AREP
GAW
Section 7
Chemical Aspects
of Air Pollution
Overview of Basic Pollutants
Ozone
Particulate Matter
Carbon Monoxide
Sulfur Dioxide
Nitrogen Oxides
AREP
GAW
Section 7 – Chemical Aspects of Air Pollution
2
AREP
GAW
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 1900’s, 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.
Section 7 – Chemical Aspects of Air Pollution
3
AREP
GAW
Los Angeles, California
(December 3, 1909)
Library of Congress Prints and Photographs Division, Washington, D. C.
Section 7 – Chemical Aspects of Air Pollution
4
AREP
GAW
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.
Section 7 – Chemical Aspects of Air Pollution
5
AREP
GAW
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
Section 7 – Chemical Aspects of Air Pollution
6
AREP
GAW
Basic Pollutants (2 of 3)
Pollutant
Abbreviation
Type
Carbon Monoxide
CO
Primary
Sulfur Dioxide
SO2
Primary
Ozone
O3
Secondary
Nitrogen Dioxide
NO2
Secondary
HC
Primary & Secondary
PM
Primary & Secondary
Hydrocarbon Compounds
(also called VOCs – volatile
organic compounds )
Particulate Matter
Section 7 – Chemical Aspects of Air Pollution
7
AREP
GAW
Basic Pollutants (3 of 3)
Section 7 – Chemical Aspects of Air Pollution
8
AREP
GAW
Basic Pollutants – Toxics (1 of 2)
●
●
Air toxics (hazardous air pollutants) are known or
suspected to cause cancer or other serious health
effects.
EPA’s 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)
Area/
Mobile
Other
25%
National air toxics emissions sources in 1996
U.S. Environmental Protection Agency, 1998
Point
24%
(nonroad)
20%
Mobile
(onroad)
31%
Section 7 – Chemical Aspects of Air Pollution
9
AREP
GAW
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
Section 7 – Chemical Aspects of Air Pollution
10
AREP
GAW
Basic Pollutants – Sources (1 of 4)
• Combustion
• Evaporation
• Natural Production
Section 7 – Chemical Aspects of Air Pollution
11
AREP
GAW
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
Section 7 – Chemical Aspects of Air Pollution
12
AREP
GAW
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)
Section 7 – Chemical Aspects of Air Pollution
13
AREP
GAW
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
Section 7 – Chemical Aspects of Air Pollution
14
AREP
GAW
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
Section 7 – Chemical Aspects of Air Pollution
15
AREP
GAW
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
Section 7 – Chemical Aspects of Air Pollution
16
AREP
GAW
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
Section 7 – Chemical Aspects of Air Pollution
17
AREP
GAW
Tropospheric Ozone Photolysis
Troposphere ozone photolysis takes place in a narrow UV window
(300-320 nm), NO2 broadly below 428
30o equinox
midday
Solar spectrum
Section 7 – Chemical Aspects of Air Pollution
18
AREP
GAW
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 yellowbrown 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)
Section 7 – Chemical Aspects of Air Pollution
19
AREP
GAW
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
Section 7 – Chemical Aspects of Air Pollution
20
AREP
GAW
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)
O(1D) + H2O  2 OH·
(~90%)
(~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
Section 7 – Chemical Aspects of Air Pollution
21
AREP
GAW
THE OH RADICAL: MAIN
TROPOSPHERIC OXIDANT
