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Section 7 Chemical Aspects of Air Pollution

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Title: Section 7 Chemical Aspects of Air Pollution


1
Section 7 Chemical Aspects of Air Pollution
  • Overview of Basic Pollutants
  • Ozone
  • Particulate Matter
  • Carbon Monoxide
  • Sulfur Dioxide
  • Nitrogen Oxides

2
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3
Photochemical Smog
  • Air pollution formed by sunlight catalyzing
    chemical reactions of emitted compounds
  • Los Angeles, California
  • Early pollution due to London-type smog.
  • 1905-1912, L.A. City Council adopts regulation
    controlling smoke
  • Early 1900s, automobile use increases.
  • 1939-1943 visibility decreases significantly.
  • Plume of pollution engulfs downtown (26 July
    1943).
  • 1943 L.A. County Board of Supervisors bans
    emission of dense smoke and creates office called
    Director of Air Pollution Control
  • 1945. L.A. Health Officer suggests pollution due
    to locomotives, diesel trucks, backyard
    incinerators, lumber mills, dumps, cars.
  • 1946. L.A. Times hires air pollution expert to
    find methods to ameliorate pollution.

4
Los Angeles, California (December 3, 1909)
Library of Congress Prints and Photographs
Division, Washington, D. C.
5
Discovery of Ozone in Smog
  • 1948 Arie Haagen-Smit (1900-1977), biochemistry
    professor at Caltech, begins to study plants
    damaged by smog.
  • 1950 Finds that plants sealed in a chamber and
    exposed to ozone exhibit similar damage as did
    plants in smog
  • Also finds that ozone caused eye irritation,
    damage to materials, respiratory problems.
  • Other researchers find that rubber cracks within
    minutes when exposed to high ozone.
  • 1952 Haagen-Smit finds that ozone forms when
    oxides of nitrogen and reactive organic gases are
    exposed to sunlight. Postulates that ozone and
    precursors are main constituents of L.A. smog.
  • Oil companies and business leaders argue that
    ozone in L.A. originates from stratosphere.
  • Measurements of low ozone over Catalina Island
    disprove this.

6
Basic Pollutants (1 of 3)
  • Categories of pollutants
  • Primary emitted directly from a source
  • Secondary formed in the atmosphere from a
    reaction of primary pollutants
  • Precursors primary pollutants (gases) that
    participate in the formation of secondary
    pollutants
  • Pollutants originate from
  • Combustion of fossil fuels and organic matter
  • Evaporation of petroleum products or compounds
    used in commercial products, services, and
    manufacturing
  • Natural production of smoke from fires, dust from
    strong winds, and emissions from the biosphere
    and geosphere

7
Basic Pollutants (2 of 3)
Type
Abbreviation
Pollutant
Primary
CO
Carbon Monoxide
Primary
SO2
Sulfur Dioxide
Secondary
O3
Ozone
Secondary
NO2
Nitrogen Dioxide
Primary Secondary
HC
Hydrocarbon Compounds (also called VOCs
volatile organic compounds )
Primary Secondary
PM
Particulate Matter
8
Basic Pollutants (3 of 3)
9
Basic Pollutants Toxics (1 of 2)
  • Air toxics (hazardous air pollutants) are known
    or suspected to cause cancer or other serious
    health effects.
  • EPAs 188 hazardous air pollutants include
  • Benzene (motor fuel, oil refineries, chemical
    processes)
  • Perchlorethylene (dry cleaning, degreasing)
  • Chloroform (solvent in adhesive and pesticides,
    by-product of chlorination processes)
  • BTEX, Dioxins, PAHs, Metals (Hg, Cr)

National air toxics emissions sources in
1996 U.S. Environmental Protection Agency, 1998
10
Basic Pollutants Toxics (2 of 2)
  • Differences between toxics and criteria
    pollutants
  • Health criteria are different
  • No AQI-like standards for toxics
  • Cancer/non-cancer benchmarks (long-term
    exposures)
  • Short-term exposure limits for some
  • A challenge to monitor
  • Usually not available in real-time
  • Example Dioxin requires 28 days of sampling to
    acquire measurable amounts in ambient air
  • Often localized near source

