九十七學年第二學期
大氣污染傳輸
TOPIC 4
Photochemical Smog
授課老師:蔡俊鴻 教授
98/03/19
Two Types of “Smog”
London
Los Angles
Time
Pollutants
1873
PM, SO2, H2SO4
Fuels
Season
Temperature
Humidity
Sunlight
O3 conc.
Time of event
Visibility
Toxicity
Coal, fuel oils
Winter
Low (<40C)
High
Weak
Low
Day-night cont.
Very low
Irritate to respiratory
1946
HC, NOx, O3, PAN, aldehyde,
ketone
Gasoline, gas, petroleum
Summer & Fall
High (240C)
Low
Strong
High
Day
Low (half-mile)
Eye, respiratory irritation, O3
damage
Reducing smog
Oxidizing smog
2
Effects of Photochemical Smog
• Toxicity to plants
– NOx: Toxicity of NOx itself is low compared to its secondary
products.
– PAN: Highest toxicity to plant, damaging vegetation at 0.02-0.05
ppm. However, PAN is normally present at low concentration.
– O3: Reduction to plant growth and yield. In California (mostly
located n san Francisco and Los Angles), crop damage from O3
and other photochemical air pollutants alone is estimated to be
millions of dollars each year.
3
Effects of Photochemical Smog
• Human health:
– There are some 100 urban areas with a combined
population of approximately 100 million people that do not
meet the existing ambient air quality standards for O3 (1992)
– O3 at 0.15 ppm causes coughing, wheezing, bronchial
constriction, and irritation to the respiratory mucous system
– Oxidant (peroxyacyl nitrates ; PAN) and aldehydes are eye
irritants
• Damage to materials:
– O3 causes cracking and aging of rubber by oxidizing and
breaking double bonds in the polymer
• Effects on the atmosphere
– Reduced visibility
4
London-Type Smog
• London-type smog occurs in the regions where
1. emission of the sulfur-containing compounds is
high (due to burning of coal to generate heat and
energy)
2. air contains high liquid water contents (e.g., fogs).
• Fog is the dispersed water drops. Fogs can be
viewed as clouds that are in contact with the Earth’s
surface. Fogs are created during cooling of air next
to the Earth’s surface either by radiation to space
(radiation fogs) or by a contact with a surface
(advection fogs).
London-Type Smog
•
•
Burning of coal produces sulfur dioxide, soot and other gases
and particulates, which are called smoke.
The typical London smog results from the accumulation of
smoke from coal burning, which has a high sulfur content. It
leads to the production of high concentrations of sulfuric acid
in fog droplets.
1. Direct dissolution of SO2 into water drop and subsequent
aqueous-phase oxidation to sulfate
2. Gas-phase conversion of SO2 to sulfuric acid gas (H2SO4),
which has a low surface vapor pressure, and therefore, easily
condenses onto particles. The gas phase conversion requires
three steps:
SO2 + OH + M -> HSO3 + M
HSO3 + O2
-> SO3 + HO2
SO3 + H2O
-> H2SO4
Photochemical Smog
• Los Angeles-type smog occurs in the regions where
1. high emissions of automobiles
2. large concentrations of reactive hydrocarbons (RH)
( from automobile exhaust or from other natural or
anthropogenic sources)
3. plenty of sunlight (high level of UV radiation)
• Photochemical smog forms primarily as a result of
interactions among nitrogen oxides (NOx = NO + NO2),
reactive hydrocarbons, and sunlight
Topography and Meterology
• Two factors influencing the formation of
photochemical smog
Topography
•
•
Very important for
formation of photochemical
smog
Restriction of air movement,
city in valley experience more
smog problem, than plains
http://www.uwsp.edu/geo/faculty/ritter/images/a
tmosphere/misc/smog.jpg
8
Temperature
Inversion
•
•
•
Increase of air temperature
with height for some distance
above ground causing the
smog trapped close to ground
Consequences
– Air becomes still and dust
and pollutants are no
longer lifted from surface
Serious problem in many cities
http://www.ec.gc.ca/cleanair-airpur
9
Nature of Photochemical Smog
Species
Polluted Area Unpolluted Air
(μg/m3)
(μg/m3)
CO
NO
HC (excluding CH4)
O3
PANs
10,000-30,000
100-400
600-3,000
50-150
50-250
<200
<20
<300
<5
<5
Most values are estimates based on data in Air Quality in Ontario 1991,
Environment Ontario, Queen’s Printer for Ontario; 1992
10
Three Ingredients Required for the
Formation of Photochemical Smog
• UV light
• Hydrocarbons
• Nitrogen oxides
Photochemical pollution level (Stern et al., 1973)
– PPL = (ROG) (NOx) (Light Intensity) (Temperature)
/ (Wind Velocity) (Inversion Height)
where
PPL = photochemical pollution level
ROG = concentration of reactive organic gases
NOx = concentration of oxides of nitrogen
11
The Origins of NO and HC:
Exhaust Gases from Internal Combustion
Engines
Engines
CO NOx Hydrcarbons
Two-stroke engine 165 0.3
Four stroke engine 127 0.7
89
7
Unit: 10-8 g/J
Examples of two-stroke gasoline-powered engines: chainsaws, law mowers,
mopeds, motorcycles, outboard motor for boats, scooters.
