Surface, indoor accumulation rates

Sinclair, J. D., L. A. Psota-Kelty, C. J. Weschler, and H. C. Shields, Measurement and Modeling of Airborne Concentrations and Indoor Surface Accumulation Rates of Ionic Substances at Neenah, Wisconsin, Atmos. Environ., 24A, 627-638 (1990b). 

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. 

To the best of our knowledge, the only measurements of surface accumulation rates of specific species on indoor surfaces have been conducted by us and our coworkers (1,4). These data can be represented in terms of contributions from fine and coarse particles using a procedure described elsewhere (0. Average accumulation rates on zinc and aluminum surfaces derived from studies at eleven urban and three rural areas are given in Table V. Also given in Table V... 

In order to meet the above-mentioned conditions and also to minimize dust entry, modern cases are predominantly highly sealed, which implies that the air exchange rate would be lowered to a minimum. It is to be assumed that the high surface to volume ratio, which especially characterizes smaller enclosures, combined with unsuitable construction materials and almost static conditions, enhance the accumulation of chemical compounds. Hence, the comparison of the indoor environment as a reaction vessel , as stated by Weschler and Shields (1997), comes to a head in museum showcases. 

The RH in most indoor environments is usually not above 70 percent and, thus, the CRH of most common metals is seldom exceeded. The time-of-wetness will be quite small. The corrosion rate is likely to be comparable to the outdoor rate (at similar contaminant levels) when the surfaces are dry. Such rates are insignificant compared to the wet rates for most metals (18). In many cases, the anions associated with deposited substances may play the dominant role in surface processes (24). The concentrations of sulfate, nitrate, and chloride, which accumulate on these surfaces, are likely to increase continuously. After 10 years exposure, total anion concentrations of five to ten /ng/cm can be expected in urban environments. These anions, especially chloride, are well known to dramatically affect the corrosion rates of many metals in aqueous solutions. This acceleration is often a result of solubilization of the surface metal oxide through complexation of the metal by the anions. Chloride, in particular, can dramatically lower the RH above which a moisture film is present on the surface, since chloride salts often have low CRHs (e.g., zinc chloride - < 10 percent calcium chloride - 30 percent and aluminum chloride - 40 percent). The combination of the low CRHs of chloride salts and the well documented ability of dissolved chloride to break down metal oxide passivation set chloride apart from the other common anions in ability to corrode indoor metal surfaces. Some nitrate salts also have moderately low CRHs (e.g., zinc nitrate -38 percent calcium nitrate - 49 percent aluminum nitrate - 60 percent). 

We have also noted that chloride accumulation on indoor zinc or aluminum surfaces can occur at dramatically different rates (0. At locations where large differences are observed, hydrogen chloride has been implicated to be the source of much of the chloride accumulation. This difference in behavior was attributed, in part, to the slightly higher acidity and higher ammonium content of the aluminum surfaces, both of which will inhibit the reaction of hydrogen chloride with aluminum and will enhance the volatilization of hydrogen chloride. 

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