METEOROLOGY
GEL-1370
Chapter Twelve
Air Pollution
What we are going to learn?
We are going to learn the answers to the following questions
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What are the major sources of pollution?
What are primary and secondary pollutants?
Why does PM-10 pose the greatest risk to human health?
Health problems associated with CO, SO2, VOCs, NOx?
What is Smog? London-type and Los Angeles-type smog?
Stratospheric vs tropospheric zone – which is beneficial?
What is PSI scale?
How does atmospheric stability influence the accumulation of
air pollutants?
Historical Background of air pollution
• Major sources of pollutants: car exhaust, chimneys, power plants,
factories, forest fires & other anthropogenic activities
• ~1/6th of world population in urban areas continuously exposed to
health hazards from air pollutants
• Air pollution is as old as the invention of fire – smoke problem
• King Edward I (1273): Declared a proclamation to stop using sea
coal, a dirty form of coal (soot and SO2)
• 1850s: London had serious “pea-soup’ fog, a thick mixture of
smoke and fog that hung over the city
• 1873: ~700 people died due to these fogs
• 1911: ~1150 people in London died of smog (combination of
smoke and fog)
• 1952: Major smog lasting for 5 days leading to 4000 people’s
death
• 1956: Passage of ‘Clean Air Act’ in London
History of Air Pollution
• In US, industrial revolution brought air pollution
• By 1940s: Air in some cities was polluted that the auto-mobile
headlights had to be turned on during the day
• 1948: Donora (PA), industrial pollutants got trapped in the
Monongahela River valley, >20 people died, thousands became ill
• 1960s: Air pollution levels dangerously high in NY city
• In LA, heavy smog due to large petroleum-processing plants &
automobiles; first alert toward the end of WW-II
• 1970: ‘Clean Air Act’ was passed; emission standards were set
• 1977, 1990: Clean Air Act was updated (stricter emission
standards; incentives to encourage companies to lower emission
standards)
• Amendments to the Act identified 189 toxic air pollutants
Easterly winds blow smoke from many large fires
over a vast area of Southern Africa
Sources of Air pollutants
• Air pollutants: Airborne substances, either solid, liquid, or
gases, that occur in high conc. Threatening the health of people,
animals, vegetation and structures
• Natural Sources of pollutants: Dust and soot from earth’s
surface (forest fires), volcanic eruption releasing tons of ash and
dust into the atmosphere
• Anthropogenic Pollutants:
– Fixed sources: Homes, office buildings, industrial complexes, power
plants, etc
– Mobile sources: Motor vehicles, ships, jet aircraft, etc.
• Primary Pollutants: Enter atmosphere directly (from
smokestacks and tail pipes)
• Secondary Pollutants: Formed when a chemical reaction
occurs between a primary pollutant and some other component
of air, such as water vapor or another pollutant
Some of the sources of Primary pollutants
Sources
Pollutants
Natural
Volcanic eruptions
Forest fires
Dust storm
Ocean waves
Vegetation
Hot springs
Particle (dust, ash), gases (SO2, CO2)
Smoke, unburned hydrocarbons (CO2, NOx, ash)
Suspended particulate matter
Salt particles
Hydrocarbons (VOCs), pollen
Sulfurous gases
Human
caused
Industrial
Paper mills
Power Plants –coal
oil
Refineries
ManufacturingH2SO4
PO4 fertilizer
Iron & steel mills
Automobiles,
fireplaces, home
Particulate matter, sulfur oxides
Ash, sulfur oxides, nitrogen oxides
Sulfur oxides, nitrogen oxides, CO
Hydrocarbons, sulfur oxides, CO
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Personal
SO2, SO3 and H2SO4
Particulate matter, gaseous fluoride
Gaseous resin
CO, NOx, VOCs, particulate matter
% of the primary pollutants in US
Primary sources of air pollutants in US
Denver (CO) on a clear day
Denver (CO) on a day when particulate matter
and other pollutants reduce the visibility
Aerosols – suspended particulate matter
• Aerosols: A group of solid particles and liquid droplets that remain
in suspension in the air; usually not poisonous; more substances
include asbestos, fibers, arsenic, droplets of H2SO4, PCBs, oil, &
pesticides
• Some particulate matter containing Pb, Ni, Cu, etc can affect
respiratory system; make breathing difficult, particularly for those
suffering from chronic respiratory disorders
• High accumulation of Pb in bones and soft tissues can cause brain
damage, convulsion, and death; at low concentrations, dangerous
to fetuses, infants and children when exposed may suffer central
nervous system damage
• PM-10: Fine particulate matter < 10 mm diameter
• > 10 mm SPM settle in < 1-day; < 1 mm remain in air for several
weeks
Suspended particulate matter – contd.
