Aerosol Chemistry and Physics - Aerosols: Measurement, Dosimetry, and Health Effects - page 156

phenanthrene) onto airborne particles (at their total suspended particle concentration 20 μg m −3 ) was
2.1 × 10 3 μg m −2 . From the data of Naumova et al. (2003), Weschler calculated the mass loading of the
same compounds per unit of particulate surface area as 7.7 × 10 3 μg m −2 in a room of volume 40 m 3 .

6.3.3.2 Indoor Materials
Morrison and Nazaroff (2000) found that the actual surface area of nylon carpet (with pad) was
about 100 times its nominal surface area, and Weschler used their data to calculate the surface load-
ing capacity for hexadecane or phenanthrene in the example room. He found that the surface loading
(7.1 × 10 3 μg m −2 ) for the carpet was similar to that found by Naumova et al. (2003) for ine particles.
Therefore, indoor surfaces, with their much higher total surface area than indoor particles, are very
likely to be important for any indoor aerosol process that depends on gas/surface interactions.
Weschler estimated that the surface area of the particles would be 1.6 × 10 −3 m 2 , compared to the
room's nominal surface area of 36 m 2 , not counting surface porosity or furnishings. The carpet and
pad contributed actual surface area of 1000 m 2 . Although the surface area of the airborne particles
is miniscule compared to the surface area of the room and its furnishings, it is the surface of the
particles that makes contact with the respiratory systems of the occupants. Particle motion provides
the vector for moving low-volatility pollutants from one indoor surface to another: Weschler showed
that particle deposition is the main route for transport of the plasticizer diethylhexylphthalate
(DEHP; vapor pressure 1.9 × 10 −10 atm at 25°C) to the indoor surfaces.

6.3.3.3 Adsorption Indoors
Indoor aerosol behavior is strongly inluenced by the presence of indoor surfaces because build-
ing materials and furnishings act as reservoirs of sorbed compounds. At equilibrium, sorption to
indoor surfaces follows Equation 6.1. Won et al. (2000, 2001) determined that partitioning coef-
icients of organic gases to indoor materials depend on p o , the subcooled liquid vapor pressure.
Weschler (2003) showed that the molecular weight and the negative log of vapor pressure are
linearly related for a large number of organic compounds found indoors, as shown in Figure 6.9.
Thus, for compounds whose vapor pressures are unknown, partitioning behavior can be predicted
from knowledge of only the molecular weight.
Table 6.7, taken from Weschler (2003), shows how a range of common organic pollutants parti-
tion in the example room. The lower the vapor pressure, the more signiicant is the particulate-bound
fraction on the transport and fate of the compound, even if that fraction is very small. Inhalation

450

400

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300

250
200
150
100
50

Y=-28.7X+ 42
R 2 = 0.88

0

-12 -10 -8 -6 -4 -2 0

Log (saturation vapor pressure, atm)

FIGURE 6.9 Molecular weight versus the log of the vapor pressure for nonpolar compounds. (From Won,
D. et al., Environ. Sci. Technol ., 34, 4193-4198, 2000; Weschler, C.J., Atmos. Environ ., 37, 5455, 2003. With
permission.)