Ozone

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Air pollutants
CO (transportation/internal combustion) asphyxiat
ion from binding hemoglobin hv smog SO2 (coal
combustion metal sulfide ores) acid rain H2SO4
aerosol impairs breathing hv smog Toxic organics
small aldehydes benzene polycyclic
aromatics all known/likely carcinogens many
other toxics (eg, metals, pesticides etc.,
are not air pollutants) Particulates
climate cooling effects, health hazards NOx
and VOCs (including CH4) primary pollutants
that generate photochemical smog, including
generation of ground-level ozone
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Levels of gas-phase pollutants are regulated to
minimize health effects
Regulation of these criteria pollutants, as well
as of lead and particulates, is through
Title II of the Clean Air Act, which regulates
mobile sources Toxic substances, including most
chemicals, lead, PCBs, asbestos are regulated
by EPA through the Toxic Substances Control Act
(1976)
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Particulates

• Particles suspended in air
• (dust, soot, mist, fog)
• Slow sedimentation depending
• on square of diameter
• Classed by size 2 nm 100 mM
• 2.5 mM cutoff defines coarse
• vs. fine particles
• Fine particles are a criteria
• pollutant under the CAA.

asbestos fibers, 0.01 0.1 m thick
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Particulates- Sources

• Coarse particles
• Mostly from natural sources
• soil dust, debris from fires, sea salt aerosols,
  pollen, volcanoes
• carbonate in coarse particles helps neutralize
  acid rain
• formed by breakup of larger particles
• Fine particles
• Significant anthropogenic component
• Vehicle exhausts, soot (carbon crystallites)
• formed by coagulation of smaller particles
• include more organics (VOCs from emissions
  combining
• with other pollutants in photochemical smog)

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Particulates- Acidity
HNO3 and H2SO4 are present in fine particles
acidity Vapor pressure of HNO3 much higher than
H2SO4, so wet deposition is much more of a
problem with the latter Can be neutralized by
reaction with NH3 (especially in agri. areas with
high fertilizer use H2SO4(aq) 2NH3(g) ?
(NH4)2SO4(aq) HNO3(aq) NH3(g) ?
NH4NO3(aq) Result in sulfate and nitrate
aerosols since both acids readily dissolve in
water
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Particulates- PM indices
PM index amount of particulate matter in a
given volume (mg/m3) PM10 concentration of
particles with D lt 10 mM PM 2.5 concentration
of particles with D lt 2.5 mM (all fine
(respirable) particles). Typically 10-20 mg/m3
in cities Ultrafine particles with D lt 0.1 mM
Size distribution of particles in an urban area
Nuclei mode from condensation of pollutant
gases Accumulation mode coagulation of
nuclei Coarse particles from mechanical
disintegration of larger particles
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Dominant sink varies for each size range
Large particles settle out (remember PSCs)
Residence times
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Particles can carry pollutants by absorption or
adsorption
Total particle surface area determines amount of
adsorbed material A large number of small
particles carry more pollutants than a smaller
number of large particles (particle volume
constant but surface area for adsorption is
larger with many small particles)
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Dose-response relationship for fine particles in
US No threshhold is apparent
SO2 aerosol soot from coal
London Smog of December 1952 from a temperature
inversion http//www.portfolio.mvm.ed.ac.uk/studen
twebs/session4/27/greatsmog52.htm
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Ground-level Air Pollution

• Urban air pollution in Mexico City
• possibly the worst in the world
• thermal inversion from Altiplano topography
• mountaintop cool air flows into valley at
  night
• this cool surface layer gets trapped under
  
• warmer air rising from the valley
• smoke fog smog
• inversion layer stable air mass (no
  mixing)

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Photochemical smog
Primary pollutants ?NO, CO, CH4, volatile
organic carbon (VOCs), SO2 sources internal
combustion engines, electric power
plants, evaporation of solvents O2 hv ? ? ?
O3, HNO3, oxidation products of organics
(aldehydes, acids, etc etc) these are the
secondary pollutants
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• How air composition
• changes during a smog
• episode.
• VOCs (HC) and ?NO are
• the primary pollutants
• (fuel combustion)
• ?NO2 and aldehydes are
• synthesized ?NO2 absorbs
• visible light
• O3 builds up only when
• ?NO concentration declines
• aldehydes are formed late
• by oxidation with O2

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Production of NOx during fuel combustion

• N2 O2 ? 2 ?NO
• Highly endothermic (stable N2, O2), needs very
  hot temperatures
• Occurs when a fuel is burned in air (internal
  combustion engine)
• Exhaust gas cools rapidly
• Reverse reaction has a high activation barrier ?
  nitrogen trapped as ?NO
• Additional ?NO is produced from oxidation of N
  contaminants in the fuel
• ?NO is oxidized to ?NO2 this species is
  responsible for yellowish haze in
• smog, since it absorbs in the 400 nm range
• How this occurs in the internal combustion
  engine
• O2 ? 2O
• O N2 ? ?NO N

