2. Air Quality Problems
• Question
– What are the air quality problems caused by pollution?
– For each problem, state the pollutant being emitted and the
activity that results in the emission.
many sources
CO, Pb, Hg, PAHs, many others
air toxics
combustion (fossil fuels, biofuel, waste incineration, etc)
SO2, NOx, soot, PAHs, fly ash,
production ofsmog, others
particulate matter
transportation in motorized vehicles
reactive VOCs, NOx (produces O3,
PAN, organic PM, nitrate PM)
photochemical
smog
burning fossil fuels (esp coal-burning power plants)
SO2, NOx (ie, NO+NO2)
acid rain
Major Source Activities
Major Chemical Pollutants
Degradation
3. Emission of Criteria Pollutants by Source Category
CO Pb NOx VOCs SO2
Emission
Rate,
x
10
3
ton/yr
0
5000
10000
15000
20000
25000
65000
70000
Fuel Combustion
Industrial
Transportation
Note: Pb emission rates are in ton/yr, not 103 ton/yr
4. U.S. Air Pollutant Trends
(The Good News)
GDP
VMT
Energy
Consumption
US
Population
Aggregate
Emissions
Source: EPA, Latest Findings on National Air Quality, “2001 Status and Trends” summary report.
5. Air Quality Problems (The Bad News)
• Lecture Question
– What two criteria pollutants are currently considered the biggest health
risks in the US?
2001 US population: 285 Million;
46.7% live in counties exposed
to levels above NAAQS
• Estimated 50,000 people in the US
per year die from air pollution.
• These are mostly the susceptible
portion of the population: the
elderly, children, and those
suffering from pre-existing
respiratory or CV problems.
• The number of annual deaths is
roughly comparable to those who
die in car accidents.
• Globally about 3 million people die
each year due to air pollution
• 5% of all annual global deaths.
• Range of estimates: 1.6 – 6
million people.
• Source: WHO
• Air pollution also associated with
increased risk of developing asthma,
COPD disease, decreased lung
capacity, etc.
6. Three Major Problems
• Photochemical smog (`ground-level ozone’)
– Primary pollutants: VOCs, NOx
– Secondary pollutants: O3, PAN, organic aerosol, nitrate aerosol, etc
• Primary pollutants are what are discharged directly into the air
• Secondary pollutants are formed from primary pollutants, and are generally the ones
that impact human and ecosystem health and welfare.
• Particulate matter (PM10 and PM2.5)
– Primary pollutants
• Direct emissions of PM (crustal material, soot)
• SO2; NOx; VOCs
– Secondary pollutants
• nitrate, sulfate, and organic component of aerosol
• Acid deposition
– Primary pollutants
• SO2; NOx
– Secondary pollutants
• H2SO4(aq), HNO3(aq)
• Problems are all related
– For example, production of smog also produces PM and acid deposition
– A common factor: photochemical oxidation in the atmosphere
7. Killer Smog Episodes
• 1930: 63 die in Meuse Valley, Belgium
– prophetic: “Proportionally the public services of London, e.g.,
might be faced with the responsibility of 3200 sudden deaths if
such a phenomenon occurred there.”
• 1948: 20 die in Donora, PA
– one-third residents ill; see
http://www.westol.com/~shawley/dhs/smog.pdf
• 1952: 4000 die in London
– leads to UK’s first Clean Air Act
• 1962: 700 die in London
– meteorological conditions similar to those in 1952 episode but
with far fewer deaths
• Sulfurous Smogs
– The above severe smog episodes are all examples of “London
smog,” or sulfurous smogs.
• Distinguished from current smog problems, called photochemical or
“LA” smog.
9. Epidemiology: Effects of Poor Air Quality
1952 London
smog episode
Environmental Epidemiology
• Looks at the correlation
between the level of an
inadvertent pollution exposure
to a population with some
measure of health impact (eg
mortality rate).
• Establishes a correlation
between exposure levels and
health effects.
• Most conclusive if supported
by data from animal and
clinical studies.
