METO 621

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METO 621

• Lesson 24

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The Troposphere

• In the Stratosphere we had high energy photons so
  that oxygen atoms and ozone dominated the
  chemistry.
• In the troposphere we have lower energy photons,
  and the chemistry is dominated by the OH and NO3
  radicals.
• OH is generated photochemically (i.e. only during
  the day), NO3 is rapidly photolyzed during the
  day, so it can only survive at night.
• NO3 is generally less reactive then OH, its peak
  concentration is higher.
• OH provides an efficient scavenging mechanism for
  both natural and anthropogenic trace constituents

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Dry and Wet Deposition

• Dry deposition removal of gases and particles
  by a direct transfer from the atmosphere to the
  surface.
• Wet deposition removal of gases and particles
  carried to the surface in water rain, snow, fog
  etc.
• Dry deposition is known for SO2, O3, CO2, and
  SO3.
• Wet deposition of gaseous species requires that
  they be water soluble. Terms used are rainout, or
  washout.
• Acid rain is an example of the rainout of
  sulfurous and nitric acids, produced in polluted
  atmospheres.

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Oxidation and Transformation

• Let us assume that no methane has been oxidized.
• Then OH is produced by the following reactions
• O3 hn ? O(1D) O2(1Dg)
• O(1D) H2O ? OH OH
• It should be noted that the O(1D) does not stay
  around for long, and is quenched to the ground
  state. The ground state then quickly combines
  with molecular oxygen to reform ozone.
• The OH formed reacts mainly with CO and CH4
• OH CO ? H CO2
• OH CH4 ? CH3 H2O

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Oxidation and Transformation

• These compounds then react with molecular oxygen
• H O2 M ? HO2 M
• CH3 O2 M ? CH3O2 M
• If the concentration of NO is very low then
  further reactions convert the peroxy radicals to
  water vapor and carbon dioxide.
• However if the nitrogen oxides are present then
  we get
• HO2 NO ? OH NO2
• CH3O2 NO ? CH3O NO2
• This then followed by
• NO2 hn ? NO O
• O O2 M ? O3 M

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Oxidation and Transformation

• Analogous reactions can be written for the higher
  hydrocarbons, e.g. C8H18 octane.
• If we assign the formula RH to these hydrocarbons
  then we get
• RH OH ? R H2O
• R O2 M ? RO2 M
• RO2 NO ? RO NO2
• This is the basis of photochemical smog.
• The photolysis of the resultant NO2 is the only
  known way of producing ozone in the troposphere.
• The RO is further reduced to aldehydes and other
  organic compounds by OH, all of which can
  eventually produce ozone.

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Ozone vs NOx for NHMC0.6 ppmc
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SMOG

• NEEDS
• Hydrocarbons and nitrogen oxides
• Strong sunlight to start reactions
• Warm temperatures to maintain reactions the
  higher the temperature the faster the rate.
• Peak ozone will be close to peak temperature
  afternoon

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Overall scheme for air pollution
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Dispersion of acid rain
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Natural levels of Acidity in Rain

• Carbon dioxide dissolves in the rain drop
• CO2(g) H2O(aq) ? H2CO3(aq)
• Henrys Law states that
• H2CO3 KHP(CO2)
• KH 3.4E-2 M ATM-1
• In the liquid some of the H2CO3 ionizes
• H2CO3(aq) ? H(aq) HCO3-(aq)
• This reaction has an equilibrium constant of
  4.2E-7 M-3
• For the overall reaction
• CO2(g) H2O(aq) ? H(aq) HCO3(aq)
• The equilibrium constant Kc is 1.43E-8 M2.ATM-1

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Natural levels of Acidity in Rain

• in a liquid H HCO3- hence
• H2 KcPCO2
• given that CO2 has a mixing ratio of 320
  ppm we get
• H 2.14E-6 and a pH of 5.67

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pH of precipitation over the US
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Acid rain

• Acid rain over the Eastern States is the highest
  most of the sulfur containing coal occurs in
  this region
• Shaded areas in the figure show where granite is
  found.
• If the soil/rocks are carbonates (chalk,
  limestone) then the acid rain can be neutralized,
  and does not change the pH of the streams and
  lakes
• If the soil/rocks are granite then acid rain is
  not neutralized, and can also leach out the heavy
  metals. Thus the pH of the lakes and streams can
  be lowered, and the heavy metal concentration
  raised.

