Factors Governing the Soiling Rate of Indoor Surfaces Due to Particle Deposition

Factors Governing the Soiling Rate of Indoor Surfaces Due to Particle Deposition

In Chapter 3, a detailed mathematical model was developed to predict the time-dependent chem- ical composition and size distribution of indoor aerosols. That model accounts in detail for the effects of emission, filtration, ventilation, coagulation, and deposition. As shown in Chapter 4, the model is capable of making accurate predictions of the airborne particle characteristics in Southern California museums based on building parameters and data on outdoor particle properties. Pre- dictions of the rate of particle deposition to surfaces in museums were shown to be reasonably con- sistent with measurement results (Chapters 2 and 5). In the present chapter, that model is applied to evaluate the effectiveness of control measures aimed at reducing the soiling rate.

Several approximations are employed for this purpose. First, only two chemical components of the airborne particulate matter—elemental carbon and soil dust—are considered. Elemental carbon is directly associated with the blackness of the airborne particulate matter (Cass et al. 1984), and so this material is a major concern as a soiling agent. Soil dust contributes

Protecting Museum Collections from Soiling Due to Deposition of Airborne Particles

much of the brown color seen in coarse airborne particle samples and thus may also lead to soil- ing. Elemental carbon is predominantly found among fine particles with a mass-median diame- ter in the vicinity of 0.1 µ m (Ouimette 1981). Airborne soil dust, by contrast, occurs primarily as coarse particles, larger than 2 µ m in diameter (see Chapter 4, Table 4.3, p. 71). Because of the rel- atively large particle size of this material, the dominant mechanism for soil-dust deposition indoors is gravitational settling onto surfaces with an upward orientation; the soiling of vertical and downward-facing surfaces by soil dust is minimal. By contrast, the gravitational settling velocity of particles with 0.1 µ m diameter, such as those of elemental carbon, is less than typical deposition velocities for particles of that size due to convective diffusion and thermophoresis; as a result, surfaces with any orientation may be soiled by the deposition of elemental carbon particles.

For the purposes of this chapter, a characteristic time for soiling, τ s , associated with the accumulation of a soiling deposit is defined as the time necessary to accumulate an effective surface-area coverage of 0.2% by the particles of interest. As explained in Chapter 5, a 0.2% surface coverage by black particles represents the approximate point at which soiling of a white surface first becomes apparent to an observer. The actual point at which soiling becomes perceptible may well depend on additional factors, such as the chemical composition and mor- phology of the particles and the optical properties of the surface of interest. Since the light- absorbing properties of elemental carbon differ from those of soil dust, the degree of surface coverage needed to produce perceptible soiling probably differs for the two components. The information needed to combine the accumulation of the two components into a single soiling rate is lacking, so the control objective shall be to increase the value of τ s associated with both soiling components of the atmospheric aerosol to as great an extent as possible for all surfaces of interest.