10. LA Smog
morning
view
afternoon
view on
same day
• First recognized in late 1940’s
• Much different in nature to
“London” smog:
• Favored by sunny, warm, dry
days
• Strongly oxidizing, eye-
watering
• Air pollution peaks in the
afternoon (not the morning)
• London smog = sulfurous smog
• LA smog = photochemical smog
11. Photochemical Smog
• What exactly is photochemical smog?
– Main component is ozone, O3
• Smog often referred to as “ground-level ozone” or “bad ozone”
– To distinguish it from the “good ozone” in the stratospheric ozone layer
– There is, of course, no chemical difference other than location. Ozone
is toxic (bad if we breathe it) but also shields us from harmful uv light
– It is a complicated mixture (secondary pollutants)
• Ozone
• Partially oxidized organics
– Alcohols (eg methanol, ethanol)
– Organic acids (eg acetic acid, formic acid)
– Ketones (eg acetone)
– Aldehydes (eg formaldehyde)
– Organic nitrates (eg peroxyacyl nitrates, PANs)
• Nitrogen dioxide, NO2
– A brown-colored gas; the source of the ozone
• Particulate matter, PM
– With high nitrate and organic components
12. Photochemical Smog
• How is photochemical smog formed?
– Smog forms only in sunlight
• And it forms more rapidly on hot, dry days
• Severe smog episodes more likely to occur in the summer months
– Primary pollutants
• Smog is formed from the following primary pollutants
– Reactive organic gases, such as the hydrocarbons in unburned fuel or in
emissions by trees
– Nitric oxide, NO
– Formation
• Smog is formed (over a period of hours) by the photochemical oxidation of
organic compounds in the presence of nitric oxide (NO)
– The oxidation process is natural
– BUT: when NO is present, NO2 is formed during the oxidation process
– Photochemical reaction of NO2 is the source of ground-level ozone
13. Evolution of Photochemical Smog
Caused by the atmospheric oxidation of reactive hydrocarbons in the
presence of NOx
Smog chamber experiments
• Reproduce the
characteristics of
photochemical smog
• Expose precursors
propene (a small reactive
hydrocarbon) and NO to
sunlight
• Ozone conc peaks about 4
hours after the expt starts
• Similar to noon/afternoon
peaks due to rush hour
traffic
• Shows appearance of some
other smog components
• NO2, PAN, aldehydes
• But nitrate/organic PM,
HNO3, and (many) other
organics are not shown.
14. Photochemical Oxidants
• OH
– The major oxidant during daylight hours (for most molecules)
– Formed by the photodissociation of a number of compounds (O3,
HONO, H2O2, aldehydes)
– Disappears at night
• NO3
– Most important nighttime oxidant, especially in polluted areas
– Formed by NO2 + O3 NO3 + O2
• O3
– Major source: photodissociation of NO2
– Attacks unsaturated HCs (day and night)
• “Unsaturated HCs contain double bonds
• Usually does not attack aromatic HCs
• Cl
– Powerful oxidant in the marine boundary layer, MBL, where it acts much
like OH
– Really only a factor in coastal areas, or over oceans
15. Initial Oxidation of “Saturated” Hydrocarbons
• H-abstraction to form alkyl radical
RH + OH R + H2O
• O2 addition to form alkylperoxy radical
R + O2 + M ROO + M
• In the presence of NO: O-abstraction to form alkoxy
radical:
ROO + NO RO + NO2
• In unpolluted areas: formation of peroxides (some of
which may photodissociate to produce RO)
ROO + HO2 ROOH + O2
(ROOH + hv RO + OH)
16. The NOx “Switch” for Smog Production
•Oxidation of HCs proceeds
differently in the presence/absence of
NOx
•When NOx is present, O3 is produced
(due to photodissociation of NO2) and
the mixture becomes progressively
more oxidizing
•When NOx is absent, oxidation
proceeds more slowly, without
producing O3. Naturally, there is also
no production of organic nitrates (eg
PAN), HNO3 or nitrate aerosol
•The “NOx switch” occurs at NOx
concentration of about 20 pptv. At this
concentration, reaction of the peroxy
radical with HO2 and NO occurs at
about equal rates.