11
Basic Pollutants Sources (1 of 4)
  • Combustion
  • Evaporation
  • Natural Production

12
Basic Pollutants Sources (2 of 4)
  • Combustion
  • Complete combustion
  • Fuel ? water and carbon dioxide (CO2)
  • Incomplete combustion
  • Fuel ? water, CO2, and other pollutants
  • Pollutants are both gases and particles

13
Basic Pollutants Sources (3 of 4)
  • Evaporation
  • Thousands of chemical compounds
  • Liquids evaporating or gases being released
  • Some harmful by themselves, some react to produce
    other pollutants
  • Many items you can smell are evaporative
    pollutants
  • Gasoline benzene (sweet odor, toxic,
    carcinogenic)
  • Bleach chlorine (toxic, greenhouse gas)
  • Trees pinenes, limonene (ozone- and particulate
    matter forming)
  • Paint volatile organic compounds (ozone- and
    particulate matter forming)
  • Baking bread, fermenting wine and beer VOCs and
    ethanol (ozone-forming)

14
Basic Pollutants Sources (4 of 4)
  • Natural Production
  • Fires (combustion) produce gases and particles
  • Winds pick up dust, dirt, sand and create
    particles of various sizes
  • Biosphere emits gases from trees, plants, soil,
    ocean, animals, microbes
  • Volcanoes and oil seeps produce particles and
    gases

15
Ozone
  • Colorless gas
  • Composed of three oxygen atoms
  • Oxygen molecule (O2)needed to sustain life
  • Ozone (O3) the extra oxygen atom makes ozone
    very reactive
  • Secondary pollutant that forms from precursor
    gases
  • Nitric oxide combustion product
  • Volatile organic compounds (VOCs) evaporative
    and combustion products

16
Solar radiation and chemistry
  • The reaction that produces ozone in the
    atmosphere
  • O O2 M ? O3 M
  • Difference between stratospheric and tropospheric
    ozone generation is in the source of atomic O
  • For solar radiation with a wavelength of less
    than 242 nm
  • O2 hv ? O O

17
Solar radiation and chemistry
  • Photochemical production of O3 in troposphere
    tied to NOx (NO NO2)
  • For wavelengths less than 424 nm
  • NO2 hv ? NO O
  • But NO will react with O3
  • NO O3 ? NO2
  • Cycling has no net effect on ozone

18
Tropospheric Ozone Photolysis
  • Troposphere ozone photolysis takes place in a
    narrow UV window
  • (300-320 nm), NO2 broadly below 428

19
Nitrogen Oxides
  • Nitrogen oxides, or NOx, is the generic term for
    a group of highly reactive gases, all of which
    contain nitrogen and oxygen in varying amounts.
  • Nitrogen dioxide is most visually prominent (it
    is the yellow-brown color in smog)
  • The primary man-made sources of NOx are motor
    vehicles electric utilities and other
    industrial, commercial, and residential sources
    that burn fuels
  • Affects the respiratory system
  • Involved in other pollutant chemistry
  • One of the main ingredients in the formation of
    ground-level ozone
  • Reacts to form nitrate particles, acid aerosols,
    and NO2, which also cause respiratory problems
  • Contributes to the formation of acid rain
    (deposition)

20
Must make NO2
  • To make significant amounts of ozone must have a
    way to make NO2 without consuming ozone
  • Presence of peroxy radicals, from the oxidation
    of hydrocarbons, disturbs O3-NO-NO2 cycle
  • NO HO2 ? NO2 OH
  • NO RO2 ? NO2 RO
  • leads to net
  • production of ozone

