METO 621
Lesson 24
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
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.
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
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
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.
Ozone vs NOx for NHMC=0.6 ppmc
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
Overall scheme for air pollution
Dispersion of acid rain
Natural levels of Acidity in Rain
• Carbon dioxide dissolves in the rain drop
CO2(g) + H2O(aq) ↔ H2CO3(aq)
• Henry’s 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
Natural levels of Acidity in Rain
[ H  ][ HCO3 ]
where K c 
P(CO2 )
•
in a liquid [H+] = [HCO3-] hence
[H+]2 = KcP[CO2]
given that CO2 has a mixing ratio of 320 ppm we get
[H+] = 2.14E-6 and a pH of 5.67
•
pH of precipitation over the US
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.
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
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.
H
•Circulation around the high
pressure center moves
pollution from points west
into the mid-Atlantic.
Fort Meade profile 6/19/2001
• 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
After Sunset
Ozone concentration
remaining constant
Ozone concentration
diminishing
Temperature
Temperature
Inversion
Daily Ozone
Cycle
Ozone production
follows a daily
cycle with maximum
concentrations
typically observed
in the late afternoon.
Ozone
Concentration
Sunrise
Sunset
Time of day
This cycle is a signatu
of the dynamic
processes of
atmospheric air
pollution
Comparison of ozone data at Fort
Meade for August 2 and 8 2002
Overplot of 2 and 8 Aug 2002 and the
difference between the two days
Difference 2 Aug minus 8 Aug*1.2
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.
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 yesterday’s
ozone
• The winds above the boundary layer are usually high.
Hence the ozone has been transported some distance
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
Aircraft measurements of ozone
Aircraft measurements of Sulfur Dioxide
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)
Effects of the August 15, 2003
Blackout on Air Quality
Selinsgrove, PA
Coming soon to GRL
Compared with
Aug 4, 2002
Effects of the 2003 Blackout
on Air Quality
CEM data indicate
reductions of 60-80%
L. Marufu, B. Taubman, B. Doddridge et al.
Effects of the 2003 Blackout
on Air Quality
In summary
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It isn’t all transport…
It isn’t all local…
It isn’t all power plants…
It isn’t 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.