17. •Initial step: attack by photochemical
oxidant to produce reactive radical
•Typical initial attack strategies: H-
abstraction; addition at unsaturated
site
•A number of partially-oxidized
intermediates are formed (molecules
in boxes). These are relatively stable;
they may react further or may be
removed thru wet/dry deposition
•Reaction of peroxy radicals (ROO)
with NO yields NO2, which
photodissociates to give atomic
oxygen (and hence ozone)
•Instead of photodissociating, every
once in a while NO2 reacts with OH to
produce HNO3
Tropospheric Oxidation
of Organics
18. Initial Attack is Rate Determining
Organic
OH
(106 cm-3)
O3
(100 ppbv)
NO3
(50 pptv)
Cl
(104 cm-3)
n-butane 5 d > 1300 y 205 d 5 d
trans-2-butene 4.3 h 35 min 35 min 4 d
acetylene 14 d > 400 d > 188 d 22 d
toluene 2 d > 400 d 138 d 20 d
formaldehyde 1.2 d > 450 d 16 d 16 d
General trends:
•OH is usually the main oxidant (in the MBL Cl is also important)
•oxidation rates of alkanes increase with increasing number of carbons (due
to stabilization of the alkyl radical)
•oxidation rate is faster for alkenes than for alkanes BUT aromatics and
alkynes are less easily oxidized than alkenes
24. NOx Emissions
• Lecture Questions
– How are nitrogen oxides (NOx) released into the air?
– List all the pollution problems caused (at least partly) by NOx
emissions into the air; be complete.
– Released through combustion
• Hot enough to cause N2 + O2 2NO
– Environmental problems associated with NOx:
• Direct effects on human and ecosystem health (NO2 toxicity)
• Photochemical smog
• Acid deposition
• Nitrate PM
• Eutrophication
• (Slight contribution to global warming and ozone depletion through
N2O formation.)
26. Acid Deposition: What is It?
• Carbon dioxide dissolves in water to form an acidic
solution
– When CO2 dissolves it forms carbonic acid, which is a weak acid
CO2 + H2O H2CO3
– Water in equilibrium with 380 ppm CO2 has a pH of about 5.6
– Deposition that results in a pH that is less (more acidic) than 5.6
is considered acid deposition
• “Acid deposition” is due to
– Hydrometeors (rain, hail, fog, snow, ice) that fall to the earth
– PM that produces acidic solutions after deposition on the surface
27. Global Distribution of Precipitation Acidity
•Values given here are averages; more severe episodes can occur
•Regions traditionally most affected by acid precipitation: North American NE region (US & Canada), largely due to
emissions from the American mid-west region; Scandinavian countries, largely due to emissions from Great
Britain.
•Remember that acidification can occur through dry deposition of acidic PM; it doesn’t have to be associated with
a precipitation event
•Snow can be acidic and can result in “pulses” of acidity in water bodies that receive springtime melt water
29. Effects of Acid Deposition
• May affect freshwater organisms
– Regions that are poorly-buffered (with underlying granite) are most affected
– Direct effects: acidity, increased mobility of toxic metals (esp Al), reproduction
• Can cause fish and other populations to plummet.
– Ecosystem effects
• Population levels can be affected even if the acidification doesn’t harm individual
organisms
• Effects on predation, reproductive success, etc
• May affect vegetation (eg forests)
– Direct effects (erosion, nutrient leaching)
– Indirect effects (soil acidification and leaching)
– Can be hard to study due to confounding factors
• Examples: phytotoxicity of SO2, NOx, O3; nutritional benefits of nitrate/sulfate PM;
leaching of essential minerals from the soil.
• Increased weathering of materials used in construction
30. regions in N America
with low soil alkalinity
Sensitivity to Acid Rain
31. Sources of Acid Deposition
• Tropospheric oxidation of SO2 and NOx produces acidic aerosol
• SO2 emission is through burning fuel that contains sulfur impurities. The
most important source is coal-burning power plants.