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Ground-level O3, as a component of photochemical
smog, requires generation of ?NO2 radical
First the ?NO2 originates from oxidation of ?NO
?NO O3 ? ?NO2 O2 (favored) (rxn 1) 2 ?NO
O2 ? 2 ?NO2 (much slower alternative to rxn 1)
Generation of O3, without need for UV photon at l
lt 241 nm to break up O2 ?NO2 hv (lt 394 nm) ?
?NO O (rxn 2) O O2 M ? O3 M heat (rxn 3)
Because ?NO recombines with O3 (rxn 1) there
cannot be net O3 buildup in excess of ?NO2 by
these reactions alone. Reactions 1, 2, 3
constitute a null cycle.
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Net O3 buildup requires the presence of excess
hydrocarbons in addition to the ?NO2 radical
HO? R-CH3 ? R-CH2? H2O R-CH2? O2 ?
R-CH2-O-O? R-CH2-O-O? ?NO ? ?NO2 R-CH2-O?
Then O3 is generated as follows ?NO2 hv (lt
394 nm) ? ?NO O (rxn 2) O O2 M ? O3 M
heat (rxn 3)
As long as there exist R-CH2-O-O? arising from
HCs as above, they can react with the ?NO to
generate more ?NO2. This removes the ?NO so it
cannot react to destroy O3 (rxn 1 on prior slide)
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Hydroxyl free radical is the key to reactivity in
the troposphere
H2O-soluble or fully oxidized molecules that
do not either photolyze or react with ?OH,
are removed from the troposphere without
reacting. Either photodecomposition or
reaction with ?OH produces a free radical
species which goes on to react further.
Example CFCs
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Net O3 buildup requires the presence of excess
hydrocarbons in addition to the ?NO2 radical
HO? R-CH3 ? R-CH2? H2O R-CH2? O2 ?
R-CH2-O-O? R-CH2-O-O? ?NO ? ?NO2 R-CH2-O?
Then O3 is generated as follows ?NO2 hv (lt
394 nm) ? ?NO O (rxn 2) O O2 M ? O3 M
heat (rxn 3)
As long as there exist R-CH2-O-O? arising from
HCs as above, they can react with the ?NO to
generate more ?NO2. This removes the ?NO so it
cannot react to destroy O3.
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Methane oxidation pathway
-Radical CH3O? can lose one H to form CH2O, so
O2 abstracts H to also form HOO? -Both CH2O and
CO are stable intermediates -CO is itself a
primary smog pollutant
methyl radical
peroxy radical
to here, same reactions as previous slide, with
RH
Buildup of aldehydes from RH and CH4, late in
the smog episode
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CO oxidation pathway
methyl radical
peroxy radical

• Generation of HOO? in CO oxidation also
• allows for oxidation of ?NO to ?NO2,
• with regeneration of ?OH (as in RH pathway)
• Other HOO? are also generated in the pathway
• Net increase in ?OH overall
• HOO? ?NO ? ?NO2 ?OH
• The ?NO2 generates further ozone

CO ?OH ? CO2 ?H ?H O2 ? HOO?
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• How air composition
• changes during a smog
• episode.
• VOCs (HC) and ?NO are
• the primary pollutants
• ?NO2 and aldehydes are
• synthesized
• O3 builds up only when
• ?NO concentration declines
• aldehydes react as in the
• methane oxidation pathway

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Photodecomposition of aldehydes R2CO hv ? R?
RCO? Then RCO? O2 ?
Finally (shown for R CH3)
Peroxyacetylnitrate (PAN) is an eye irritant
with cytotoxicity in plants Produced from a
side-reaction between radicals
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Photochemical smog reactions
Volatile organic compounds (VOCs) like ethene and
its derivatives, are also primary pollutants,
and react also with free radicals Reaction rates
vary with detailed hydrocarbon structure ?OH
attack on CC bond is fast
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Technology forcing through the Clean Air Act

• Title II of the CAA (A) tailpipe emission
  standards (B) requires cleaner fuels
• Requirements of 1970 Act
• 90 reduction in VOCs and in CO by 1975
• 90 reduction in NOx by 1976
• At the time, there were no available technologies
  to achieve these reductions
• The targets were not met by 1975-1976, but they
  were met by 1985.
• (Development of the catalytic converter)
• __________________________________________________
  ________________
• CAFÉ standards (corporate average fuel
  efficiency)
• (from the 1975 Energy Policy and Conservation
  Act)
• US vehicle fleets must meet 27.5 mpg average fuel
  efficiency
• Light trucks, including SUVs, need only meet 20.7
  mpg
• Energy Policy Act of 2007 requires 35 mpg average
  for all categories, by 2020

