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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
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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 SO 2)
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
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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
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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:
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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
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% 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
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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 m diameter
> 10 m SPM settle in < 1-day; < 1 m remain in air for several weeks
Suspended particulate matter – contd.
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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 m 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
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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
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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 high-temperature combustion of fuel
NOX & Ozone
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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
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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
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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
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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.
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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
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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
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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;
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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, SO 2, NO2, O3 & SPM)
Factors that affect air pollution
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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
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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