21
The Hydroxyl Radical
  • produced from ozone photolysis
  • for radiation with wavelengths less than 320 nm
  • O3 hv ? O(1D) O2
  • followed by
  • O(1D) M ? O(3P) M (O2?O3) (90)
  • O(1D) H2O ? 2 OH (10)
  • OH initiates the atmospheric oxidation of a wide
    range of compounds in the atmosphere
  • referred to as detergent of the atmosphere
  • typical concentrations near the surface 106 -
    107cm-3
  • very reactive, effectively recycled

22
THE OH RADICAL MAIN TROPOSPHERIC OXIDANT
  • Primary source
  • O3 hn ? O2 O(1D) (1)
  • O(1D) M ? O M (2)
  • O(1D) H2O ? 2OH (3)
  • Sink oxidation of reduced species leads to
    HO2(RO2) production
  • CO OH ? CO2 H
  • CH4 OH ? CH3 H2O
  • HCFC OH
  • Global Mean OH 1.0x106 molecules cm-3

Major OH sinks
23
Oxidation of CO - production of ozone
  • CO OH ? CO2 H
  • H O2 M ? HO2 M
  • NO HO2 ? NO2 OH
  • NO2 hv ? NO O
  • O O2 M ? O3
  • CO 2 O2 hv ? CO2 O3

24
Carbon Monoxide
  • Odorless, colorless gas
  • Caused by incomplete combustion of fuel
  • Most of it comes from motor vehicles
  • Reduces the transport of oxygen through the
    bloodstream
  • Affects mental functions and visual acuity, even
    at low levels

25
What breaks the cycle?
  • Cycle terminated by
  • OH NO2 ? HNO3
  • HO2 HO2 ? H2O2
  • Both HNO3 and H2O2 will photolyze or react with
    OH to, in effect, reverse these pathways
  • but reactions are slow (lifetime of several days)
  • both are very soluble - though H2O2 less-so
  • washout by precipitation
  • dry deposition
  • in PBL they are effectively a loss
  • situation is more complicated in the upper
    troposphere
  • no dry deposition, limited wet removal

26
Ozone Chemistry
  • Summary of ozone chemistry
  • NO2 Sunlight ? NO O Production
  • O O2 ? O3 Production
  • NO O3 ? NO2 O2 Destruction
  • VOC OH ? RO2 H2O Production of NO2 without
    the
  • RO2 NO ? NO2 RO Destruction of O3

ROReactive Organic compound such as VOC
  • Key processes
  • Ample sunlight (ultraviolet)
  • High concentrations of precursors (VOC, NO, NO2)
  • Weak horizontal dispersion
  • Weak vertical mixing
  • Warm air

27
Day and Night Chemistry
28
Ozone Precursor Emissions (1 of 2)
  • Man-made sources
  • Oxides of nitrogen (NOx) through combustion
  • VOCs through combustion and numerous other
    sources
  • Natural sources (biogenic)
  • VOCs from trees/vegetation
  • NOx from soils (Midwest fertilizer)
  • Concentration depends on
  • Source location, density, and strength
  • Meteorology

29
NOx EMISSIONS (Tg N yr-1) TO TROPOSPHERE
30
An example of gridded NOx emissions
31
Mapping of Tropospheric NO2
  • From the GOME satellite instrument (July 1996)

