Download 637Lesson18

METO 637
LESSON 18
Schematic of ozone production from VOC
SMOG
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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
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
Ozone isopleths (NMHC vs NOx)
Ozone vs NOx for NHMC=0.6 ppmc
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
Back Trajectories, Hysplit
AUG 2, 2002
AUG 8, 2002
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
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
Dispersion of acid rain
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.
Sources of pollutants
in the USA
Sulfur Dioxide emissions
2%
3%
4%
7%
Fuel Combustion
- Electric Utility
17%
67%
Fuel Combustion
- Industrial
Fuel Combustion
- Other
Metals
Processing
Non-Road
Vehicles
All Other
Flight area
Annual mean haze, United States
IMPROVE network
Detection Techniques
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In situ via UV pulsed fluorescence.
– Modified commercial instrument
– Detection limit ~70 ppt ()
– Response time ~3 min
Satellite via UV (315 to 327 nm ) spectroscopy
(modified DOAS)
SO2 over the Eastern US
Summer, 2002 (over 100 profiles)
650
700
Pressure (hPa)
750
Scale height ~1.1 km
800
850
900
950
1000
1050
0
1
2
3
4
Median (25% 75%) Mixing Ratio (ppb)
5
6