Lecture 21
Urban Chemistry: Smog
ATOC/CHEM 5151
1
What is smog?
• Term was coined in England in early
1900s to refer to the dark “clouds”
observed over British towns
SMoke + fOG = smog
• Today, we recognize two types:
– London-type smog
– Los Angeles-type smog
2
London smog
• Mixture of smoke, fog, ash, soot, SO2, and
H2SO4
• Requires a temperature inversion
3
Temperature Inversion
4
Temperature Inversions and Smog
5
Consequences of Smog
• London, 1952
• 4000 people died
from respiratory and
heart ailments
attributed to breathing
smog
• Most pollution
components were
from coal burning
6
London Smog is not exclusive to
London: Donora, PA (1948)
• Donora lies along the Monongahela River
• Industry: zinc smelters and steel mills
• 26-31 Oct 1948: 20 people died; 43% of
population became ill
7
London smog chemistry
• Pretty simple – carbon and sulfur:
– Carbon burns to become CO2 or CO,
depending upon extent of combustion
– Some carbon remains unburned → soot
• Particles about 0.1 µm in diameter
– Sulfur in coal also burns
•
•
•
•
S + O2 → SO2
SO2 + OH → HSO3
HSO3 + O2 → HO2 + SO3
SO3 + H2O(ℓ) → H2SO4 (ℓ)
8
Los Angeles smog
• Meteorology is also important – inversions trap
pollutants in stagnant air and give them time to
“cook”
9
Marine inversion
Subsidence inversion
“Classic” LA smog
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Primary Pollutants
Things that are directly emitted
Nitrogen Oxides (NOx)
Hydrocarbons (VOCs)
Carbon Monoxide (CO)
11
Secondary Pollutants
• Formed by
photochemical reactions
of primary pollutants
• Key elements are:
– NO2
– O3
– Peroxyacetyl nitrate (PAN)
12
Comparison of London and LA type smogs
13
Ozone formation
• Provided there is sufficient NOx (> 50-100 ppt),
hydrocarbon oxidation will make O3.
• Recall CH4 oxidation:
CH4 + 10 O2 → CO2 + H2O + 5 O3 + 2 OH
• “Branched chain” reaction
– Several pathways
– Produces radicals as net products (which can go on
to oxidize other hydrocarbons)
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Typical alkane oxidation scheme
15
Photochemistry: Smog Chamber
16
Diurnal behavior of Ozone
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Ozone
production and
transport
18
Types of Hydrocarbons
19
General Reaction Scheme
20
Hydrocarbon Reactivity
21
“Group Reactivity”
22
Incremental Ozone production
23
Maximum O3 production
24
Biogenic Hydrocarbons
25
Isoprene Chemistry
Each HCHO
leads to 3 O3
and 2 OH!
26
Particulate
Matter
27
Sulfate and Nitrate: Dominant Components
28
Soot Particles: Host for Organics
29
Urban pollution to global scale
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