoor concentrations of these substances through the relationships among the indoor and outdoor concentrations and the indoor deposition velocities of these species. A substantial amount of data is available on outdoor concentrations (4-13). Simultaneous measurements of outdoor and indoor concentrations are less numerous. Very few measurements of indoor deposition velocities have been made. Estimated ratios of outdoor to indoor concentrations will be used that are based on field data, where available, or best judgments. From the limited experimental measurements, taking into account the relative variations in outdoor deposition velocities as a function of particle size, indoor deposition velocities will be estimated. Using these approximate indoor/outdoor ratios and deposition velocities, the indoor surface accumulation rates for substances contained in airborne particles can then be estimated from prevailing outdoor concentrations. 

Estimating the deposition velocities of gaseous species is considerably more complex than estimating those for substances in particles, in part due to the uncertainties in the sticking and reaction probabilities. Such estimates have not been made but the potential effects of some of the typical gases can be surmised from available data on surface accumulation rates, e.g. sulfate accumulation on indoor zinc and aluminum surfaces is predominantly a result of particulate sulfate deposition rather than a corrosion reaction involving sulfur dioxide (0. 

In order to obtain surface accumulation rates from the indoor concentrations, indoor deposition velocities are needed. These are expected to be considerably lower than outdoor deposition velocities, primarily because of reduced turbulence. Data from the authors (4) and other sources (16) suggest that indoor deposition velocities for substances associated with particles are approximately a factor of 100 lower than outdoor values this factor has been used to estimate values where experimental data are not available. Values for substances in airborne particles are summarized in Table IV. As discussed above, data are not included for gaseous species. 

As discussed in Chapter 4, not only the concentration of pollutants but also the air velocity determines the dry deposition velocity of corrosion stimulants. Since indoors the air velocity is decreased, significantly lower dry deposition velocities will take place. 

In general, the dry deposition velocity will be the combined effect of both resistances. However, at highly turbulent air flow conditions R =0 and the dry deposition velocity is dependent only on the surface processes. Alkaline surfaces, such as lead peroxide or triethanolamine, are ideal absorbers of SO2 for which = 0. In this case, the dry deposition velocity if dependent on the aerodynamic processes. Typical ranges for dry deposition velocities onto various materials imder outdoor and indoor conditions are given in Table 2.1. 

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