Sample Collection

Deposition surfaces for optical analysis that were included in the Southern California museum experiments described in Chapter 2 consisted of white artists’ canvas and Whatman 42 filter paper. Some of the paper and canvas samples were clamped into aluminum frames, similar to those described in Chapter 2 for the quartz cloth deposition plates. Those quartz cloth deposition surfaces will be used here as the basis for measurement of black carbon-particle deposits. The quartz deposition surfaces were left in place for one full year, beginning in June 1987. Sets of the canvas and paper deposition surfaces were left in place for a summer period (July 2 through August 31, 1987), a winter period (November 23, 1987, through January 30, 1988), and for the entire year. During the summer and winter periods, airborne particle samples were collected over 24-hour periods every sixth day.

Measurements of the diffuse reflectance spectra of the canvas and paper deposition surfaces were made before and after use with a Diano Match-scan II reflectance spectro- photometer (Diano Corp., Woburn, Mass.). The measurements were made at three specific, recorded positions on the sample surface in order to maximize the ability to detect slight color changes. Each sample was analyzed a few days prior to placement at the sampling site, then reanalyzed after removal. Measurements were made in triplicate to determine the variability of measurements taken at the same position on the sample, at different positions on the sample, and at the same position but on different analysis days. Field blanks also were analyzed and used to correct for instrument drift over the sample collection period. The initial color of the samples and the color after exposure were calculated in terms of the tristimulus values X, Y, and Z, and also the L*, a*, b* color scale (CIE 1978, Billmeyer and Saltzman 1981) for the CIE 2 ° standard observer (CIE 1931) using illuminant C. Small changes in the white deposition surfaces associ- ated with slight degrees of soiling are easily interpreted using this system. The ∆

E value, indi-

cating the overall color change of a sample over the course of the experiment, also was determined.

The minimum color change detectable was determined by the variability in the samples and blanks. The largest variability was found for the L* coordinate and for the paper

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

measurements were taken of the canvas samples, while ∆

E values of 0.5 units or greater were found to be statistically significant when measurements were taken of the paper samples.

Results and Discussion

Over the sampling periods of three months to a year, the canvas and paper surfaces experienced

a measurable color change at every site. In most cases, the color change consisted of a combina- tion of darkening and yellowing of the surface. At all sites, there was a greater color change observed with the horizontally oriented samples than with the vertically oriented samples. The Sepulveda House and the Southwest Museum are the only two sites studied at which darkening of the vertically oriented surface was detected during a one-year exposure period. These optical data are presented in detail in the final project report (Nazaroff et al. 1990).

Canvas-covered deposition plates that had been placed inside a display case at the Southwest Museum and inside the isolated display bedroom at the Sepulveda House also were analyzed optically. The nearly airtight enclosure used at the Sepulveda House to protect the artifacts both from pollutants and from temperature and humidity fluctuations was extremely effective in reducing the darkening of horizontally oriented white surfaces. By con- trast, the display cases at the Southwest Museum were not airtight. Air gaps of approximately 4 mm existed between each of the panels of glass that enclosed the display case. The color change experienced by the horizontally oriented canvas deposition plates inside the display case after one year was roughly twice those present on the horizontally oriented canvas samples placed in the main portion of the same building after 22 weeks. Thus, the display cases at this site afforded no significant protection from soiling.

The fluxes of black elemental-carbon particles to indoor horizontal surfaces, as measured by combustion analysis applied to the quartz fiber cloth deposition surfaces, are presented in Table 5.1 (p. 94) for four of the five sites. (The horizontal deposition plate at the remain- ing site, the Norton Simon Museum, was inexplicably removed from the museum during the course of the study.) The fluxes of elemental carbon were highest at the two sites lacking in par-

ticle filtration equipment. Deposition velocities (v d ) for elemental carbon also are presented in

Table 5.1. These deposition velocity values are computed from the following expression: (flux to surface ( µ g/m2 ⋅ s)

V d (m/s) = airborne concentration ( µ g/m3)

using the indoor yearlong average of airborne elemental carbon concentrations measured during the experiments described in Chapter 2.

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

Elemental Carbon

Deposition velocity Site

Flux

( µ g/m 2 • day)

(10 -4 m/s)

Getty Museum

< 0.7 Scott Gallery

7 ± 3 0.9 ± 0.4 Southwest Museum

19 ± 3 1.0 ± 0.2 Sepulveda House

Table 5.1. Fluxes and deposition velocities of elemental carbon to horizontal surfaces inside museums.

At all sites, the quantity of elemental carbon deposited to vertical surfaces in a one-year period was below the detection limit of the analytical method. Using that information, an upper limit in µ g/(m2 day) can be placed on the elemental carbon flux to vertical surfaces, based upon the elemental-carbon detection limit of 0.1 µ g/cm2. This value yields an upper limit to the elemental-carbon deposition velocity at the Sepulveda House of 5 x 10-6 m/s. By comparison, the measured deposition velocities for sulfate, nitrate, and ammonium ions to vertical surfaces at the Sepulveda House were 4.8, 3.3, and 4.9 x 10-6 m/s, respectively. This comparison indicates that the deposition velocity of elemental carbon on vertical surfaces at that site was not appreciably larger than the velocities of ionic species measured. The upper limits for the elemental carbon deposition velocities at the other sites are higher and less informative, due to lower indoor elemental-carbon concentrations. However, the deposition velocity for other submicron-size particle species (e.g., sulfates) at these sites is known from ion chromatographic analysis of airborne particle concentrations and resultant deposition fluxes (Chapter 2). The size-dependent deposition velocities for the overall aerosol also are known as a result of SEM analysis of airborne and deposited particles (Chapter 2). These deposition velocity data were combined with a knowledge of the size distribu- tion of airborne elemental-carbon particles in Pasadena and Los Angeles (Ouimette 1981) to arrive at a best-estimate value for the elemental-carbon deposition velocity at all sites. These values, when multiplied by the measured ambient elemental-carbon concentrations, give an estimate of the elemental carbon deposition flux to vertical surfaces. These estimated fluxes and deposition velocities for elemental carbon are presented in Table 5.2. (p. 95).

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

Deposition velocity Site

Flux

( µ g/m -2 • day)

(10 -6 m/s)

Norton Simon Museum 0.08 1.5 Scott Gallery

0.15 2.1 Getty Museum

0.20 5.5 Southwest Museum

0.55 3.0 Sepulveda House

Table 5.2. Estimated fluxes and deposition velocities of elemental carbon to vertical surfaces inside museums.

Sample Collection