• Primary source:
• O3 + hn  O2 + O(1D)
• O(1D) + M  O + M
• O(1D) + H2O  2OH
(1)
(2)
(3)
• Sink: oxidation of reduced species –leads
to HO2(RO2) production
• CO + OH  CO2 + H
• CH4 + OH  CH3 + H2O
• HCFC + OH
Major
OH sinks
• Global Mean [OH] = 1.0x106 molecules
cm-3
Section 7 – Chemical Aspects of Air Pollution
22
AREP
GAW
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  O 3
CO + 2 O2 + hv  CO2 + O3
Section 7 – Chemical Aspects of Air Pollution
23
AREP
GAW
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
Section 7 – Chemical Aspects of Air Pollution
24
AREP
GAW
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
Section 7 – Chemical Aspects of Air Pollution
25
AREP
GAW
Ozone Chemistry
Summary of ozone chemistry
Meteorology
• NO2 + Sunlight  NO + O Production
• O+ O2  O3
Production
• NO + O3  NO2 + O2
• VOC + OH  RO2 + H2O
• RO2 + NO  NO2 + RO
Emissions
Chemistry
Destruction
Production of NO2 without the
Destruction of O3
RO=Reactive 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
Section 7 – Chemical Aspects of Air Pollution
26
AREP
GAW
Day and Night Chemistry
Section 7 – Chemical Aspects of Air Pollution
27
AREP
GAW
Ozone Precursor Emissions (1 of 2)
●
Man-made sources
– Oxides of nitrogen (NOx) through
combustion
– VOCs through combustion and
numerous other sources
●
Meteorology
Emissions
Chemistry
Natural sources (biogenic)
– VOCs from trees/vegetation
– NOx from soils (Midwest fertilizer)
●
Concentration depends on
– Source location, density, and
strength
– Meteorology
Section 7 – Chemical Aspects of Air Pollution
28
AREP
GAW
NOx EMISSIONS (Tg N yr-1) TO TROPOSPHERE
Stratosphere
0.2
Soils
5.1
Biomass
Burning
5.2
Lightning
5.8
Fossil Fuel
23.1
Biofuel
2.2
Aircraft
0.5
Section 7 – Chemical Aspects of Air Pollution
29
AREP
GAW
An example of gridded NOx emissions
Section 7 – Chemical Aspects of Air Pollution
30
AREP
GAW
Mapping of Tropospheric NO2
From the GOME satellite instrument (July 1996)
Section 7 – Chemical Aspects of Air Pollution
31
AREP
GAW
GOME Can Provide Info on Daily Info
Section 7 – Chemical Aspects of Air Pollution
32
AREP
GAW
Lightning Flashes Seen from Space
DJF
JJA
2000 data
Section 7 – Chemical Aspects of Air Pollution
33
AREP
GAW
Global Budget of CO
Section 7 – Chemical Aspects of Air Pollution
34
AREP
GAW
Satellite Observations of Biomass
Fires (1997)
Section 7 – Chemical Aspects of Air Pollution
35
AREP
GAW
Daily Los Angeles Emission (1987)
Gas
Emission (tons/day)
Percent of total
Carbon monoxide
Nitric oxide
Nitrogen dioxide
Nitrous acid
Total NOx+HONO
Sulfur dioxide
Sulfur trioxide
Total SOx
Alkanes
Alkenes
Aldehydes
Ketones
Alcohols
Aromatics
Hemiterpenes
Total ROGs
Methane
9796
754
129
6.5
889.5
109
4.5
113.5
1399
313
108
29
33
500
47
2429
904
69.3
Total emission
14,132
6.3
0.8
27.2
6.4
100
Section 7 – Chemical Aspects of Air Pollution
36
AREP
GAW
Percent Emission by Source-LA
Source Category
Stationary
Mobile
Total
CO(g)
2
98
100
NOx(g)
24
76
100
Section 7 – Chemical Aspects of Air Pollution
SOx(g)
38
62
100
ROG
50
50
100
Table 4.2
37
AREP
GAW
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
Section 7 – Chemical Aspects of Air Pollution
Table 4.4
38
AREP
GAW
Lifetimes of ROGs Against Chemical
Loss in Urban Air
ROG Species
n-Butane
trans-2-butene
Acetylene
Formaldehyde
Acetone
Ethanol
Toluene
Isoprene
Phot.
