L.A. photo smog
London smog at daylight
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ʻsmogʼ: view reduction and bad air quality caused by traffic, industrial and further
anthropogenic emissions, appearing like fog but consisting of gases and
particles (e.g. Los Angeles, London, Mexico City, Athens)
Relation to death rate during the London smog in 1952
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1)London – type smog, consists mainly of aerosol particles (soot &
sulfates from combustion) + elevated SO2 levels, known since 1800s.
–  Particles and SO2 both cause respiratory problems
–  Centralized heating, catalytic converters and cleaner fuels have significantly
reduced the occurrence of this type of smog in industrial countries.
–  However, some measures may lead to an increase in aerosol particle
number even if the particle mass goes down (”more but smaller particles”),
this may have adverse health effects.
–  SO2 and particle emission developing world are increasing ⇒ more smog
problems there.
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2)Photochemical (Los Angeles – type) smog, characterised by high O3
and PAN levels as well as particulate matter, discovered around 1940.
–  Actually not a new phenomena, ”blue haze” smog is formed also from
biogenic terpene emissions (R. Reagan: ”trees pollute more than cars do”).
–  Gas-phase chemistry more complicated than London smog.
–  Harder to regulate because of nonlinear feedbacks in the NOx/ VOC/O3
system ⇒ still a problem in many western cities, e.g. Los Angeles.
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SO2 in otherwise clean air causes health problems for ξ > 1 ppm.
–  Below about 25 ppm, problems confined to upper respiratory tract.
–  Effects include chronic and acute bronchitis, pleurisy and
ephysema (chronic obstructive lung disease).
The simultaneous presence of aerosol particles makes the effects of
SO2 much worse ⇒ health problems at lower concentrations.
Aerosol particles from combustion often contain carcinogens (e.g. PAH;
polyaromatic hydrocarbons); linked to e.g. lung cancer.
British coal (used for heating) was high in sulfur, and also contained
tars & other hydrocarbons which produced a lot of particles when
burned. Hence the infamous London smog.
Due to new legislation, SO2 concentrations e.g. in London dropped by a
factor of 6 between 1974-1994.
Fresh
urban
Aged
urban
rural
remote
Warneck [1999]
Note: Concentrations especially of larger particles decrease rapidly with height.
D.J. Jacob
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PM10 (particles > 10 µm)
PM2.5 (particles > 2.5 µm) Red circles indicate violations of national air quality standard:
50 µg m-3 for PM10
15 µg m-3 for PM2.5 D.J. Jacob
PM2.5 (aerosol particles < 2.5 µm diameter) Site
PM 2.5 [µg m-3]
Urban / rural
Umeå (S)
8/7
Malmö (S)
14 / 10
Oslo (N)
11 / 7
Kuopio (FIN)
11 / 8
Amsterdam (NL)
25 / 27
Berlin (D)
31 / 26
Katowice (PL)
36 / 44
Cracow (PL)
35 / 34
Prague (CZE)
32 / 30
Teplice (CZE)
45 / 20
Pisa (I)
38 /42
Athens (GR)
59 / 30
EU air quality standard
2005:
PM10 = 40 µg m-3 (annual mean)
50 µg m-3 (daily mean, 35 days/yr.)
2010:
PM10 = 20 µg m-3
50 µg m-3 (daily mean, 7 days/yr.)
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Schaap, 2003
DIESEL
DOMESTIC
COAL BURNING
BIOMASS
BURNING
Chin et al. [2000]
D.J. Jacob
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Main pollutants of photochemical smog:
–  Ozone
•  Respiratory problems (aggravation of asthma, reduced lung
function, respiratory infections, inflammation).
•  Plant damage.
–  PAN
•  Powerful lacrymator (causes tears) already at low levels.
•  Respiratory damage at higher levels.
•  Very phytotoxic (plant damage).
–  Particles and/or SO2 often also involved.
Often, total oxidant (O3 + PAN) levels are used instead of O3 alone.
–  Adverse health effects for short-term exposure are observed
around 150-200 ppb for e.g. asthmatics, 250-300 ppb more
generally. WHO guidelines are < 76-100 ppb for 1 h exposure and
< 50-60 ppb for 8-hour exposure.
