Environmental Engineering Reference
In-Depth Information
mostly concentrated on the fi nest fraction, although
some researchers (e.g. Kappos
et al
. 2004) have
reported diffi culties in assessing the health effects of
ultra fi ne particles i.e. those less than 0.5
in the previous paragraph have generally taken place
fi rst in investigations on lead, and since the now
widespread introduction of unleaded fuels, have sub-
sequently been applied to other trace elements.
However, it comes as no surprise that historically,
the largest body of published geochemical research
on urban particulate materials is concerned with
lead, a fact that is refl ected in the following
sections.
m. Some
of the trace elements of toxicological concern in the
atmospheric aerosol include As, Cd, Cr, Hg, Mn, Ni,
Pb, and V. Exposure to airborne compounds of these
elements in occupational environments has been sus-
pected of causing effects ranging from sinusitis,
asthma and chronic bronchitis to pneumonia, lung
hemorrhage, lung cancer, and brain hemorrhage
(Doadrio 1984; Sadiq & Mian 1994; Crosby 1998).
Although the possible health effects of exposure to
those elements and compounds in open, urban
atmospheres are diffi cult to ascertain, their potential
toxicity has nonetheless encouraged much scientifi c
research into the sources and levels of particulate
trace elements in urban aerosols. Among the most
relevant emission sources of urban suspended parti-
cles, the following can be cited.
μ
4.2.1 Urban aerosols
Concern over the quality of urban air has driven the
need for targets to improve emissions with legislative
controls implementing the agreed improvements
(Williams 2004). These targets have been based on
the sources of emissions, beginning in the UK in the
1950s as a reaction to the “London smogs” of 1952
and concentrating mainly on smoke. However, sus-
pended particles in an urban aerosol can have their
origin outside the city limits or in typically urban
sources (i.e. vehicular traffi c, domestic heating
systems, etc.) and much research has concentrated on
attempts to differentiate natural from anthropogenic
contributions (discussed later in section 4.2.2). There
is a general agreement (cf Van Dingenen
et al.
2004)
that the size of urban suspended particles follows a
bimodal distribution, in which particulate matter of
a “natural” origin (resuspended soil and mineral par-
ticles) constitutes the coarsest fraction of the urban
aerosol, while particles emitted from anthropogenic
sources (combustion processes, in most cases) are
smaller, with a diameter usually below 2
4.2.1.1 Traffi c
The emission source most thoroughly researched in
urban environments is automotive traffi c. The parti-
cle size distribution of exhaust aerosols is strongly
affected by driving patterns; for example, freeway
exhaust particles usually exhibit median diameters
close to 0.1
m, whereas urban driving causes a dis-
tinct shift towards coarser particle sizes, probably
around 5
μ
m and larger. Urban traffi c has in the past
contributed large amounts of lead to the atmospheric
aerosol as a consequence of the use of leaded petrol
in internal combustion engines. Kowalczyk
et al.
(1978) concluded that the absolute concentration of
Pb associated with motor vehicle particles could
range from about 40% if there is little contribution
from diesel traffi c, to 4% when the contribution of
diesel traffi c is signifi cant (the large amount of car-
bonaceous particles emitted by diesel vehicles exerts
a noticeable diluting effect). However, the gradual
shift from leaded to unleaded petrol has drastically
reduced vehicular emissions of this element, to the
extent in fact that some countries have dropped lead
from their atmospheric monitoring programs, con-
centrating instead on Zn and Cu from the original
fi ve metals of concern which included Cd, Pb, and
Ni (Foster & Charlesworth 1996). Although studies
have shown a reduction in the lead concentration in
μ
m.
Investigations in different cities have concluded that
the dominant particle size in urban environments lies
in the sub-micron size range (Oberdörster
et al.
1995; Kasparian
et al.
1998).
Size and chemical composition determine the
potential health effects of atmospheric particles.
Particulate matter with a diameter below 10
μ
m
(PM
10
) is considered “inhalable”, whereas atmos-
pheric particles with a diameter less than 2.5
μ
m
(PM
2.5
) are regarded as “respirable”. The PM
2.5
frac-
tion has been found to be associated with adverse
health effects, such as mortality and asthma (Kappos
et al.
2004), as well as with ambient air quality
problems, including visibility reduction (Larson
et al.
1989; Lin & Tai 2001). Consequently, research
efforts on the geochemistry of the urban aerosol have
μ