------7h
23 d
-------
OH
22 h
52 m
3d
6h
9.6 d
19 h
9h
34 m
HO2 O
1000 y 18 y
4y
6.3 d
--2.5 y
1.8 h 2.5 y
----------6y
--4d
NO3
29 d
4m
--2d
----33 d
5m
O3
650 y
17 m
200 d
3200 y
----200 d
4.6 h
Table 4.3
Section 7 – Chemical Aspects of Air Pollution
39
AREP
GAW
Summary
Section 7 – Chemical Aspects of Air Pollution
40
AREP
GAW
Ozone Meteorology – Key Processes
•
•
•
•
•
•
•
•
Dispersion (horizontal mixing)
Vertical mixing
Sunlight
Transport
Weather pattern
Geography
Diurnal
Season
Meteorology
Emissions
Chemistry
Section 7 – Chemical Aspects of Air Pollution
41
AREP
GAW
Ozone Precursor Emissions (2 of 2)
Wind speed (WS)
S
S
Concentration  S/WS
Vertical mixing (VM)
Concentration  S/VM
●
Key processes
– Source location, density, and strength
– Dispersion (horizontal mixing) - wind speed
– Vertical mixing - inversion
Courtesy of New Jersey
Department of Environmental Protection
Section 7 – Chemical Aspects of Air Pollution
42
AREP
GAW
Daily Variation
Section 7 – Chemical Aspects of Air Pollution
43
AREP
GAW
0.3
Central Los Angeles
August 28, 1987
0.2
NO2
NO
O3
0.1
0
0
6
12
18
Hour of day
24
Volume mixing ratio (ppmv)
Urban center
Volume mixing ratio (ppmv)
Volume mixing ratio (ppmv)
Source/Receptor Regions in Los Angeles
Sub-urban
0.3
San Bernardino
August 28, 1987
0.2
O3
NO2
0.1
0
NO
0
Section 7 – Chemical Aspects of Air Pollution
6
12
18
Hour of day
24
72
Figure 4.10
44
AREP
GAW
Ozone Isopleth Plot
0.32
0.32
0.16
0.24
0.24
0.08
0.16
0.1
0.4
3
0.15
0.08 = O (g), ppmv
NO
x x (ppmv)
0.2
NO
(g) (ppmv)
0.25
0.05
0
0
0.5
1
1.5
ROG (ppmC)
Contours are ozone (ppmv)
Section 7 – Chemical Aspects of Air Pollution
2
Figure 4.9
45
AREP
GAW
THIS OZONE BACKGROUND IS A SIZABLE INCREMENT
TOWARDS VIOLATION OF U.S. AIR QUALITY STANDARDS
(even more so in Europe!)
Europe
(8-h avg.)
Europe
(seasonal)
0
preindustrial
20
40
U.S.
(8-h avg.)
60
80
U.S.
(1-h avg.)
100
120 ppbv
present
background
Section 7 – Chemical Aspects of Air Pollution
Slide courtesy of D. Jacob
46
AREP
GAW
EU/USA
SURFACE OZONE ENHANCEMENTS CAUSED BY
ANTHROPOGENIC EMISSIONS FROM DIFFERENT CONTINENTS
GEOS-CHEM
model, July 1997
North America
Europe
Asia
Li et al. [2002]
Section 7 – Chemical Aspects of Air Pollution
47
AREP
GAW
Section 7 – Chemical Aspects of Air Pollution
48
AREP
GAW
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 < 5 µg/m3 *
Background concentrations can be higher
due to dust and smoke
Concentrations range from 0 to 500+ µg/m3 *
●
Health concerns
●
●
–
–
–
–
Ultra-fine fly-ash or
carbon soot
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
*
24-hour average
Section 7 – Chemical Aspects of Air Pollution
49
AREP
GAW
Particulate Matter (2 of 3)
Particles come in different shapes and sizes
Particle sizes
• Ultra-fine particles (<0.1 μm)
• Fine particles (0.1 to 2.5 μm)
• Coarse particles (2.5 to 10 μm)
Crustal material
PM10
Carbon chain agglomerates
Section 7 – Chemical Aspects of Air Pollution
50
AREP
GAW
Particulate Matter (3 of 3)
A clear (left) and dirty (right) PM filter
Section 7 – Chemical Aspects of Air Pollution
51
AREP
GAW
Particulate Matter Composition (1 of 3)
PM is composed of a mixture of primary and secondary
compounds.
●
Primary PM (directly
emitted)
–
–
–
–
–
–
Suspended dust
Sea salt
Organic carbon
Elemental carbon
Metals from combustion
Small amounts of sulfate
and nitrate
●
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
Section 7 – Chemical Aspects of Air Pollution
52
AREP
GAW
Particulate Matter Composition (3 of 3)
Most PM mass in urban and nonurban areas is composed of a
combination of the following chemical components
•
•
•
•
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
•
•
•
•
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
Section 7 – Chemical Aspects of Air Pollution
Chow and Watson (1997)
53
AREP
GAW
PM Emissions Sources (1 of 4)
Point – generally a major facility emitting pollutants from identifiable
sources (pipe or smoke stack). Facilities are typically permitted.