32
GOME Can Provide Info on Daily Info
33
Lightning Flashes Seen from Space
  • 2000 data

34
Global Budget of CO
35
Satellite Observations of Biomass Fires (1997)
36
Daily Los Angeles Emission (1987)
Gas Emission (tons/day) Percent of total Carbon
monoxide 9796 69.3 Nitric oxide 754 Nitrogen
dioxide 129 Nitrous acid 6.5 Total
NOxHONO 889.5 6.3 Sulfur dioxide 109 Sulfur
trioxide 4.5 Total SOx 113.5 0.8 Alkanes 1399
Alkenes 313 Aldehydes 108 Ketones 29 Alcohol
s 33 Aromatics 500 Hemiterpenes 47 Total
ROGs 2429 27.2 Methane 904 6.4 Total
emission 14,132 100
37
Percent Emission by Source-LA
Source Category CO(g) NOx(g) SOx(g) ROG
Stationary 2 24 38 50 Mobile 98 76 62 50 Total 10
0 100 100 100
Table 4.2
38
Most Important Gases in Smog in Terms of Ozone
Reactivity and Abundance
1. m- and p-Xylene 2. Ethene 3. Acetaldehyde 4.
Toluene 5. Formaldehyde 6. i-Pentane 7.
Propene 8. o-Xylene 9. Butane 10.
Methylcyclopentane
Table 4.4
39
Lifetimes of ROGs Against Chemical Loss in Urban
Air
ROG Species Phot. OH HO2 O NO3 O3
n-Butane --- 22 h 1000 y 18 y 29 d 650
y trans-2-butene --- 52 m 4 y 6.3 d 4 m 17
m Acetylene --- 3 d --- 2.5 y --- 200
d Formaldehyde 7 h 6 h 1.8 h 2.5 y 2 d 3200
y Acetone 23 d 9.6 d --- --- --- --- Ethanol ---
19 h --- --- --- --- Toluene --- 9 h --- 6 y 33
d 200 d Isoprene --- 34 m --- 4 d 5 m 4.6 h
Table 4.3
40
Summary
41
Ozone Meteorology Key Processes
  • Dispersion (horizontal mixing)
  • Vertical mixing
  • Sunlight
  • Transport
  • Weather pattern
  • Geography
  • Diurnal
  • Season

42
Ozone Precursor Emissions (2 of 2)
  • Key processes
  • Source location, density, and strength
  • Dispersion (horizontal mixing) - wind speed
  • Vertical mixing - inversion

43
Daily Variation
44
Source/Receptor Regions in Los Angeles
Urban center
Sub-urban
45
Ozone Isopleth Plot
0.32
0.08
0.24
NOx (ppmv)
0.16
Figure 4.9
Contours are ozone (ppmv)
46
Slide courtesy of D. Jacob
47
EU/USA
48
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49
Particulate Matter (1 of 3)
  • Complex mixture of solid and liquid particles
  • Composed of many different compounds
  • Both a primary and secondary pollutant
  • Sizes vary tremendously
  • Forms in many ways
  • Clean-air levels are lt 5 µg/m3
  • Background concentrations can be higher due to
    dust and smoke
  • Concentrations range from 0 to 500 µg/m3
  • Health concerns
  • Can aggravate heart diseases
  • Associated with cardiac arrhythmias and heart
    attacks
  • Can aggravate lung diseases such as asthma and
    bronchitis
  • Can increase susceptibility to respiratory
    infection

Ultra-fine fly-ash or carbon soot
24-hour average
50
Particulate Matter (2 of 3)
  • Particles come in different shapes and sizes
  • Particle sizes
  • Ultra-fine particles (lt0.1 µm)
  • Fine particles (0.1 to 2.5 µm)
  • Coarse particles (2.5 to 10 µm)

PM10
Carbon chain agglomerates
Crustal material
51
Particulate Matter (3 of 3)
A clear (left) and dirty (right) PM filter
52
Particulate Matter Composition (1 of 3)
PM is composed of a mixture of primary and
secondary compounds.
  • Secondary PM (precursor gases that form PM in the
    atmosphere)
  • Sulfur dioxide (SO2) forms sulfates
  • Nitrogen oxides (NOx) forms nitrates
  • Ammonia (NH3) forms ammonium compounds
  • Volatile organic compounds (VOCs) form organic
    carbon compounds
  • Primary PM (directly emitted)
  • Suspended dust
  • Sea salt
  • Organic carbon
  • Elemental carbon
  • Metals from combustion
  • Small amounts of sulfate and nitrate