• Total SPM emitted over US: 6.6 million tons; ~40% emitted from
industrial processes; highway vehicles ~17%
• Arctic Haze: Sooty particles formed over the arctic during late
winter and early spring
• These find SPM serves as nuclei for cloud droplets & ice crystals;
when the particles are in 0.1-1.0 mm size, haze particles effectively
scatter incoming sunlight
• CO: A major pollutant of city air; odorless, poisonous gas formed
during the incomplete combustion of carbon containing fuels;
most abundant primary pollutant;
• Over 60 million metric tons/yr of CO enter the air in US; CO
levels have decreased by ~40% since early 1970’s;
• CO is quickly removed from the atmosphere by microorganisms in
the soil; human hemoglobin prefers CO to oxygen, so if CO is
present, brain will starve of oxygen leading to headache, fatigue,
drowsiness, and even death may result
SO2 & VOCs
• Sulfur dioxide sources: burning of sulfur containing
fossil fuels (coal & oil); primary source includes power
plants, heating devices, smelters, petroleum refineries &
paper mills; volcanic eruptions and sulfate particles from
ocean spray other major sources
• Inhalation of high concentrations aggravate respiratory
problems, such as asthma, bronchitis & emphysema
• VOCs: Organic compounds that are mainly hydrocarbons
– individual organic compounds composed of hydrogen
and carbon; occur as solids, liquids and gases at room
temperature
• Methane is the most abundant; Other VOCs include
benzene, formaldehyde & some chlorofluorocarbons
VOCs & NOX
• 18 million metric tons of VOCS emitted into the air in
US; 34% from vehicles used for transportation; ~50%
from industrial processes
• Certain VOCs are carcinogens (cancer-causing agents) –
benzene, benzo-a-pyrene (a product of burning wood,
tobacco & barbecuing);
• Many VOCs are harmless, but combining with nitrogen
oxides in the presence of sunlight to produce secondary
pollutants that are harmful to human health
• NOX: Oxides of Nitrogen, primarily Nitrogen oxide
(NO2) and Nitric Oxide (NO); forms when some of the
nitrogen in the air reacts with oxygen during the hightemperature combustion of fuel
NOX & Ozone
• NOx are produced by natural bacteria; concentration in
urban area is 10-100 times greater than nonurban areas
• In moist air, nitrogen dioxide reacts with water vapor to
form nitric acid --- acid rain
• Sources of NOx: motor vehicles, power plants, and waste
disposal systems
• Problems: Contribute to heart & lung problems, &
lowering body’s resistance to respiratory infections; may
encourage the spread of cancer; NOx are highly reactive
gases & plays a key role in producing O3 in the air
• Ozone (O3): Noxious substance with unpleasant odor
• Smog means smoke + fog; used to refer smog that forms
in large cities (e.g., LA)
Smog
• Photo-chemical smog (or LA smog): Smog formed by
the chemical reactions in the presence of sunlight
• London-type smog: When smog is composed of
sufurous and foggy air
• Ozone forms naturally in the stratosphere through the
combination of molecular oxygen and atomic oxygen;
stratospheric ozone provides a protective shield against
sun’s harmful UV rays
• Tropospheric ozone: Near the surface, in polluted air,
ozone is formed from a complex series of chemical
reactions (with sunlight) involving pollutants, such as
NOx and VOCs; normally higher during the afternoons
and during the summer months
Ozone
• Problems with ozone: Irritates eyes & mucous membranes
of the respiratory system, aggravating chronic diseases, such
as asthma and bronchitis; exposure to low concentrations
for 6-7 hrs during periods of moderate exercise can
significantly reduce lung function; other symptoms include
chest pain, nausea, coughing, and pulmonary congestion
• Ozone Production: Sunlight (< 410 nm) breaks down NO2
into NO and atomic oxygen
– NO2 + Solar Radiation -------- NO + O
• O2 + O + M -------- O3 + M (M: third molecule)
• Ozone is destroyed by combining with nitric oxide
• O3 + NO ----------- NO2 + O2
Average (2-yrs, 6 cities) hourly concentrations of
Ozone
Ozone – contd.