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Standards

• Ozone 1-hour 125 ppbv, 8-hour 85 ppbv
• The 8-hour standard is much more stringent, and
  encompasses many areas where transport is the
  only issue (e.g Shenandoah National Park)
• PM2.5 daily 65 mg/m3, yearly 15 mg/m3
• Most areas will have trouble only with the annual
  standard
• Visibility a 60 year glide path back to
  natural conditions

parts per billion by volume
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A Typical Day in a Pollution Episode

• A common severe pollution weather pattern occurs
  when high pressure is centered just west of the
  Mid Atlantic region.
• Circulation around the high pressure center moves
  pollution from points west into the mid-Atlantic.

H
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Fort Meade profile 6/19/2001
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After Sunset

• If the weather remains the same, the temperature
  inversion forms again after dark.
• Ozone concentrations above the inversion remain
  at a constant, relatively, high level.
• Ozone trapped under the inversion reacts with
  other pollutants, and the surface the ozone
  concentration diminishes.

Altitude
Ozone concentration remaining constant
Temperature Inversion
Ozone concentration diminishing
Temperature
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Daily Ozone Cycle
Ozone production follows a daily cycle with
maximum concentrations typically observed in the
late afternoon. This cycle is a signature of the
dynamic processes of atmospheric air pollution
Ozone Concentration
Sunrise
Sunset
Time of day
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Comparison of ozone data at Fort Meade for August
2 and 8 2002
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Overplot of 2 and 8 Aug 2002 and the difference
between the two days
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Difference 2 Aug minus 8 Aug1.2
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Comparison of Aug 2 and 8, 2002

• Ozone data for August 8 is typical for local
  pollution on a clear warm day.
• The NOx and VOC are emitted early in the morning
  and the ozone amount slowly increases as the
  temperature increases. The peak production is at
  about 3-4 in the afternoon when the temperature
  at the ground is a maximum.
• The back trajectory shows fast upper level winds,
  which start at a high altitude and then subside
  to boundary levels at Baltimore.
• Small probability of upper air being polluted.

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Comparison of Aug 2 and 8, 2002

• On the 2nd of August the back trajectories show
  that the air is moving slowly at the boundary
  layer, and the probability of this air being
  polluted is high.
• The nocturnal inversion typically breaks down at
  about 10-11 in the morning.
• Hence the peak in ozone at this time must come
  from downward transport.
• The overall shape of the ozone data on Aug 2 is a
  combination of locally produced ozone peaking at
  about 3 pm and a downward movement of ozone from
  above at about 10.00 am.
• This ozone above the boundary layer is
  yesterdays ozone
• The winds above the boundary layer are usually
  high. Hence the ozone has been transported some
  distance

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Westerly transport is often present when the
highest ozone is observed in the mid-Atlantic.
24 hr. Back-trajectories on days of 1-hr. ozone
exceedances from 1997 through 2002, Baltimore area
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Aircraft measurements of ozone
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Aircraft measurements of Sulfur Dioxide
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Modes of Transport
All three modes of transport are important when
the highest pollution values are observed in the
mid-Atlantic.

• Large scale 800 km (70-100 ppbv)
• (Much of the Eastern US)

• Medium scale 200- 800 km
• (Carolinas to New England Region)

• Small scale 100 km
• (N. Virginia to Baltimore,
• Research Triangle to NC/VA border)

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Effects of the August 15, 2003 Blackout on Air
Quality
Selinsgrove, PA
Compared with Aug 4, 2002
Coming soon to GRL
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Effects of the 2003 Blackout on Air Quality
CEM data indicate reductions of 60-80
L. Marufu, B. Taubman, B. Doddridge et al.
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Effects of the 2003 Blackout on Air Quality
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In summary

• It isnt all transport
• It isnt all local
• It isnt all power plants
• It isnt all automobiles
• The problem will not be solved by addressing any
  one of these problems individually. ALL of these
  will have to show significant reductions for us
  to breathe clean air.