53
Particulate Matter Composition (3 of 3)
Most PM mass in urban and nonurban areas is
composed of a combination of the following
chemical components
  • NaCl salt is found in PM near sea coasts and
    after de-icing materials are applied
  • Organic Carbon (OC) consists of hundreds of
    separate compounds containing mainly carbon,
    hydrogen, and oxygen
  • Elemental Carbon (EC) composed of carbon
    without much hydrocarbon or oxygen. EC is black,
    often called soot.
  • Liquid Water soluble nitrates, sulfates,
    ammonium, sodium, other inorganic ions, and some
    organic material absorb water vapor from the
    atmosphere
  • Geological Material suspended dust consists
    mainly of oxides of Al, Si, Ca, Ti, Fe, and other
    metal oxides
  • Ammonium ammonium bisulfate, sulfate, and
    nitrate are most common
  • Sulfate results from conversion of SO2 gas to
    sulfate-containing particles
  • Nitrate results from a reversible gas/particle
    equilibrium between ammonia (NH3), nitric acid
    (HNO3), and particulate ammonium nitrate

Chow and Watson (1997)
54
PM Emissions Sources (1 of 4)
  • Point generally a major facility emitting
    pollutants from identifiable sources (pipe or
    smoke stack). Facilities are typically
    permitted.

55
PM Emissions Sources (2 of 4)
  • Area any low-level source of air pollution
    released over a diffuse area (not a point) such
    as consumer products, architectural coatings,
    waste treatment facilities, animal feeding
    operations, construction, open burning,
    residential wood burning, swimming pools, and
    charbroilers

56
PM Emissions Sources (3 of 4)
  • Mobile
  • On-road is any moving source of air pollution
    such as cars, trucks, motorcycles, and buses
  • Non-road sources include pollutants emitted by
    combustion engines on farm and construction
    equipment, locomotives, commercial marine
    vessels, recreational watercraft, airplanes, snow
    mobiles, agricultural equipment, and lawn and
    garden equipment

57
PM Emissions Sources (4 of 4)
  • Natural biogenic and geogenic emissions from
    wildfires, wind blown dust, plants, trees,
    grasses, volcanoes, geysers, seeps, soil, and
    lightning

58
COMPOSITION OF PM2.5 IS HIGHLY VARIABLE (NARSTO
PM ASSESSMENT)
59
ORIGIN OF THE ATMOSPHERIC AEROSOL
Aerosol dispersed condensed matter suspended in
a gas Size range 0.001 mm (molecular cluster) to
100 mm (small raindrop)
Soil dust Sea salt
Environmental importance health (respiration),
visibility, radiative balance, cloud formation,
heterogeneous reactions, delivery of nutrients…
60
Particulate Matter Chemistry (1 of 4)
Coagulation Particles collide and stick
together.
Cloud/Fog Processes Gases dissolve in a water
droplet and chemically react. A particle exists
when the water evaporates.
Sulfate
Chemical Reaction Gases react to form particles.
61
Particulate Matter Composition (2 of 3)
  • PM contains many compounds

Primary Particles (directly emitted)
Secondary Particles (from precursor gases)
VOCs
Organic Carbon
Carbon (Soot)
SO2
Metals

Ammonium Sulfate
Composition of PM tells us about the sources and
formation processes

Crustal (soil,dust)
Ammonium Nitrate
Other (sea salt)
Ammonia
Gas
NOx
Particle
62
Sulfur Dioxide
  • Sulfur dioxide (SO2) belongs to the family of
    sulfur oxide (SOx) gases.
  • Gases are formed when fuel containing sulfur
    (mainly coal and oil) is burned and during metal
    smelting and other industrial processes.
  • Affects the respiratory system
  • Reacts in the atmosphere to form acids, sulfates,
    and sulfites
  • Contributes to acid rain

