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Air Pollution
GEOL 1350: Introduction To Meteorology
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Overview
• Types and Sources of Air Pollutants
• Factors That Affect Air Pollution
• Air Pollution and the Urban Environment
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•  Air pollutants are airborne substances (either
solids, liquids, or gases) that occur in
concentrations high enough to threaten the
health of people and animals, to harm
vegetation and structures, or to toxify a given
environment.
•  Air pollutants come from both natural sources
and human activities.
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Smoke from massive wild fires across southern California out over the Pacific Ocean
•  Air pollutant natural sources include:
•  Wind picking up dust and soot from the earth’s surface
•  Volcanoes belching tons of ash and dust into the
atmosphere
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•  Smoke produced from forest fires
•  Human-induced pollution enters the atmosphere from
•  Fixed sources: Industrial complexes, power plants,
homes, office buildings.
•  Mobile sources: Motor vehicles, ships, jet aircraft.
•  Certain pollutants are called primary air pollutants: They
enter the atmosphere directly -- from smokestacks and
tail pipes.
•  Other pollutants, known as secondary air pollutants,
form only when a chemical reaction occurs between a
primary pollutant and some other component of air (e.g.
water vapor or another pollutant).
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•  Carbon monoxide (CO), Sulfur Oxides (SOx), nitrogen
oxides (NOx), volatile organic compounds (VOC), and
particulate matter (PM) are the most prevalent primary
pollutants.
•  Transportation and fuel combustion in stationary
sources are the primary sources for these pollutants.
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Sources of Primary Air Pollutants
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•  Particulate matter (PM) represents a group of solid
particles and liquid droplets that are small enough to
remain suspended in the air.
•  PM includes particles such as soot, dust, smoke, and
pollen, etc. It also includes tiny liquid droplets of
sulfuric acid, oil, and various pesticides.
•  PM often dramatically reduces visibility in urban
environment. PM collected in cities includes iron,
copper, nickel, and lead. This type of pollution can
immediately influence the human respiration system.
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Denver, Colorado, on a clear day and on a day when PM and other pollutants greatly reduce visibility.
•  Particulates are classified as having diameters < 10 µm
(PM-10) and < 2.5 µm (PM-2.5).
•  PM-10 pollutants settle out of the atmosphere relatively
quickly compared with PM-2.5.
•  PM-10 and PM-2.5 poses greatest health risks. They are
small enough to penetrate the lung’s natural defense
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mechanisms. They also reduce visibility.
Cumulus clouds and a thunderstorm rise above the thick layer of haze that frequently covers the
eastern half of the United States on humid summer days.
•  Many suspended particles are hygroscopic, as water vapor readily
condenses onto them. As a thin film of water forms on the particles,
they grow in size.
•  When they reach a diameter between 0.1 and 1.0 µm, these wet haze
particles effectively scatter incoming sunlight to give the sky a
milky white appearance.
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Cumulus clouds and a thunderstorm rise above the thick layer of haze that frequently covers the
eastern half of the United States on humid summer days.
•  The particles are usually sulfate or nitrate particulate matter (PM)
from combustion processes, such as those produced by diesel
engines and power plants.
•  The hazy air mass may become quite thick, and on humid summer
days it often becomes cumulus clouds as shown in the figure.
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•  Carbon monoxide (CO), a major pollutant of city air, is a
colorless, odorless, poisonous gas.
•  It is formed during the incomplete combustion of
carbon-containing fuels.
•  It is most plentiful of the primary pollutants.
•  Environmental Protection Agency (EPA) estimate that
over 60 million metric tons of CO enter the air annually
over the United States alone -- more than half is from
highway vehicles.
•  CO is removed from the atmosphere by microorganisms
in the soil.
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•  Sulfur dioxide (SO2) is a colorless gas that comes
primarily from the burning of sulfur-containing fossil
fuels (such as coal and oil).
•  Its primary source includes power plants, heating
devices, smelters, petroleum refineries, and paper mills.
•  It can also enter atmosphere naturally during volcano
eruptions and as sulfate particles from ocean spray.
•  SO2 can be oxidized to form the secondary pollutants
sulfur trioxide (SO3) and in moist air high corrosive
sulfuric acid (H2SO4).
•  SO2 can cause respiratory problems.
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•  Volatile organic compounds (VOCs) represent a class of
organic compounds that are mainly hydrocarbons -individual organic compounds composed of hydrogen
and carbon.
•  Methane (CH4) is the most abundant hydrocarbons.
•  Other hydrocarbons include benzene, formaldehyde,
and some chlorofluorocarbons.
•  EPA estimates that over 18 million metric tons of VOCs
are emitted into the air over the USA each year, with
about 34% from vehicles used for transportation and
50% from industrial processes.
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•  Nitrogen oxides are gases that form when some of the
nitrogen in the air reacts with oxygen during high
temperature combustion of fuel.
