Environmental Engineering Reference
In-Depth Information
distance x is given by the following equation:
exp
H 2
σ
2 Q a l
1
2
c
(
x
) =
(9.9)
1
/
2
z
(
2
π)
σ z u
average mass emission rate per square meter, g m 2
s 1
Q a =
l
=
width of the urban area along the wind axis, m
Equation (9.9) can only be used at downwind distances x
l . It is also necessary that the
crosswind dimension of the urban area be larger than
σ y (
l
)
.
9.2.5
Photo-oxidants
Photo-oxidants are a class of secondary air pollutants formed from some of the primary pollutants
emitted by fossil fuel combustion. The name arises because these chemicals are formed under the
influence of sunlight, and all of them have strong oxidizing capacity. They irritate and destroy
(oxidize) the respiratory tract, eyes, skin, animal organs, vegetation tissues, and materials and
structures. The major representative of this class of chemicals is ozone, O 3 , but other compounds
are included: ketones, aldehydes, alkoxy radicals (RO) peroxy radicals, (RO 2 ), peroxyacetyl nitrate
(PAN), and peroxybenzoyl nitrate (PBN). The symbol R denotes a hydrocarbon fragment, with one
hydrogen missing.
Tropospheric ozone (the “bad” ozone) is to be distinguished from stratospheric ozone (the
“good” ozone). Tropospheric ozone is mainly formed as a consequence of fossil fuel combustion,
while stratospheric ozone is formed naturally by photochemical reactions under the influence of
solar ultraviolet radiation. 5 Some of the tropospheric ozone may be due to intrusion of stratospheric
ozone into the troposphere. This constitutes a background level of ozone. However, compared to
concentrations of ozone in an urban polluted atmosphere, the background ozone is a small fraction
of that concentration.
The current NAAQS (see Section 9.2.2) for O 3 is 120 parts per billion by volume (ppbV),
1-hour maximum concentration. During the 1990s, this standard was exceeded many times per
year in most major metropolitan areas of the United States, especially in cities with high insolation
and temperature and where topographic conditions are preventing good ventilation, such as Los
Angeles, Salt Lake City, Phoenix, Houston, Dallas-Fort Worth, Denver, Atlanta, and other cities.
High ozone levels are observed in most metropolitan areas of the world, especially areas with
high insolation, such as Mexico City, Sao Paulo, Jakarta, Bombay, Cairo, Istanbul, Rome, Athens,
Madrid, and other cities. But rural and remote areas that are downwind from metropolitan areas
are also experiencing elevated ozone concentrations. The high concentrations in rural and remote
areas may contribute to degradation and die-back of plants and trees.
5 As is well known, stratospheric ozone has been depleted over the past decades due to the penetration into
the stratosphere of long-lived industrial chemicals, the chlorofluorocarbons, creating the so-called “ozone
hole.” Stratospheric ozone shields humans, animals, and vegetation from the penetration to the surface of the
earth of harmful ultraviolet radiation. It is an irony that mankind, on one hand, creates tropospheric ozone
by emissions of combustion products of fossil fuel and, on the other hand, destroys stratospheric ozone by
emissions of some other chemicals.
 
 
Search WWH ::




Custom Search