63
Particulate Matter Chemistry (2 of 4)
  • Sulfate Chemistry
  • Virtually all ambient sulfate (99) is
    secondary, formed within the atmosphere from SO2
    during the summer.
  • About half of SO2 oxidation to sulfate occurs in
    the gas phase through photochemical oxidation in
    the daytime. NOx and hydrocarbon emissions tend
    to enhance the photochemical oxidation rate.
  • At least half of SO2 oxidation takes place in
    cloud droplets as air molecules react in clouds.
  • Within clouds, soluble pollutant gases, such as
    SO2, are scavenged by water droplets and rapidly
    oxidize to sulfate.
  • Only a small fraction of cloud droplets deposit
    out as rain most droplets evaporate and leave a
    sulfate residue or convective debris.
  • Typical conversion rate 1-10 per hour

64
Mechanisms of Converting S(IV) to S(VI)
Why is converting to S(VI) important? It allows
sulfuric acid to enter or form within cloud drops
and aerosol particles, increasing their
acidity Mechanisms 1. Gas-phase oxidation of
SO2(g) to H2SO4(g) followed by condensation of
H2SO4(g) 2. Dissolution of SO2(g) into liquid
water to form H2SO3(aq) followed by aqueous
chemical conversion of H2SO3(aq) and its
dissociation products to H2SO4(aq) and its
dissociation products.
65
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66
Particulate Matter Chemistry (3 of 4)
  • Nitrate Chemistry
  • NO2 can be converted to nitric acid (HNO3) by
    reaction with hydroxyl radicals (OH) during the
    day.
  • The reaction of OH with NO2 is about 10 times
    faster than the OH reaction with SO2.
  • The peak daytime conversion rate of NO2 to HNO3
    in the gas phase is about 10 to 50 per hour.
  • During the nighttime, NO2 is converted into HNO3
    by a series of reactions involving ozone and the
    nitrate radical.
  • HNO3 reacts with ammonia to form particulate
    ammonium nitrate (NH4NO3).
  • Thus, PM nitrate can be formed at night and
    during the day daytime photochemistry also forms
    ozone.

67
Particulate Matter Chemistry (4 of 4)
Meteorological Processes
68
Particulate Matter Meteorology
How weather affects PM emissions, formation, and
transport
69
ANNUAL MEAN PARTICULATE MATTER (PM)
CONCENTRATIONS AT U.S. SITES, 1995-2000 NARSTO PM
Assessment, 2003
PM10 (particles gt 10 mm)
PM2.5 (particles gt 2.5 mm)
Red circles indicate violations of national air
quality standard 50 mg m-3 for PM10
15 mg m-3 for PM2.5
70
AEROSOL OPTICAL DEPTH (GLOBAL MODEL)
Annual mean
71
AEROSOL OBSERVATIONS FROM SPACE
Biomass fire haze in central America yesterday
(4/30/03)
Fire locations in red
Modis.gsfc.nasa.gov
72
BLACK CARBON EMISSIONS
DIESEL
DOMESTIC COAL BURNING
BIOMASS BURNING
Chin et al. 2000
73
RADIATIVE FORCING OF CLIMATE, 1750-PRESENT
IPCC 2001
Kyoto also failed to address two major
pollutants that have an impact on warming  black
soot and tropospheric ozone.  Both are proven
health hazards.  Reducing both would not only
address climate change, but also dramatically
improve people's health. (George W. Bush, June
11 2001 Rose Garden speech)
74
Particles Impact Human Health and MORE
75
EPA REGIONAL HAZE RULE FEDERAL CLASS I AREAS TO
RETURN TO NATURAL VISIBILITY LEVELS BY 2064
…will require essentially total elimination of
anthropogenic aerosols!

moderately polluted day
clean day
Acadia National Park
http//www.hazecam.net/
76
ASIAN DUST INFLUENCE IN UNITED STATES Dust
observations from U.S. IMPROVE network
April 16, 2001 Asian dust in western U.S.
April 22, 2001 Asian dust in southeastern U.S.
0 2 4
6 8 mg m-3
Glen Canyon, AZ
April 16, 2001 Asian dust!
Clear day
77
Aerosols Link Air Quality, Health and Climate

Dirtier Air and a Dimmer Sun
Anderson et al., Science 2003
Smith et al., 2003
He et al., 2002
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