Environmental Engineering Reference
In-Depth Information
predicted source compositions must be non-negative
which means the sum of the predicted elemental mass
fractions for each source must be less than or equal
to 1 (Henry 1987).
Source apportionment based on CMB does not
suffer from the same problems. CMB models assume
that, in addition to the composition of the aerosol at
the receptor site, the elemental composition of the
emissions from the different sources is known. The
individual contribution of each source at a particular
receptor site can then be estimated by solving a
system of linear equations (Gatz 1975; Kowalczyk
et
al.
1978, 1982; Batterman
et al
. 1988; Cornille
et al
.
1990; Adgate
et al
. 1998). Although CMB source
apportionment has certain advantages over multi-
variate methods, its application is restricted to sites
where the emission profi les of the main sources in
the area are known, and the reliability of the results
is limited by the accuracy in the estimates of the
emission profi les.
Depending on its physical characteristics, the air-
borne particulate material may settle onto a surface
of some kind in the urban area. The next sections
follow these particles onto the street and thence
indoors and assess their eventual risk to the environ-
ment as a whole and to those who live in it in
particular.
of the urban aerosol are not the only source of street
dust, which also incorporates a large amount of dis-
placed urban soil as well as particles that never
become fully suspended after they are emitted.
Studies of particle size are important, however, in
identifying whether they pose a health hazard to the
city's population. According to Horowitz (1991),
there is a strong positive correlation between the
decrease in the size of particles and the increase in
the concentration of trace elements, depending on
the greater surface area of the particle and the
increase of the cation-exchange capacity (CEC).
Particles less than 100
m can reach the respiratory
system by inhalation through the mouth or nose, but
of that, only the fraction less than 10
μ
m can reach
the alveoli of the lungs where they can cause irrita-
tion and disease. This is further explored in section
4.3.
Street dust does not remain deposited in place for
a long time. In fact a study by Allott
et al
. (1990) in
a coastal town in northwest England, using
137
Cs,
found that the half-life of street dust was between
190 and 370 days. It is easily resuspended back into
the atmospheric aerosol, to which it contributes a
signifi cant amount of trace elements (Maxwell &
Nelson 1978), or precipitation washes it away
becoming an important component of the suspended
and dissolved solids in street run-off (Vermette
et al
.
1991 and references therein). Consequently, the tem-
poral variability of the concentration of trace ele-
ments in street dust is high (Duggan 1984), and most
studies do not monitor for long enough to evaluate
it. Street dust also presents a pronounced small-scale
heterogeneity (Duggan 1984; Leharne
et al.
1992), a
refl ection not only of the mobility and of rapid envi-
ronmental alteration of street dust, but also of the
heterogeneity in the distribution of its urban sources.
As was mentioned in section 4.2.1, the two main
sources of street dust, and consequently of the trace
elements found therein, are deposition of previously
suspended particles (atmospheric aerosol) and urban
soil. However, there are several point sources whose
emissions contribute directly to the street-dust load
in their proximity (Harrison 1979; Hopke
et al.
1980; Schwar
et al
. 1988). The most relevant among
them is vehicular traffi c.
Car exhaust emissions are responsible for elevated
concentrations of Pb, Zn, Cd, Cu, and Ba in the
vicinity of roads. Lead is obviously associated with
μ
4.2.3 Street dust
Whether a solid particle remains airborne or settles
down onto an urban surface (pavement, soil, roof,
window ledge, playground area, etc.) is related to its
aerodynamic diameter and to weather conditions,
the fi nest materials staying suspended for longer
periods. Fine particles are preferentially removed
from the urban aerosol by wet deposition, whereas
coarse particles are sedimented by dry deposition
(Jaffé
et al.
1993). Solid particles that accumulate on
outdoor, impervious materials are collectively
referred to as “street dust”, whereas particles found
inside urban dwellings are commonly termed “house
or indoor dust” (see section 4.2.4), which suggests
that all this material is extremely fi ne (i.e. less than
10
m) and therefore inhalable. However, studies of
various urban environments (e.g. Sansalone
et al
.
1998) have found that most urban sediments are
greater than 400
μ
m by mass, and therefore the term
“dust” may be inappropriate. The largest particles
μ
