1. Tropospheric Air Quality
Smog, PM, Acid Deposition

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.

3. Air Quality Problems Regulation of Air Quality Clean Air Act of 1970
Established six criteria pollutants
Initially PM, SO2, O3, NOx, CO and hydrocarbons
Later, lead (Pb) replaced hydrocarbons (redundant, as it is an ozone precursor)
Later, PM was redefined as PM10
Still later, PM2.5 was added
The criteria pollutants are regulated by their ambient concentrations
National Ambient Air Quality Standards (NAAQS)
There are also many (currently 189) other regulated pollutants
The hazardous air pollutants, HAPS
examples: mercury, benzene
Regulated by emission standards (NESHAPS)

5. U.S. Air Pollutant Trends (The Good News)
GDP
VMT
Energy Consumption
US Population
Source: EPA 2001 “Status and Trends” summary report
Emissions trends shown are for the six criteria pollutants: NO2, O3, SO2, PM, CO, Pb. Not sure about O3, since it is a secondary pollutant.
Aggregate
Emissions
Source: EPA, Latest Findings on National Air Quality, “2001 Status and Trends” summary report.

6. Air Quality Problems (The Bad News)
Lecture Question
What two criteria pollutants are currently considered the biggest health risks in the US?
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.
2001 US population: 285 Million; 46.7% live in counties exposed to levels above NAAQS
air quality in the US is generally superior to that of most other countries

7. 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
SO2; NOx
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
smog consists of a mixture of ozone, NO2, and various partially-oxidized organic intermediates: aldehydes, ketones, organic acids, organic nitrates. PANs are one such. These organics may coalesce into droplets (ie, organic aerosol). The NO2 will continue to be oxidized to nitric acid and nitrate aerosol.
PAN = peroxyacetyl nitrate or peroxyacyl nitrates. The general structure is RC(O)OONO2; when R is the methyl group then you have peroxyacetyl nitrate; the general family name is peroxyacyl nitrates (unfortunately both are abbreviated PAN at times).

8. But First…Carbon Monoxide
What are the (human) Sources of CO?
Motor vehicle exhaust (approx 60% total) and nonroad vehicles do most of the damage
In cities, as much as 95% is from motor vehicles
1990 CAAA mandated oxygenated fuel during winter in problem areas
What are the National Trends in CO concentration?
Decreasing despite a 23% increase in VMT over the last 10 years
Urban sites have higher CO than rural

9. But First…Carbon Monoxide
Seasonal Trend
Highest in winter months due to greater vehicular emissions and more frequent local inversions
Home heating also contributes in winter

10. 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
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.
source: “London’s Historic Pea-Soupers,” EPA Journal, Summer 1994 (look on web site)
a 1873 London smog episode caused 268 deaths from bronchitis
one 1879 smog episode lasted four months
1902-3: average visibility from St. Paul’s Cathedral of Westminister Tower estimated at ½-mile
prediction about London (based on Belgium episode) made by J. Firket, “Fog along the Meuse Valley,” Trans. Faraday Soc., 1936, 32,
sulfurous smogs primary pollutants are SO2 and soot; the secondary pollutants are H2SO4 and sulfate aerosols. Caused by burning load-quality coal (ie containing high sulfur content)
solution is simple: (i) switch fuels: cleaner coal; natural gas; oil, and/or (ii) use emission controls on smokestacks to limit SO2 and soot emissions
question (before seeing next slide): how can they estimate how many die due to the smog? Get students to think about it.

11. Donora at Noon during Killer Smog Episode

12. Epidemiology: Effects of Poor Air Quality
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.
1952 London smog episode
Types of toxicological effects:
acute vs chronic. Acute effects manifest shortly, within 48 hours. Chronic effects are due to long-term exposure.
also: lethal vs nonlethal

13. LA Smog morning view 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
afternoon view on same day
London smogs were historically well known; a new type of smog was recognized shortly after WWII: LA Smog. Describe it.
LA smogs are more generally referred to as photochemical smogs
photochemical smog primary and secondary pollutants given earlier. Caused by both power plants and motor vehicles. Can be minimized by increasing fuel efficiency, using oxygenated fuel additives, and using catalytic converters to minimize NOx and VOC emissions.

