Estimates of the Time Required for Soiling to Occur

Estimates of the Time Required for Soiling to Occur

To calculate the rate of surface area coverage by dark elemental-carbon or soil-dust particles, the size distribution of the particles must be known. Size distributions of outdoor fine elemental-car- bon particles are available for downtown Los Angeles and for Pasadena (Ouimette 1981). The elemental-carbon size distribution for Los Angeles is representative of the size distribution near vehicular sources, with predominantly small (0.1 µ m) particles, while the Pasadena data repre- sent an aged size distribution. The Los Angeles size-distribution data were used to represent ele- mental carbon at the Getty Museum and Sepulveda House sites, while the Pasadena size- distribution data were used to represent elemental carbon at the Norton Simon Museum and the Scott Gallery. An arithmetic mean of the two size distributions was used to represent elemental carbon at the Southwest Museum. The assumption was made that the elemental-carbon size dis- tribution indoors was the same as that outdoors. These size distributions were scaled to match the measured fine elemental-carbon concentrations inside these museums as reported in Chap- ter 2. The measured elemental carbon deposition rates to horizontal surfaces reported in Table

5.1 and the best-estimate deposition velocities from Table 5.2 then were used to calculate elemen- tal carbon mass fluxes. These fluxes were converted into projected area coverages by assuming

a monolayer coverage of spherical particles of density 2.3. The time required for the accumula- tion of a 0.2% surface coverage by elemental carbon alone was calculated. The results are pre- sented in Table 5.3 (p. 96) for both vertical and horizontal surfaces.

Table 5.3 indicates that soiling of horizontal surfaces due to black elemental- carbon particles takes place rapidly, even inside the museums with particle filtration. The times required for a perceptible elemental carbon deposit to accumulate ranges from about one month to one-and-a-half years. If the additional effects of deposited organic matter and soil dust are taken into account, the computed times before perceptible soiling occurs are even shorter.

Measurements of the Rates of Soiling Inside Museums Due to Deposition of Airborne Particles

Site Vertical Horizontal

Norton Simon Museum 18 1.4 Scott Gallery

7.2 0.9 Getty Museum

4.8 1.3 Southwest Museum

2.0 0.3 Sepulveda House

0.3 0.1 Perceptible soiling defined as 0.2% surface coverage by elemental carbon.

Deposition of other colored particles (e.g., soil dust) will act to further decrease the time to reach the onset of perceptible soiling.

Table 5.3. Best estimate of the time (years) required for perceptible soiling to occur on indoor vertical and horizontal white surfaces due to elemental-carbon deposition.

As expected, the estimated time required for perceptible soiling is longer for the three sites with particle filtration systems than for the two sites without them. Among the sites studied, the greatest protection from soiling exists at the Norton Simon Museum, where nearly two decades are required for perceptible soiling of vertical surfaces by elemental carbon parti- cles. At both the Getty Museum and Scott Gallery, deposition fluxes are higher than at the Norton Simon Museum, in part because these facilities have higher infiltration rates of unfiltered air through open doors in the building.

The soiling time estimates presented in Table 5.3 on the basis of experimentally determined deposition velocities and indoor elemental carbon concentrations are in good agree- ment with estimates made for three of the same sites on the basis of the indoor air-quality modeling calculations discussed in Chapter 4. Furthermore, these estimates are in excellent agreement with visual observations of the deposition surfaces. The treatment in Table 5.3 pre- dicts that horizontal surfaces at the Sepulveda House and Southwest Museum will become vis- ibly darkened in much less than one year, that horizontal surfaces of the Scott Gallery will be just barely darkened in a year, but that deposits at the Getty and Norton Simon museums will not be visible at the end of a year. Further, Table 5.3 predicts that Sepulveda House is the only site at which visible deposits will be seen on vertical surfaces within a one-year period. Recorded visual observations of the quartz deposition surfaces after exposure for one year were as follows: For the horizontal surfaces, reports indicated Sepulveda House with “deposit quite dark (grey)”; Southwest Museum with “grey, visible deposit”; Scott Gallery with “deposit faintly visible”; and the Getty Museum with “no visible deposit.” For the vertical surfaces, reports indicated “no vis- ible deposit except Sepulveda House.”