ESM 202

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ESM 202

• Air Quality

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Reactive Atmosphere

• High energy environment
• Lots of oxidants (O2, O3, H2O2)
• Hydroxyl radical (HO.) produced by photolysis of
  water
• H2O HO. H
• H2O hn HO. H
• very short life but very reactive

light
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Reactive Atmosphere

• OH. also formed by photolysis of ozone
• O3 hn O O2
• O H2O 2 HO.
• Radical oxidation reactions
• CH4 HO. .CH3 H2O
• RH HO. .R H2O
• CO HO. CO2 H.

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Ozone Formation

• Molecular oxygen can be split by very high energy
  light (high UV light)
• O2 hn O O
• Atomic oxygen can react with molecular, but a
  third body is needed
• O2 O M O3 M

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Ozone Formation
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Ozone Formation

• Maximum natural ozone concentration occurs in
  stratosphere at 20-40 km
• About 350,000 metric tons of ozone formed and
  destroyed every day
• Maximum concentration levels are 10 ppm
• Ozone is only a very small fraction of the gases
  in stratosphere
• The actual thickness of the ozone layer is only a
  few mm!

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UV Absorption
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Ozone Formation

• Ozone absorbs radiation in the region of 220-330
  nm (UV-B)
• Only UV-A radiation (320-440 nm) penetrates into
  the lower troposphere
• Absorbed energy is eventually converted into heat
• Most of energy absorption is in upper
  stratosphere
• hotter than the lower stratosphere

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CFCs

• Chlorofluorocarbons used as
• refrigerants
• aerosol propellants
• foam blowing agents
• solvents
• Most common are
• Freon12 CCl2F2 500 ppt
• Freon11 CCl3F 300 ppt
• CCl4 120 ppt

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CFC lifetimes
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CFCs

• Within a few years, troposphere concentration is
  quite uniform - good mixing
• Transport to the stratosphere is slow, but
  eventually build up
• High energy environment can cause
  photodissociation of CFCs
• CCl2F2 hv Cl. .CClF2

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CFCs

• Radicals react very fast with other molecules
  present
• Cl. O3 ClO. O2
• ClO. O Cl. O2
• O3 O 2 O2
• Typically destroy several thousand O3 molecules
  for every Cl.

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CFCs

• The cycle can be broken if Cl. reacts with CH4 to
  form hydrochloric acid
• Cl. CH4 HCl .CH3
• Reaction of HCl with hydroxyl radicals in the
  atmosphere can regenerate the Cl.
• HO. HCl H2O Cl.

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CFCs
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CFCs

• CFCs are also very strong absorbers of long wave
  radiation (e.g. radiation reflected from Earths
  surface)
• Their contribution to increased heat trapped in
  the troposphere is many times (gt 1000) greater on
  a molecule per molecule basis than CO2

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CFCs

• First ban on CFCs was in 1979 for use as
  propellants in aerosol spray cans
• Montreal Protocol
• Complete phase-out of production in 24
  industrialized nations by 2000
• Developing countries can continue to produce and
  purchase CFCs and carbon tetrachloride for use
  until 2010 and methyl chloroform until 2015

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Montreal Protocol

• Long residence times mean CFC problem will be
  here for a while

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Ozone Hole

• Why do we see the hole in the Antarctic Spring?
• Chlorine from CFCs
• Stored in stratospheric clouds during long polar
  night
• Released to attack O3 when sun returns in spring.
  
• Normal ozone levels restored in summer
• Plenty of sunlight to produce O3
• Antarctic atmosphere also mixes with ozone-rich
  air from other latitudes

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Ozone Hole
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Ozone Hole
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Ozone Hole
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Ozone Hole
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Effects of UV-B

• Forms lots of hydroxyl radicals
• Delivers energy for oxidation reactions
• Can lead to mutagenic and carcinogenic effects,
  since it affects cellular DNA
• Destroys plankton
• Produces skin cancer

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Nitrous Oxide

• N2O is produced by bacteria in soil and water
• Very stable in troposphere (lifetime 150
  years), so can rise to stratosphere
• N2O reacts with excited O
• O N2O NO NO
• N2O is a very good absorber of longwave radiation
  ( 200 times better than CO2)

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NOx

• NO and NO2 form another cycle of reactions that
  destroys ozone
• NO O3 NO2 O2
• NO2 O NO O2
• O3 O 2 O2
• Eventually NO2 forms HNO3, which is then removed
  by rain from the troposphere

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Nitrous Oxide
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Urban Smog
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Tropospheric Smog

• Local to regional issue
• Oxidants, irritating vapors, visibility reduction
• Strongly influenced by atmospheric conditions
• 2 major components incomplete combustion of
  hydrocarbons and NOx
• Ozone is a secondary pollutant

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Tropospheric Smog
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Air-Fuel Ratio
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Air-Fuel Ratio
NO nitric oxide
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Urban Ozone

• NO is non-toxic and colorless
• NO converts to NO2
• O3 NO ? NO2 O2
• NO2 has a brownish color and can absorb light at
  many wavelengths
• NO2 hv ? NO O
• Formation of Ozone
• O2 O M ? O3 M
• In principle no net O3 is formed, but there is a
  time lag between these reactions

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Tropospheric Smog
Source Env. Chem. Baird
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Tropospheric Smog
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Urban Ozone
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Tropospheric Smog

• Ozone is a good oxidant and thus a strong
  irritant even at 0.15 ppm
• Ozone can also attack plastic materials, in
  particular rubber
• Many other organics are formed during a smog
  event
• aldehydes, ketones
• intermediate free radicals
• Can also form smog from forest and grassland
  burning

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Catalytic Converter
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Tropospheric Smog
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Air Quality Standards
From Environmental Chemistry (Baird, 1999)
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Particulates
From Environmental Chemistry (Baird, 1999)
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Particulates
From Environmental Chemistry (Baird, 1999)
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Particulates
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Particulates
Polyaromatic hydrocarbons (PAHs)
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Particulates
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Solutions?

• Catalytic converters
• Particle traps
• Filter bags
• Electrostatic precipitators
• Fuel cell vehicles
• Liquid vs. gaseous fuel (H2)
• Distribution of fuel
• Mechanics out, chemists in
• Incentives policies to promote RD, marketing
• Hybrid vehicles with small cleaner diesel
  engine?