Environmental Engineering Reference
In-Depth Information
9.2.7
Regional Haze and Visibility Impairment
Small particles (also called fine particles) less than 1-2
m in diameter settle very slowly on
the ground and can travel hundreds to thousands of kilometers from their emitting sources.
A part of the fine particles are emitted directly from industrial, commercial-residential, and
transportation sources. These are called primary particles. However, the majority of fine parti-
cles is a product of gas-to-particle transformation processes, including photochemical processes,
where the precursor gases are emitted from the aforementioned sources. These are called sec-
ondary particles. The particles can envelope vast areas, such as the northeastern United States and
southeastern Canada, California and adjoining states, western and/or central Europe, and south-
eastern Asia. Satellite photos often show continental areas covered with a blanket of particles,
sometimes stretching out hundreds of kilometers over the ocean. This phenomenon is called
re-
gional haze
. The haze is mostly associated with stagnating anticyclones, when a high barometric
pressure cell remains stationary over a region, sometimes over a period of a week or more.
The high-pressure cell causes air to circulate in a clockwise fashion around the cell, drawing
in increasingly large amounts of emissions from the surrounding sources, forming the regional
haze. Usually, the regional haze period is terminated by a cold front moving through the re-
gion with accompanying convective clouds, thunderstorms, and precipitation that washes out the
particles.
The composition of the fine particles varies from region to region, depending on the precursor
emissions. In the northeastern United States, central Europe, and southeastern Asia, more than half
of the composition is made up of sulfuric acid and its ammonium and sodium salts, largely due to
high sulfur coal and oil combustion. The rest is made up of nitric acid and its salts, carbonaceous
material (elemental and organic carbon), and crustal matter (fine dust of soil, clay, and rocks). In
the western and southwestern United States and in some other urbanized/industrial areas of the
world, nitrate and carbonaceous matter makes up the majority of the composition of fine particles.
This is due to heavy automobile traffic, chemical industries, refineries, gas-fired power plants, and
other urban/industrial sources. A part of the haze may be due to biogenic sources, such as the
emissions of isoprene, terpene, pinene, and other isoprene derivatives from coniferous forests, but
the predominant cause of regional haze is the emission of gaseous and particulate matter from
anthropogenic sources, mostly from fossil fuel combustion. Condensed water is also an ingredient
of fine particles, as the precursor gases and the formed particle nuclei attract water molecules from
the air, especially during high humidity periods.
The small particles are efficient scatterers of light. The scattering efficiency is dependent
upon the wavelength. Maximum scattering efficiency for visible light (400-750 nm) occurs with
particles less than 1
µ
m in diameter, the so-called submicron particles. Light scattering prevents
distant objects from being seen. This is called
visibility impairment
. During regional haze periods,
one cannot distinguish distant mountains on the horizon, and occasionally one cannot see objects
farther than hundreds of meters, such as the other wall of a river valley or buildings a few city
blocks away.
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Also, the increasing concentration of particles in urbanized parts of the continents
causes the loss of visibility of the starlit nocturnal sky. These days, small stars, less than fifth order
of magnitude, rarely can be seen from populated areas of the world.
µ
9
Visibility impairment became a significant problem in some national parks in the United States. For example,
on some hazy days the northern rim of the Grand Canyon in Arizona cannot be seen from the southern rim.
