Section 7 Chemical Aspects of Air Pollution

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Section 7Chemical Aspects of Air Pollution

• Overview of Basic Pollutants
• Ozone
• Particulate Matter
• Carbon Monoxide
• Sulfur Dioxide
• Nitrogen Oxides

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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.

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Los Angeles, California (December 3, 1909)
Library of Congress Prints and Photographs
Division, Washington, D. C.
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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.

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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

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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
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Basic Pollutants (3 of 3)
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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
1996U.S. Environmental Protection Agency, 1998
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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

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Basic Pollutants Sources (1 of 4)

• Combustion
• Evaporation
• Natural Production

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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

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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)

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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

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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

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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

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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

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Tropospheric Ozone Photolysis

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

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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)

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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

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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

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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
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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

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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

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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

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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

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Day and Night Chemistry
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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

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NOx EMISSIONS (Tg N yr-1) TO TROPOSPHERE
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An example of gridded NOx emissions
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Mapping of Tropospheric NO2

• From the GOME satellite instrument (July 1996)

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GOME Can Provide Info on Daily Info
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Lightning Flashes Seen from Space

• 2000 data

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Global Budget of CO
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Satellite Observations of Biomass Fires (1997)
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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
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Percent Emission by Source-LA
Source Category CO(g) NOx(g) SOx(g) ROG
Stationary 2 24 38 50 Mobile [phone] Total 10
0 100 100 100
Table 4.2
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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
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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
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Summary
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Ozone Meteorology Key Processes

• Dispersion (horizontal mixing)
• Vertical mixing
• Sunlight
• Transport
• Weather pattern
• Geography
• Diurnal
• Season

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Ozone Precursor Emissions (2 of 2)

• Key processes
• Source location, density, and strength
• Dispersion (horizontal mixing) - wind speed
• Vertical mixing - inversion

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Daily Variation
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Source/Receptor Regions in Los Angeles
Urban center
Sub-urban
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Ozone Isopleth Plot
0.32
0.08
0.24
NOx (ppmv)
0.16
Figure 4.9
Contours are ozone (ppmv)
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Slide courtesy of D. Jacob
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EU/USA
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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
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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
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Particulate Matter (3 of 3)
A clear (left) and dirty (right) PM filter
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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

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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)
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PM Emissions Sources (1 of 4)

• Point generally a major facility emitting
  pollutants from identifiable sources (pipe or
  smoke stack). Facilities are typically
  permitted.

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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

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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

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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

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COMPOSITION OF PM2.5 IS HIGHLY VARIABLE (NARSTO
PM ASSESSMENT)
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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
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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.
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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
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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

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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

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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.
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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.

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Particulate Matter Chemistry (4 of 4)
Meteorological Processes
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Particulate Matter Meteorology
How weather affects PM emissions, formation, and
transport
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ANNUAL MEAN PARTICULATE MATTER (PM)
CONCENTRATIONS AT U.S. SITES, 1995-2000NARSTO 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
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AEROSOL OPTICAL DEPTH (GLOBAL MODEL)
Annual mean
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AEROSOL OBSERVATIONS FROM SPACE
Biomass fire haze in central America yesterday
(4/30/03)
Fire locations in red
Modis.gsfc.nasa.gov
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BLACK CARBON EMISSIONS
DIESEL
DOMESTIC COAL BURNING
BIOMASS BURNING
Chin et al. 2000
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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)
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Particles Impact Human Health and MORE
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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/
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ASIAN DUST INFLUENCE IN UNITED STATESDust
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
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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