Air PollutionTropospheric Ozone - PowerPoint PPT Presentation

Title: Air PollutionTropospheric Ozone


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• Air Pollution-Tropospheric Ozone

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Good Ozone and Bad Ozone

• Stratospheric ozone protect lives on Earth from
  harmful effects of UV radiation.
• Tropospheric ozone
• Causing respiratory distress and eye irritation
• Destroying plants
• Producing cracks in rubber

Ozone is a strong oxidant, reacts with molecules
containing CC double bonds, forming epoxides.
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Two types of air pollutants primary vs.
secondary

• Primary pollutants released directly from
  sources
• Examples CO, SO2, NOx
• Secondary pollutants formed through chemical
  reactions of the primary pollutants and the
  constituents of the unpolluted atmosphere in the
  air.
• Example O3

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Formation of ozone
NO2 is capable of absorbing visible light (lt400
nm) to produce O.
NO2 hv ? NO O (1) O O2 M ? O3
M (2) NO O3 ? NO2 O2 (3)
No net O3 formation
NO2 hv ? NO O (1) O O2 M ? O3
M (2) HO2. NO? NO2 OH (4) RO2. NO ? NO2
RO. (5)
O3 is formed
Net of (1)(2)(4) RO2. O2 ? O3 RO.
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Sources of RO2. Oxidation of hydrocarbons
RH OH ? R. H2O R. O2 ? RO2.
A single organic radical can produce many peroxy
radicals by successive rounds of O2 combination
and fragmentation.
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Example Oxidation of carbon monoxide

• CO .OH O2 ? CO2 HO2.
• HO2. NO ? NO2 .OH
• NO2 hv ? NO O
• O O2 M ? O3 M
• Net CO 2 O2 hv ? CO2 O3

The net reaction can be viewed as a catalytic
oxidation of CO to CO2. Net formation of O3
occurs.
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Example Oxidation of methane

• CH4 .OH O2 ? CH3OO. H2O
• CH3OO. NO ? CH3O. NO2
• CH3O. O2 ? HCHO HO2.
• HO2. NO ? .OH NO2
• NO2 hn ? NO O (2x)
• O O2 M ? O3 M (2x)
• Net CH4 4 O2 ? HCHO H2O 2 O3

The net reaction is that for each mole of methane
oxidized, 2 moles of O3 is produced.
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Necessary ingredients for ozone formation

• Sunlight
• NOx (NO, NO2)
• Hydrocarbons (VOCs volatile organic carbon)

Production of O atom
Production of RO2, which reacts with NO so that
O3 could accumulate.
VOCs NOx hn ? O3 other pollutants
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Necessary ingredients for ozone formation

• CH4 .OH O2 ? CH3OO. H2O
• CH3OO. NO ? CH3O. NO2
• CH3O. O2 ? HCHO HO2.
• HO2. NO ? .OH NO2
• NO2 hn ? NO O (2x)
• O O2 M ? O3 M (2x)
• Net CH4 4 O2 ? HCHO H2O 2 O3

VOC
Sunlight
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Formation of oxidants other than O3

• Formation of aldehydes (e.g. formaldehyde)
• Formation of PAN (peroxyacetyl nitrate) and its
  analogs

PAN
ROO. NO2 ? ROONO2 (peroxyalkyl nitrate)
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Photochemical smog

• Smog derives from a combination of the words
  smoke and fog.

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London smog and Los Angeles smog

• London smog is characterized by high SO2 and
  particle concentration in the presence of fog.
• Also referred as sulfurous smog
• Los Angeles smog is characterized by high
  oxidants (mainly O3). It was first recognized in
  the Los Angeles area.
• The term smog is misleading in this case, as
  smoke and fog are not key components.
• The appropriate term is photochemical air
  pollution.

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Photochemical air pollution
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HO2. Radical Interconversion of .OH and HO2.

• OH and HO2 are interconverted through a series
  of reactions involving hydrocarbons and oxides of
  nitrogen.

HO2. NO ? .OH NO2 .OH RCH3 ? H2O
RCH2. RCH2. O2 ? RCH2OO. RCH2OO. NO ? NO2
RCH2O. RCH2O. O2 ? RCHO HO2.
Sources of OH are in effect sources of HO2. under
most tropospheric conditions.
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Sources for .OH radicals Photolysis of O3

• Photolysis of O3 forms O1D, followed by its
  reaction with water.

O3 hn ? O1D O2 l lt 320 nm O1D H2O ? 2 .OH
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Sources for .OH radicals Photolysis of HONO

• HONO hn ? .OH NO l lt 400 nm

• Possible sources for HONO include
• NO2 H2O
• OH NO
• NO NO2 H2O
• HO2 NO2 reaction (possibly a contribution from
  a minor channel of this reaction)
• direct emissions, for example, from automobiles.

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Sources for .OH radicals Photolysis of H2O2

• H2O2 hn ? 2 .OH l 360 nm

H2O2 is formed from the reaction HO2. HO2. ?
H2O2 O2
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Sources for HO2. Radicals formaldehyde

• Formaldehyde photolysis is a major source of
  HO2. during the daylight hours.

HCHO hn ? H. HCO. l lt 370 nm H. O2 M ?
HO2. M HCO. O2 ? HO2 . CO
Note Any process that produces HCO. or H. is a
source of HO2. in the troposphere.
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Nighttime sources for .OH/HO2.

• Ozone oxidation of alkene species
• Ethene O3 ? 0.12 OH
• Isoprene O3 ? 0.27 OH
• Thermal decomposition of Peroxyactyl nitrate
  (PAN) and its analogs of higher carbon.
• CH3C(O)OONO2 Û CH3C(O)OO. NO2
• CH3C(O)OO. NO ? CH3C(O)O. NO2
• CH3C(O)O. ? CH3. CO2
• CH3. O2 ? CH3OO.
• CH3OO. NO ? CH3O. NO2
• CH3O. O2 ? HCHO HO2.

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Nighttime sources for .OH/HO2. (Continued)

• NO3 reaction with hydrocarbons

NO3 RH ? HNO3 R. R. O2 ? ROO. ROO. NO ?
RO. NO2 RO. O2 ? HO2. RCHO
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Various sources of .OH/HO2. as a function of the
time of day
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Control strategies for ozone

• O3 is a secondary pollutant? control of O3
  requires control of its precursors.
• Control of VOCs
• General too abundant to be brought low enough to
  be the limiting factor.
• In certain areas, VOCs from biological sources
  could be significant.
• Control of NOx
• Difficult to control as efficient energy
  conversion requires high combustion temperature.