• NOx emission is through any combustion process that is sufficiently hot,
which drives the reaction
N2 + O2 2NO
• A small portion of acid deposition is due to organic acids
3
[O]
4
2
[O]
2
HNO
NO
SO
H
SO
32. Tropospheric Oxidation of NOx and SO2
• Two main types of oxidation of gaseous NOx to aqueous
HNO3
– Daytime gas-phase oxidation of NO2 by OH
– Nighttime hydrolysis of N2O5 on PM
– Nighttime H-abstraction from HCs by NO3
• Two main routes for oxidation of gaseous SO2 to
aqueous H2SO4
– Gas-phase oxidation by OH, followed by dissolution of SO3
– Dissolution of SO2, followed by aqueous-phase oxidation (mostly
by aqueous H2O2)
33. Global Emissions of SO2 and NOx
Source SO2 NOx
Natural
Oceans 22 1
Soil & plants 2 43
Volcanoes 19 -
Lightning - 15
Subtotals 43 59
Anthropogenic
Fossil fuel combustion 142 55
Industry (ore smelting) 13 -
Biomass burning 5 30
Subtotals 160 85
TOTALS 203 144
Humans: almost 80% SO2 emissions, almost 60% NOx emissions.
37. Significance of Atmospheric Aerosol (PM)
• Atmospheric composition & reactions
• Cloud formation and properties
• Absorption & scattering of light
• Climate
• Health
– PM-10 has been shown in many epidemiological studies to be
strongly related to mortality rates. Some show a 1% increase in
mortality for as little as a 10 mg/m3 increase in PM-10.
• Visibility
38. Important Properties of PM
• Concentration (diameter, surface area)
• Size distribution
• Chemical composition
Important questions:
• What are the processes by which the atmospheric aerosol is
formed?
• How do human activities impact the nature (concentration, size
distribution, composition) of the atmospheric aerosol?
39. Particulate Matter Concentrations
NAAQS for PM
•PM10: 50 mg/m3 (annual)
and 150 mg/m3 (24-hr)
•PM2.5: 15 mg/m3 (annual)
and 65 mg/m3 (24-hr)
Number densities of urban
aerosol commonly exceeds
105 cm-3
40. Particulate Matter Size
• PM is classified by size and composition
– Diameter < 2.5 mm: fine PM
• Formation
– Coagulation of still smaller particules
– Condensation of gases on smaller particles
– Much fine PM is a secondary pollutant
• Removal
– Sedimentation (removal by gravity) is slow
– Main removal mechanisms: scavenging, coagulation, adsorption
• Effects
– More important in atmospheric chemistry
– More important in terms of health effects
– Diameter > 2.5 mm : coarse PM
• Formation
– Mechanical breaking up of larger particles
– Both natural and direct anthropogenic sources
• Removal by sedimentation is rapid
41. Particulate Matter: Size Distribution
Idealized distribution has three modes.
• Not all modes need be present
• Transient nuclei constant the most
numerous faction while coarse particles
constitute the heaviest faction.
43. PM Composition
• Carbonaceous aerosol
– Elemental carbon (soot) directly emitted by combustion processes
– Organic aerosol: direct emission of condensed phase organic material
(combustion, biogenic), and some condensation of secondary organic
formed by atmospheric oxidation
• Nitrate aerosol
– Formed from dissolution/neutralization of atmospheric HNO3
• Sulfate aerosol
– Formed from dissolution of gaseous SO3 or aqueous-phase oxidation of
dissolved SO2
• Crustal material
– Mechanical formation (wind erosion)
– Consists of Si, O, Al, Fe, Mn, etc
• Chloride aerosol (seaspray)
– Mechanical formation in oceans (waves, bubbles)
– Consists of Cl, Na, K, Mg, SO4
2-, etc
49. Effect of Pollution on Fine PM
(Concentration and Composition)
Note that more polluted urban air has (a) more PM (7-fold) and (b) greatly
increased sulfate and carbon PM fractions.