Section 7 – Chemical Aspects of Air Pollution
54
AREP
GAW
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
Section 7 – Chemical Aspects of Air Pollution
55
AREP
GAW
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
Section 7 – Chemical Aspects of Air Pollution
56
AREP
GAW
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
Section 7 – Chemical Aspects of Air Pollution
57
AREP
GAW
COMPOSITION OF PM2.5 IS HIGHLY VARIABLE (NARSTO PM
ASSESSMENT)
Sulfate
Esther (1995-99)
Egbert (1994-99)
4.6 ug m -3
8.9 ug m -3
Nitrate
Toronto (1997-99)
12.3 ug m -3
Ammonium
Black carbon
Abbotsford (1994-95)
Organic carbon
7.8 ug m -3
Soil
Other
St. A ndrews (1994-97)
5.3 ug m -3
Fresno (1988-89)
39.2 ug m -3
Quaker City OH (1999)
12.4 ug m -3
Kern Wildlife Refuge (1988-89)
23.3 ug m -3
Los Angeles (1995-96)
Arendstville PA (1999)
10.4 ug m -3
Mexico City Netzahualcoyotl (1997)
55.4 ug m -3
Washington DC (1996-99)
14.5 ug m -3
30.3 ug m -3
Colorado Plateau (1996-99)
3.0 ug m -3
Mexico City - Pedregal (1997)
24.6 ug m -3
Yorkville (1999)
14.7 ug m -3
Section 7 – Chemical Aspects of Air Pollution
Atlanta (1999)
19.2 ug m -3
58
AREP
GAW
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…
Section 7 – Chemical Aspects of Air Pollution
59
AREP
GAW
Particulate Matter Chemistry (1 of 4)
Coagulation: Particles collide and stick together.
Condensation: Gases condense onto a small solid particle
to form a liquid droplet.
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.
Section 7 – Chemical Aspects of Air Pollution
60
AREP
GAW
Particulate Matter Composition (2 of 3)
PM contains many compounds
Primary Particles
(directly emitted)
Secondary Particles
(from precursor gases)
VOCs
Carbon
(Soot)
Organic
Carbon
SO2
Metals
Ammonium
Sulfate
Crustal
(soil,dust)
Other
(sea salt)
Ammonium
Nitrate
Ammoni
a
Composition of PM
tells us about
the sources and
formation processes
Gas
NOx
Particle
Section 7 – Chemical Aspects of Air Pollution
61
AREP
GAW
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
Impact of low soil
pH on agriculture
in Victoria
German sandstone
statue, 1908, 1969
Section 7 – Chemical Aspects of Air Pollution
Low crown density
of spruce trees
62
AREP
GAW
Particulate Matter Chemistry (2 of 4)
Sulfate Chemistry
●
●
●
●
●
●
Heterogeneous Oxidation
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
Husar (1999)
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
Section 7 – Chemical Aspects of Air Pollution
63
AREP
GAW
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.
Section 7 – Chemical Aspects of Air Pollution
64
AREP
GAW
Section 7 – Chemical Aspects of Air Pollution
65
AREP
GAW
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.
Section 7 – Chemical Aspects of Air Pollution
66
AREP
GAW
Particulate Matter Chemistry (4 of 4)
Sources
PM
Formation
Emissions
PM Transport/Loss
Sample
Collection
Chemical Processes
Mechanical
• Sea salt
• Dust
Combustion
• Motor vehicles
• Industrial
• Fires
Particles
• NaCl
• Crustal
Particles
• Soot
• Metals
• OC
Measurement
Issues
transport
Gases
• NOx
• SO2
• VOCs
• NH3
Other gaseous
• Biogenic
• Anthropogenic
gases condense onto particles
cloud/fog processes
condensation and
coagulation
sedimentation
(dry deposition)
• Inlet cut points
• Vaporization of
nitrate, H2O, VOCs
• Adsorption of VOCs
• Absorption of H2O
wet deposition
photochemical production
cloud/fog processes
Gases
• VOCs
• NH3
• NOx
Meteorological Processes
Winds
Clouds, fog
Winds
Temperature
Temperature
Solar radiation
Vertical mixing
Temperature
Relative humidity
Solar radiation
Precipitation
Relative humidity
Winds
Section 7 – Chemical Aspects of Air Pollution
67
AREP
GAW
Particulate Matter Meteorology
How weather affects PM emissions, formation, and transport
Phenomena
Emissions
PM Formation
PM Transport/Loss
Aloft Pressure
Pattern
No direct impact.
No direct impact.