–  These limits are often greatly exceeded in polluted urban areas!
1 – ozone fumigation
2 – ozone and isoprene (VOC)
3 – before treatment
1
A - tobacco
B - birch
Loreto et al., 2001
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3
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Information threshold
human health risk
European Environment Agency, 2005
World’s record: Mexico City
with a maximum of 441 ppbv
(≅ 850 µg m-3)
European Environment Agency,
2005
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Asia
Europe
European Environment Agency, 2005
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Basically a ”grotesquely exaggerated form of the oxidation and
transformation chemistry of the unperturbed troposphere” (Wayne).
Starts with NO and hydrocarbon emissions from e.g. traffic.
Hydrocarbon oxidation (mainly by OH) produces peroxyradicals RO2
and HO2, also acylperoxyradicals RO.O2.
NO reacts with these radicals to form NO2; NO2 photolysis leads to
ozone formation and reaction with RO.O2 leads to PAN formation.
Typically NO and hydrocarbon concentrations build up during earlymorning rush hours, followed by NO2, O3 and PAN formation later in the
day (when photolysing radiation is more intensive).
Alkenes and aldehydes more effective than alkanes in producing smog.
–  Recall that RO.O2 is produced from the oxidation of aldehydes:
RCHO + OH•  RCO• + H2O
RCO• + O2  RO.O2•
RO.O2• + NO2  PAN
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Blue haze is a bluish appearance similar to smog caused by newly formed
particles, formed over forests.
F.W. Went (1960): … caused
by terpenes released from the
vegetation and ozone.
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•  Main termination reactions for HOx radicals in the troposphere:
OH + NO2 + M  HNO3 + M (1)
HO2 + HO2  H2O2 + O2 (2)
RO2 + HO2  ROOH + O2
(3)
•  Unless NOX is low, reaction 1 is the main termination step for radicals.
•  In effect, NO2 and VOC compete for OH. Reaction 1 is about 5.5 times
faster than the average VOC + OH reaction ([VOC] given in ppmC).
•  If [VOC]/[NO2] > 5.5, OH will react mainly with VOC.
–  The VOC + OH reaction chain generates more radicals than it
consumes (e.g. due to photolysis of intermediate products). More
radicals ⇒ more O3 formation. O3 production increases with [NO2]
⇒ ”NOx – limited” conditions.
•  If [VOC]/[NO2] < 5.5, OH will react mainly with NO2.
–  NO2 thus acts to remove radicals, and an addition of NOX therefore
decreases radical concentrations and thus ozone formation. Thus,
O3 production decreases with [NO2] ⇒ ”VOC – limited” conditions. NOx-limited
Ridge
NOx-
saturated/
Hydrocarbon limited
D.J. Jacob
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In general an increase in VOC will always lead to an increase in O3 (or
have no effect).
However, an increase in NOX may increase or decrease O3 depending
on the VOC concentration.
For each VOC concentration there exists an ”optimal” NOX
concentration that leads to a maximum production of O3.
–  This corresponds to the ridge in the ozone isopleth plots shown
earlier.
–  The optimal ratio naturally depends on the precise VOC mixture
present (the value 5.5 given earlier is a rough average for typical
urban air).
City centers and sites downwind of them tend to have low [VOC]/[NO2]
ratios (due to high NOX emissions)
Rural environments usually have high [VOC]/[NO2] ratios because of
the longer lifetimes of VOCs compared to NOX.
Next, letʼs look at a derivation of the ozone production rate as a
function of [VOC] and [NOX] in a highly simplified system.
R1: RO2• + NO → RO• + NO2
R2: NO2 + hν → NO + O(3P)
R3: O(3P) + O2 → O3
HOxfamily
RO2
hν
O3
O3
RH
PHOx
OH
NO
5
4
6
7
NO
NO2 9
HNO3
RO
O2
HO2
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O3
H2O2
RH or hydrocarbons = Volatile Organic Compounds (VOCs)
RO2 = peroxy radicals formed e.g. by a RH reaction with OH (- H), adding oxygen (+O2)
RO = alkoxy radical. from which a oxygen atom is abstracted by reaction (NO)
D.J. Jacob
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•  Reactions in the VOC – NOx – HOx – O3 system:
R1-R3 on previous slide (O3 formation from NO2).