•  Two primary nitrogen pollutants are nitrogen dioxide
(NO2) and nitrogen oxide (NO), which together are
referred as NOx or oxides of nitrogen.
•  Both NO and NO2 can be produced by natural bacterial
action. Primary sources of NO are motor vehicles, power
plants, and waste disposal systems.
•  In moist air, NO2 reacts with water vapor to form
corrosive nitric acid (HNO3), a substance that adds to
the problem of acid rain.
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•  Ozone (O3) is a noxious substance with an unpleasant
odor that irritates eyes and mucous membranes of the
respiratory system.
•  Ozone forms naturally in the stratosphere through
combining of molecular oxygen and atomic oxygen.
•  Stratospheric ozone provides a protective shield against
the sun’s harmful ultraviolet rays.
•  Near the surface, in polluted air, ozone often referred as
tropospheric ozone. It is a secondary pollutant. It is
formed from a complex series of chemical reactions
involving other pollutants.
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•  Air quality in the USA may have been at its worst in the 1970s, but
programs implemented by the Clean Air Act have helped the USA
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move toward primary ambient air quality standards.
•  To indicate the air quality in a particular region,
the EPA developed the air quality index (AQI).
•  AQI includes the pollutants carbon monoxide,
sufur dioxide, nitrogen dioxide, particulate
matter, and ozone.
•  When the pollutant’s value is the same as the air
quality standard, the pollutant is assigned an
AQI number of 100.
•  A pollutant is considered unhealthful if AQI >
100.
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•  Wind
•  Stability and Inversion
•  Topography
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•  Role of Wind:
•  Wind speed plays a role in diluting pollution.
•  Wind speed determines how quickly the pollutants mix
with the surrounding air.
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•  Smoke puffs are closer together and more concentrated when wind
speed is low.
•  Smoke puffs are farther apart and more diluted when wind speed is
greater.
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•  Role of Stability and Inversions
•  Atmospheric stability determines the extent to which air
will rise.
•  Unstable atmosphere favors vertical air currents
•  Stable atmosphere strongly resists upward vertical
motions
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•  Radiation temperature inversions, lasting only a few hours at
morning with warm air above cold, creates a stable atmosphere and
traps pollutants at the surface.
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•  Subsidence inversions may last for several days, which can create
major pollution threats by reducing the mixing depth and layer,
forcing a build-up of unwanted pollutants in the urban environment.
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•  Role of Topography
•  Shape of landscape plays an important part in trapping
pollutants.
•  At night, cold air tends to bring pollutants into the
valleys and trapped in there.
•  Pollution problem in several large cities are partly due to
topography. For example, Los Angeles is surrounded on
three sides by hills and mountains. Cool air settles in
the basin, trapping pollutants from industry and millions
of autos.
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•  Air movement from higher hills to valleys can strengthen
preexisting surface inversions, as well as carry pollutants downhill,
particularly in the colder months.
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Air Pollution and the Urban Environment
•  Cities are generally warmer than surrounding rural
areas. This region of city warmth is known as urban heat
island.
•  In cities, where less vegetation and exposed soil exists,
the majority of the sun’s energy is absorbed by urban
structures. During daylight, surface temperature at cities
rise higher than in rural areas. During night, the release
of heat energy is retarded by the tall vertical city walls
that do not allow infrared radiation to escape. The slow
release of heat tends to keep night time city temperature
higher than that in rural areas.
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Air Pollution and the Urban Environment
•  Cities are generally warmer than surrounding rural
areas. This region of city warmth is known as urban heat
island.
[Streutker, 2002; Remote Sensing of Environment]
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Air Pollution and the Urban Environment
•  Products of combustion, such as oxides of sulfur and
nitrogen can slowly settle to the ground in dry form (dry
deposition) or they can be removed from the air during
the formation of cloud particles and then carried to the
ground in rain, called Acid rain (wet deposition).
•  Acid deposition encompasses both dry and wet acidic
substances.
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Air Pollution
•  Atmospheric concentrations of
CO2 make rainfall slightly acidic
(H2CO3) with PH within 5.0-5.6.
•  Power plant emissions of SO2
and car exhaust of NOx form acid
rain (H2SO4, HNO3) with PH
within 4-4.5.
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Air Pollution
•  Acid precipitation is not confined to the Northeast; the acidity of
precipitation has increased rapidly during the past 30 years in the
southeastern states too.
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Air Pollution
•  Acidic particles causes
serious deficiencies in
certain elements necessary
for the tree’s growth.
•  Trees are thus weakened and
become susceptible to
insects and drought.
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SUMMARY
1.  Primary air pollutants enter the atmosphere directly.
Secondary pollutants form by chemical reactions that
involve other pollutants.
2.  Most air pollution episodes occur when the winds are
light, skies are clear, the mixing layer is shallow, the
atmosphere is stable, and a strong inversion exists.
These conditions usually prevail when a high pressure
area stalls over a region.
3.  Urban environments tend to be warmer and more
polluted than the rural areas that surround them.
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