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Catalytic converter technology provides
effective control of NOx emissions 2 chambers
(i) reduction of NO (ii) oxidation of
CO, HC
2 NO 2 H2 ? N2 2 H2O Rh catalyst H2 made
by reacting HC w/H2O 2 CO O2 ? 2 CO2 Pt/Pd
catalyst C2H4 3O2 ? 2 CO2 2 H2O (etc)
Pt/Pd catalyst Sulfur deactivates the catalyst
metals by binding to them Reductions in diesel
fuel S content have been recently mandated
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Acid Rain

• http//www.enviroliteracy.org/article.php/2.html
• Unpolluted rain is mildly acidic (about pH
  5.6)
• CO2 (g) H2O (aq) ? H2CO3 ? H HCO3-
• Rain polluted with H2SO4 and HNO3 is at pH 5.0 or
  lower
• The primary pollutants are SO2 and ?NO these are
  carried to
• some distance by winds ? an international
  problem

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Acid Rain Sources and removal of sulfur

• Background SO2 level (volcanoes, plant
  decomposition) about 1 ppb
• Anthropogenic sources from coal (mainly) and oil
  combustion
• H2S is also an anthropogenic source (from oil and
  CH4 processing
• natural gas reservoirs often contain
  significant H2S)
• H2S and SO2 levels can be simultaneously reduced
  (Claus reaction)
• 2H2S (g) SO2 (g) ? 3S 2H2O (elemental S is
  nontoxic)
• In the atmosphere, H2S is rapidly oxidized to SO2
• Sulfide ores are also a source of SO2 when
  processed to yield the
• metal oxide, for example
• 2NiS(s) 3O2(g) ? 2NiO(s) 2 SO2(g)
• The SO2 can be oxidized to SO3, then converted to
  commercial H2SO4
• SO2 in power plant emissions is scrubbed by
  reaction with CaCO3
• to yield CaSO4 (CO2)

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How is SO2 in emissions converted to H2SO4?
Homogeneous gas phase conversion first step
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Second step
Photochemical smog
SO3
Prediction scheme for the fates of free
radicals in the atmosphere.
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Acid Rain
First step
Second step
Then SO3 H2O ? H2SO4 (g) fast H2SO4 (g)
H2O ? H2SO4 (aq)
Overall reaction consumes ?OH and generates
HOO? Feeds into further generation of smog

• The gas-phase mechanism results in only a small
  amount of oxidation
• Rates are slower than dry deposition of SO2

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First step
Second step
Then SO3 H2O ? H2SO4 (g) fast H2SO4 (g)
H2O ? H2SO4 (aq)
Overall reaction consumes ?OH and generates HOO?
Combine with production of ?NO2 under smog
conditions of high ?NO, ?OH
?NO HOO? ? ?NO2 ?OH
Overall reaction is ?OH-catalyzed SO2 ?NO
O2 H2On ? ?NO2 H2SO4 (aq)
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pH of Rainwater
Dissolution of CO2 in water
CO2 (g) H2O (aq) H2CO3 (aq)

• This equilibrium is governed by Henrys Law gas
  dissolving in liquid
• KH H2CO3(aq) / PCO2 KH is a property of
  the (dilute) solution
• If CO2 380 ppm ? PCO2 380 x 10-6 atm
• Also, KH for CO2 in H2O 0.032 M/atm
• ?H2CO3 1.26 x 10-5 M
• In the raindrop H2CO3 H HCO3- Ka 4 x 10-7
  M
• Since Ka ltlt H2CO3 ? H2 KaH2CO3
• pH 5.7
• Ka2 for carbonic acid is very small and
  contributes negligibly

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Acid Rain
Dissolution of SO2 in water
SO2 (g) H2O (aq) H2SO3 (aq)
KH H2SO3(aq) / PSO2 If SO2 0.1 ppm
? PSO2 10-7 atm Also, KH for SO2 in H2O 1.0
M/atm ?H2SO3 1.0 x 10-7 M In the raindrop
H2SO3 H HSO3- Ka 1.7 x 10-2 M H2SO3 is
determined by equilibrium with the gas phase, but
high Ka drives the dissolution of much more
gas Ka 1.7 x 10-2 HHSO3-/H2SO3 ? H
HSO3- 4 x 10-5 M ? pH 4.4
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Acid Rain - Ecological Effects

• Acidification occurs both by precipitation and by
  dry deposition

• Acidification of lakes rivers
• Soil neutralization

-Requires high soil alkalinity -Critical load
some soils are better able to withstand acid
deposition CaCO3(s) H(aq) ? Ca2(aq) HCO3-
(aq) HCO3-(aq) H(aq) ? H2CO3(aq) ? CO2(g)
H2O(aq) Problem leaching of Ca2 and other
metals from the soil inhibits plant
growth Al3 leaching from rocks causes toxic
effects