Comparing Modeling and Measurement Results
Comparing Modeling and Measurement Results
Figure 2.10 (pp. 51–54) presents a comparison of particle deposition velocity predictions and mea- surements for each of the five study sites. While the bulk sulfate deposition data are shown as a horizontal bar over the diameter range 0.1–1.0 µ m, it is noteworthy that the size distribution of Los Angeles outdoor sulfate aerosols typically peaks strongly within the diameter range 0.5–0.7 µ m. The vertical bars for deposition velocities measured by single-particle analysis reflect estimates of the + 1 σ measurement uncertainty, incorporating factors such as the effect of substrate differences between air samples and deposition plates, and statistical variation due to the small numbers of particles detected. The particle concentrations within the Norton Simon Museum were very low.
Characteristics of Airborne Particles Inside Southern California Museums
A statistically significant number of particles were detected on the mica plate for only two size ranges at this site. Considering the predictions alone, the highest particle deposition velocities are obtained for the homogeneous turbulence airflow regime, and the lowest values are obtained for the laminar forced-flow regime. For any particle size, the differences in deposition velocities among sites for a given flow regime are generally smaller than the differences that would occur for alternative flow regimes at a given site. This observation can be explained as follows: For a given flow regime, the particle-deposition velocity varies as a function of near-wall air velocities. The average near-wall air velocities for the five sites vary over a narrow range—less than a factor of three. However, for a given near-wall mean velocity, particle transport to the surface for the homogeneous turbulence flow regime is much more rapid—characteristically an order of mag- nitude—than for the laminar forced flow regime due to the contribution of eddy diffusion.
The differences between predictions for winter and summer arise from the ther- mophoretic effect. Seasonal differences are greatest at the Norton Simon Museum and at the Scott Gallery, where the mean value of T wall –T air changes sign from summer to winter.
Available information on the factors driving airflow and the corresponding comparison between measured and predicted particle-deposition velocities may be combined as
a basis for deciding the most likely flow regime for particle deposition onto the wall studied at each site. Our appraisal of the information produces the following results: Getty Museum— homogeneous turbulence from 0800–1800 hours and natural convection at other times; Norton Simon Museum—laminar forced flow; Scott Gallery—geometric mean of homogeneous turbu- lence and laminar forced flow; Sepulveda House—either homogeneous turbulence at all times or homogeneous turbulence during 1000–1500 hours daily with natural convection at other times; Southwest Museum—homogeneous turbulence. Best-estimate values of particle deposi- tion velocity as a function of particle size, obtained from modeling predictions for these flow regimes, are presented for several particle sizes in Table 2.4. The range of deposition velocities among sites for a given particle size is approximately 15–30.
Further discussion is warranted regarding the comparison between measure- ment results and predictions corresponding to these best-estimate values. At the Getty and Southwest museums, measurement results shown for the size ranges of the smallest particles are lower than the predicted values. The measurement of particles in these size ranges may be more uncertain than is reflected in the error bars. A large evaporative loss of very fine particles may occur during vapor deposition of carbon onto a sample in preparation for SEM analysis. If the losses are not constant from one sample to another, then the measured deposition velocity will
be inaccurate in general. Since the measurement is based on the ratio of particles detected on only one deposition plate to those detected on several filter samples, the occurrence of large error is more likely to yield a result that is too low rather than too high.
Characteristics of Airborne Particles Inside Southern California Museums
Scott Norton Simon
Gallery Museum
0.8 0.1 Obtained as the mean predicted deposition velocity for summer and winter periods, using the following flow
regimes: Southwest Museum—T; Getty Museum—NC/T; Sepulveda House—range of values with lower limit cor- responding to NC/T and upper limit corresponding to T; Scott Gallery—geometric mean of L and T; Norton Simon Museum—L. See Table 2.3 for definition of flow regimes.
Table 2.4. Best-estimate values of the annual average particle-deposition velocity (x 10 -6 ms -1 ) to the wall studied at each museum site.
At the Norton Simon Museum, the deposition-velocity data based on single- particle analysis for the smallest particles agree well with predictions for forced laminar flow. This flow regime is consistent with the general flow conditions in that building: Air is discharged into the galleries through perforated ceiling tiles and returned to the mechanical ventilation sys- tem at low velocity through the hallways. The measured deposition velocity for sulfates and for particles of 0.3 mm diameter is higher than that predicted for a laminar flow regime alone. Infor- mation about the size distribution of sulfates within this museum is lacking; furthermore, because of the low sulfate concentrations generally present in this museum, the sulfate-deposi- tion velocity is based on samples that are close to the detection limit.
At the Scott Gallery and the Sepulveda House, the agreement between best- estimate predictions and measurements is very good. The agreement is excellent for the deposi- tion plate mounted on a stretched canvas frame at the Sepulveda House.