Aerosol Chemistry and Physics - Aerosols: Measurement, Dosimetry, and Health Effects - page 136

materials and furnishings have much higher exposed surface areas than indoor aerosols. Lots of
recent research shows that aerosols can also form indoors when indoor ozone reacts with smog
precursor compounds like terpenes from cleaning products and fragrances. Figure 6.2 does not show
sorption and desorption of semi-volatile organic compounds (SVOCs) to and from indoor surfaces,
airborne particles, and settled dust, but these important processes are discussed in Sections 6.2
through 6.4 and by Weschler and Nazaroff (2010).
UFP from indoor combustion (cooking, heating, smoking, etc.) and indoor chemical reactions
inevitably coagulate by collision with each other and iniltrated particles of outdoor origin. Particles
are removed by deposition to surfaces, and those larger than 0.8 μm can also be resuspended by
indoor activities of the occupants (walking, vacuuming).
Because of building characteristics, how people spend time indoors also strongly inluences
the concentrations and characteristics of PM 2.5 to which they are exposed. For example, Reff
et al. (2005) found infrared signatures of meat cooking (amides) on indoor PM 2.5 in many homes,
and the signatures were even higher in the personal cloud. When there are no indoor sources of
PM, such as in unoccupied buildings, indoor PM concentrations are typically less than half of
outdoor concentrations. Blondeau et al. (2005) found this to be the case in a study of classrooms
in France. During school days the levels of PM increased, and the larger the particles, the
greater the increase. The data showed that the school day activities of the occupants caused
resuspension of particles that had already settled out on the loors and other surfaces, even
when there were no apparent indoor sources of PM (Parker et al., 2008). Similar increases in
concentrations of coarse particles were reported for mechanically ventilated schools in Utah by
Parker et al. (2008).
Although no U.S.-wide indoor PM speciation trends network operates, more than 20 years of
research (Weschler, 2011a,b) has uncovered intriguing differences between indoor and outdoor PM,
as illustrated in the following. The irst example shows how an ordinary unoccupied home conditions
iniltrating aerosol; the second points to the inluence of indoor materials on attempts to characterize
indoor PM, while the third provides evidence for the counterintuitive notion that ine particles are
produced indoors by practices that residents often consider protective and health promoting.
6.1.5.1 Building Envelope
Using real-time concentration and ventilation measurements in an unoccupied house in Fresno,
California, Lunden et al. (2003a,b) found that indoor particulate sulfate and soot (black carbon)
acted as conservative tracers for iniltration of outdoor PM 2.5 , but indoor PM 2.5 had much less ammo-
nium nitrate than predicted from the penetration factors for elemental carbon. They also found that
the indoor ammonia and nitric acid concentrations were usually lower than the outdoors. The obser-
vations are consistent with disruption of the NH 4 NO 3 gas/particle equilibrium as the indoor surfaces
took up nitric acid and ammonia, as predicted by thermodynamics (Seinfeld and Pandis, 2006). The
afinity of indoor surfaces for these species led to evaporative dissociation of iniltrated particulate
ammonium nitrate.
6.1.5.2 Indoor Materials
Several groups of investigators have found that conventional measurements of concentrations of
indoor carbonaceous particles can exceed indoor PM 2.5 mass concentrations (Landis et al., 2001;
Pang et al., 2002). Pang et al. showed that conservation of mass was not actually violated and that
the measured indoor particulate carbon concentrations were reduced substantially when denuders
adsorbed semi-volatile organic gases upstream of the particle collection medium (quartz iber ilter).
Lunden et al. (2008) conirmed that accounting for the indoor sampling artifacts was essential for
understanding gas-to-particle and gas-to-surface partitioning indoors.
Meanwhile, indoor building materials and furnishings have been found to emit organic gases
with a wide range of volatilities (e.g., Hodgson et al., 2000, 2002). Rudel et al. (2003) reported
high indoor concentrations of phthalate esters (from vinyl looring). Other indoor air measurements