14. 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

15. 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

16. 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.
characteristics of LA smog reproduced in a smog chamber that is a mixture of propene and NO that is exposed to sunlight. Note the rising concentrations of ozone, PAN and aldehydes. Nitric acid concentrations (not shown) also rise.
this is a smog chamber: a C3H6-NO mixture irradiated with sunlight. Products: ozone, aldehydes, PAN, HNO3.

17. Photochemical OxidantsOH
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
Sources of tropospheric O3
Photodissociation of NO2, obviously (occurs at less than 420 nm)
Transport from the stratosphere (folds in the tropopause). Photolytic lifetime of O3 is about 1 hour in the troposphere during the day, but lifetime of Ox is over one year in unpolluted areas.
note that PANs are a NOy reservoir and are a source of nighttime NOx, since they undergo thermal decomposition

18. 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)
species other than OH can abstract a hydrogen: Cl and NO3 are good examples.
ROO can also react with NO2 to produce peroxynitrate species: ROO + NO2 + M  ROONO2 + M. This is a reversible reaction: ie, peroxynitrates thermally decompose to yield starting products
production of smog from photodissociation of NO2, which requires light below 420 nm. Note that atomic oxygen is produced from NO2 in the troposphere, not from O2 photodissociation (ie Chapman cycle).
ozone: photodissociates to produce singlet oxygen, most of which deactivates but some of which reacts to form OH

19. 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.

20. Tropospheric Oxidation of Organics
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

21. 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
trans-2-butene
4.3 h
35 min
4 d
acetylene
14 d
> 400 d
> 188 d
22 d
toluene
2 d
138 d
20 d
formaldehyde
1.2 d
> 450 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

22. Ground-level Ozone Trends
1 hour averages

23. Ground-level Ozone Trends
8 hour averages

24. Ozone in the National Parks

25. VOC Sources

26. VOC Emissions

27. 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.)

28. NOx Emissions

29. 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

30. 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

31. Acid Precipitation in the US

32. 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
effects on forests: direct effect of acid precipitation (fog is the worst) can cause plant erosion and leach nutrients from leaves. Acidity can affect soil productivity (acidic soil is generally less fertile because of reduced CEC, among other things).

33. Sensitivity to Acid Rain
regions in N America
with low soil alkalinity

34. 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

35. 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)

36. Global Emissions of SO2 and NOx
Source
SO2
NOx
Natural
Oceans 22 1
Soil & plants 2 43
Volcanoes 19 -
Lightning 15
Subtotals 59
Anthropogenic
Fossil fuel combustion
142 55
Industry (ore smelting) 13
Biomass burning 5 30
160 85
TOTALS
203
144
Sources:
Spiro et al, “Global inventory of sulfur emissions with 1o x 1o resolution,” Journal of Geophysical Research, 1992, 97 (D5), 6023
EPA, Air Quality Criteria for Oxides of Nitrogen, EPA/600/8-91/049aA.
Humans: almost 80% SO2 emissions, almost 60% NOx emissions.

37. Anthropogenic Emission Sources
NOx emissions
SO2 emissions

38. SO2 Trends: Ambient Concentrations

39. SO2 Trends: Emission Rates

40. 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

41. 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?
these properties affect processes in which PM are involved (eg cloud formation or heterogeneous reactions)

42. Particulate Matter Concentrations
Number densities of urban aerosol commonly exceeds 105 cm-3
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)

43. 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
Mechanical breaking up of larger particles
Both natural and direct anthropogenic sources
Removal by sedimentation is rapid

44. 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.
Figure also gives some idea of the formation processes

45. Effect of Size on Residence Time

46. PM Composition Carbonaceous aerosol Nitrate aerosol Sulfate 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, SO42-, etc

47. Idealized Evolution of Fine PM

48. Anthropogenic Emission Sources
Coarse PM
Wind erosion
Travel on unpaved roads
Agricultural operations
Construction
High wind events
Fine PM
Directly emitted by combustion sources
Fuel and forests
Secondary PM
Sulfate PM: from SO2 emitted by power plants and boilers
Nitrate and ammonium PM
From NOx emitted by combustion (eg fossil fuels)
Ammonia (NH3) emitted by livestock operations
Organic PM
Unburned fuel partially oxidizes and forms PM
Other combustion sources

49. Anthropogenic Emission Sources

50. Trend in PM-10 in the US

51. Trend in Fine PM in the US

52. 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.

53. Urban Fine PM Concentration and Composition

54. Rural Fine PM Concentration and Composition