Ridges tend to produce conditions conducive for accumulation of PM2.5.
Troughs tend to produce conditions conducive for dispersion and removal of PM and
ozone.
In mountain-valley regions, strong wintertime inversions and high PM2.5 levels may not be
altered by weak troughs.
High PM2.5 concentrations often occur during the approach of a trough from the west.
Winds and
Transport
No direct impact.
In general, stronger winds disperse
pollutants, resulting in a less ideal
mixture of pollutants for chemical
reactions that produce PM2.5.
Strong surface winds tend to disperse PM2.5 regardless of season.
Strong winds can create dust which can increase PM2.5 concentrations.
Temperature
Inversions
No direct impact.
Inversions reduce vertical mixing and
therefore increase chemical
concentrations of precursors. Higher
concentrations of precursors can
produce faster, more efficient
chemical reactions that produce
PM2.5.
A strong inversion acts to limit vertical mixing allowing for the accumulation of PM2.5.
Rain
Reduces soil and fire emissions
Rain can remove precursors of
PM2.5.
Rain can remove PM2.5.
Moisture
No direct impact.
Moisture acts to increase the
production of secondary PM2.5
including sulfates and nitrates.
No direct impact.
Temperature
Warm temperatures are associated
with increased evaporative,
biogenic, and power plant
emissions, which act to increase
PM2.5. Cold temperatures can also
indirectly influence PM2.5
concentrations (i.e., home heating
on winter nights).
Photochemical reaction rates
increase with temperature.
Although warm surface temperatures are generally associated with poor air quality
conditions, very warm temperatures can increase vertical mixing and dispersion of
pollutants.
Warm temperatures may volatize Nitrates from a solid to a gas.
Very cold surface temperatures during the winter may produce strong surface-based
inversions that confine pollutants to a shallow layer.
Clouds/Fog
No direct impact.
Water droplets can enhance the
formation of secondary PM2.5. Clouds
can limit photochemistry, which limits
photochemical production.
Convective clouds are an indication of strong vertical mixing, which disperses pollutants.
Season
Forest fires, wood burning,
agriculture burning, field tilling,
windblown dust, road dust, and
construction vary by season.
The sun angle changes with season,
No direct impact.
Section 7 – Chemical Aspects
of Air Pollution
which changes the amount of solar
radiation available for
photochemistry.
68
AREP
GAW
ANNUAL MEAN PARTICULATE MATTER (PM) CONCENTRATIONS AT
U.S. SITES, 1995-2000
NARSTO PM Assessment, 2003
PM10 (particles > 10 mm)
PM2.5 (particles > 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
Section 7 – Chemical Aspects of Air Pollution
69
AREP
GAW
AEROSOL OPTICAL DEPTH (GLOBAL MODEL)
Annual mean
Section 7 – Chemical Aspects of Air Pollution
70
AREP
GAW
AEROSOL OBSERVATIONS FROM SPACE
Biomass fire haze in central America yesterday (4/30/03)
Fire locations
in red
Modis.gsfc.nasa.gov
Section 7 – Chemical Aspects of Air Pollution
71
AREP
GAW
BLACK CARBON EMISSIONS
DIESEL
DOMESTIC
COAL BURNING
BIOMASS
BURNING
Section 7 – Chemical Aspects of Air Pollution
Chin et al. [2000]
72
AREP
GAW
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
Section 7 – Chemical Aspects of Air Pollution
Garden speech)
73
AREP
GAW
Particles Impact Human Health and MORE
AREP
GAW
EPA REGIONAL HAZE RULE: FEDERAL CLASS I AREAS TO RETURN TO
“NATURAL” VISIBILITY LEVELS BY 2064
…will require essentially total elimination of anthropogenic aerosols!
•
clean day
moderately polluted day
Acadia National Park
Section 7 – Chemical Aspects of Air Pollution
http://www.hazecam.net/
75
AREP
GAW
ASIAN DUST INFLUENCE IN UNITED STATES
Dust observations from U.S. IMPROVE network
April 16, 2001
Asian dust in western U.S.
0
2
April 22, 2001
Asian dust in southeastern U.S.
4
mg m-3
6
8
Glen
Canyon,
AZ
Section 7 – Chemical Aspects of Air April
Pollution16,
Clear day
2001: Asian dust!
76
AREP
GAW
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
Section 7 – Chemical Aspects of Air Pollution
77