• Certain VOCs in polluted air allow ozone conc. to increase by
preventing nitric oxide from destroying the ozone as rapidly as it
forms
• VOCs react with oxygen and nitrogen dioxide to produce other
desirable contaminants (such as peroxy-acetyl nitrate)--- harmful
to vegetation & irritates eyes
• Ozone production should decrease in most areas when emissions
of NOx and VOCs are reduced
• Ozone is most dense at ~25 km from earth surface; at this
altitude, 12 ozone molecules/million air molecules (12 ppm)
• UV radiation < 300 nm has enough energy to cause cancer
• Decrease of ozone in the upper atmosphere leads to the
following: a) Increase in the number of cases of skin cancer &
eye cataracts and sun burning; b) suppression of human immune
system; c) decrease in the ocean productivity & d) cooling of the
stratosphere & changes in stratospheric wind pattern
Average distribution of ozone above the earth’s
surface in the middle latitude
Ozone - contd
• Nitrous oxide from fertilizers & chlorofluorocarbons (CFCs) drifting to the atmosphere through the breaks in the tropopause in
the vicinity of jet streams & during thunderstorms penetrating
into the lower stratosphere
• One single chlorine atom released from CFCs removes as many
as 100,000 ozone molecules before it is removed by combining
with other substances
• Average lifetime of a CFC molecule ~50-100 yrs
• After 1987 Montreal Protocol, ~1% decrease in the total
concentrations of ozone destroying chlorine & bromine
compounds
• >5 billion kg of CFCs have been released into the troposphere
• Ozone layer thinned by ~3% during the summer from 1979-1991
over heavily populated areas of the Northern Hemisphere
Ozone molecule adsorbing UV radiation –
dissociates into molecular and atomic oxygen
Ozone – contd.
• Ozone levels dropped to record low levels over much of the globe –
mainly due to ozone-destroying chemicals and from the 1991
Mt.Pinatubo volcanic eruption (tons of sulfur dioxide gas into the
atmosphere, formation of H2SO4)
• Ozone Hole: Sharp drop in the stratrospheric ozone level observed
over the Antarctic during the spring
• Clean Air Act of 1970 – Since then, most of the pollutant levels have
fallen off substantially; many amendments thereafter; 198 million
autos on the road in a day
• Primary ambient air quality standards are set to protect human
health and secondary standards to protect human welfare
• Nonattainment Areas: Areas that do not meet air quality standards
(80 million Americans are breathing air that does not meet at least
one of the standards)
Emission estimates of six pollutants in the US from
1940-1995
Pollutant Standard Index
• Pollutant Standards Index (PSI): Measure of air quality
in a particular region; the highest value in one day is
used; pollutant (CO, SO2, NO2, O3, SPM) measurement
is converted to a number that ranges from 0 to 500;
• Pollutant value = primary ambient air quality standard,
then, the pollutant is assigned PSI number 100; >100:
unhealthful; 50-100: moderate; < 50: Good
• Total emissions of toxic chemicals spewed out into the
skies over US declined steadily since EPA began the
inventory of chemicals
Number of unhealthful days (by county) when PSI
>100 (any of CO, SO2, NO2, O3 & SPM)
Factors that affect air pollution
• Role of wind: in diluting the pollutants
• Most pollution occurs when the winds are light, skies
are clear, mixing is shallow, atmosphere is stable and a
strong inversion exists
• Urban areas are more polluted than the rural areas
• Urban areas tend to be warmer than urban areas
• Acid rain is a major problem
• Stability of atmosphere is determined by the way air
temperature changes with height
Smoke from the shorter stack is trapped
within the inversion while smoke from taller
stack, above inversion, rises and mixes
Stability and Inversions
• An inversion (temp decreases slowly as we ascend or
increases with height) represents extremely stable
atmosphere where warm air lies over the cold air – lid
on the vertical motion
• Pollutants from a shorter stack does not rise very high,
but spreads out, contaminating the area while longer
stacks release and the pollutants get mixed well and
travel farther distances (dilution is the solution to the
pollution!)
As vertical temp profile changes, the pattern of
emitted smoke also changes
Inversion layer preventing the escape of
pollutants
Thick layer of polluted air trapped in the
valley
Cold air pollutants drain downhill and settle in
the low-lying area
A weak country breeze carries pollutants from
the outskirts into the city
pH Scale
Chapter Summary
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Smoke in London, CA
CO, NOx, SPM, how CO affect our body
Volcano – source of what contaminant
VOC, photochemical smog
Primary and secondary pollutants
Acid rain, pH of rain water
Mixing depth
PSI-scale
Aerosols, VOCs, carcinogens, ozone, sulfur dioxide