R4: RH + OH•  (+ O2)  RO2• + H2O
R5: RO2• + NO  RO• + NO2
R6: RO• + O2  RʼCHO + HO2•
R7: HO2• + NO  OH• + NO2
R8: HO2• + HO2•  H2O2 + O2
R9: NO2 + OH• + M  HNO3 + M
•  In polluted air, cycling is fast, and we can assume that chain propagarion is
efficient:
•  Rate(R4) =Rate(R5) = Rate(R6) = Rate(R7)
•  Rate of O3 production P(O3) = k5[RO2][NO] + k7[HO2][NO] = 2k7[HO2][NO]
•  Steady state for OH:
k4[RH][OH] = k7[HO2][NO] ⇒ [OH] = k7[HO2][NO]/k4[RH]
•  Steady state for HOx family: source P(HOx) must equal sink:
P(HOX) = k8[HO2]2 + k9[NO2][OH][M]
Low NOx: k8[HO2]2 > k9[NO2][OH][M]; P(HOX) ≈ k8[HO2]2 High NOx: k8[HO2]2 << k9[NO2][OH][M]; P(HOX) ≈ k9[NO2][OH][M]
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Collecting the results and expressing P(O3) in terms of VOC and/or NOX:
P(O3) = 2k7[HO2][NO]
[OH] = k7[HO2][NO]/k4[RH]
Low NOx: P(HOX) ≈ k8[HO2]2 ⇒ [HO2] ≈ (P(HOX)/k8)0.5
 PHOx
⇒ P(O3 ) = 2k7 ⋅ 
 k8
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O3 production increases with increasing [NOX]
High NOx: P(HOX) ≈ k9[NO2][OH][M]
⇒ [OH] ≈ P(HOX)/k9[NO2][M] = k7[HO2][NO]/k4[RH]
⇒ [HO2] ≈ P(HOX)k4[RH]/k7k9[NO2][NO][M] ⇒ P(O3 ) =
•
1
2
 ⋅ [NO ]

2k 4 PHOx [RH ]
k9 [NO2 ][M ]
O3 production decreases with increasing [NOX]
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Ozone production efficiency
P(O3)
L(NOx)
HO2,RO2,O3
NO
hv
NO2
OH, O3
HNO3
Emission
Deposition
Define ozone production efficiency (OPE) as the total number of O3 molecules
produced per unit NOx emitted.
Assuming NOx steady state, efficient HOx cycling, and loss of NO2 by reaction with OH:
OPE =
OPE 
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P (O3 )
2k [ HO2 ][ NO ] 2k4 [ RH ]
= 7
=
L( NO x ) k9 [ NO2 ][OH ]
k9 [ NO2 ]
as NOx 
 strong nonlinearity$
D.J. Jacob
Necessary factors for the formation of photochemical smog:
1.  NOx emissions (high NOx concentrations)
2.  VOC emissions (high hydrocarbon concentrations,
anthropogenic (traffic, heating, industry) or biogenic
(vegetation))
3.  Sunlight Furthermore, any area surrounded by hills increases the
probability of temperature inversions, which prevent dilution of the
polluted air and enhances the occurrence of smog remarkably
(L.A., London, Mexico city).
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al., 2005
Lin Jonson
et al.et[2001]
al., 2005
Lin Jonson
et al.et[2001]
14
Jonson et al., 2005
D.J. Jacob
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In the 1960-80s, legislation focused on reducing VOC emissions. E.g.
VOC emissions decreased 12% in the US from 1980-1995, but NOx
emissions stayed constant.
–  Some control measures (e.g. ”lean-burn” in California, 1966)
actually increased NOx emissions while decreasing VOC.
–  Less smog in city centers (VOC-limited) but more pollution
downwind!
Newer atmospheric chemistry models indicate that most of the U.S.
(even urban regions) is actually NOx – limited, not VOC – limited.
–  Older models underestimated automobile VOC emissions and also
did not account for natural VOC sources.
–  More efficient NOx emission reductions needed to decrease ozone
pollution. –  Modern three-way catalytic converters are pretty good at removing
both VOC and NOx from vehicle exhaust but this requires very
accurate control of the fuel:air ratio.
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