12
Basics of Photochemical Reactions
• Energy of radiation increases as wavelength
(λ) decreases
E = hν = hc/λ
• Example: Energy at λ = 100 nm
– Per photon:
E = hc/λ = (6.6x10-34 J*s) x (3x108 ms-1)/(100x10-9) m
= 2x10-18 J
– Per mole of photos:
E = 2.0 x 10-18 x 6.02 x 1023 = 1200 KJ/mol
where
h = Plank constant = 6.6x10-34 J*s
c = speed of light = 3x108 ms-1
N = Avogadro number = 6.02 x 1023
13
Basics of Photochemical Reactions
• Energy of photos at different wavelengths
λ (n m )
E (K J /m o l)
200 – 300
400 – 700
2000 - 5000
5 9 7 .2 – 3 9 8 .4
2 9 8 .9 – 1 7 0 .8
5 9 .7 – 2 3 .9
U V
V IS
In fr a r e d
• Chemical bond energy > 167.44 KJ/mole
• Therefore, only UV-VIS will make photochemical
reactions occur, infrared light cannot due to its low
energy
14
Chemical Bond Energy
(KJ/mol)
H-H: 436
H-F: 563
H-Cl: 432
H-Br: 366
H-I: 299
C=C
615
C-H: 415
C-C: 344
C-Cl: 328
C-Br: 276
C-O: 350
C-N: 292
C≡C
812
N-H: 391
N-N: 159
N-O: 175
N-F: 270
N-Cl: 200
O-H: 463
O-O: 143
O-F: 212
S-H: 368
S-S: 266
C=O
724
N=N
418
F-F: 158
Cl-Cl: 243
Br-Br: 193
I-I: 151
Cl-F: 251
Br-Cl: 218
N≡N
946
15
Formation of Photochemical Smog
• NO
• HC
UV
• O3
• PAN
• Other oxidants
(aldehyde, etc)
The key to the formation is via
photochemical reactions
16
Photochemical Smog Chemistry
Photochemical Smog Chemistry
RH + OH - > R. + H2O
R. + O2 + M - > RO2. + M
RO2. + NO - > RO. + NO2
NO2 + hv - > NO + O , at wavelength < 0.42 mm
O + O2 + M - > O3 + M
RH + HO + NO + hn - > … - > O3 + NO2 + HC
Typical Daily Concentration Change
in Photochemical Smog
(St. Louis, Missouri, 1962; Hydrocarbons data not shown)
Concentration (ppm)
0.35
0.3
O3
0.25
0.2
NO
0.15
0.1
0.05
NO2
0
4
8
12
16
20
24
Time of Day
23
Reaction of NOx in the Atmosphere
Chemical Species:
Light Absorbing vs. Non-Absorbing
•
•
In stratosphere, O3 in ozone layer absorbs light < 290 nm completely
In troposphere, pollutants absorb the remaining light between 300700 nm (equivalent to 398 - 167 kJ/mol)
Light absorbing species (300 – 700 nm):
NO2, SO2, HNO3, RONO2, HNO2, RONO, aldehyde,
ketone, ROOR’, O3
Light non-absorbing species (300 – 700 nm):
N2, O2, H2O, CO, CO2, NO, SO3, H2SO4, CH, alcohol,
organic acids
25
Types of Photochemical Reactions
•
•
•
•
•
•
•
•
Physical quenching: O2* + M ⌫ O2 + M (higher energy)
– * denote electronically excited molecule
Dissociation: O2* ⌫ O + O
– O denote atomic oxygen
Direct reaction with another species: O2* + O3 ⌫ 2O2 + O
Luminescene: NO2* ⌫ NO2 + hν
– Fluorescene, phosphorescence, chemiluminescene
Intermolecular energy transfer: O2* + Na ⌫ O2 + Na*
Intramolecule transfer: XY* ⌫ XY†
– where † denotes another excited state of the same molecule
Spontaneous isomerization:
– o-nitrobenzaldehyde + hν ⌫ o-nitrosobenzoic acid
Photoionization: N2* ⌫ N2+ + e26
Reactive and Unstable Species
• There are three types of reactive and
unstable species which are strongly involved
in the atmospheric photochemical reactions:
– Electronically excited molecules (NO2*, O2*)
– Atoms with unshared electrons (O)
– Molecular fragment with unshared
electrons (OH•, CH3•, CH3O•, CH3O2•)
27
Free Radicals:
The Key to Photochemical Smog
•
•
•
•
•
•
The most important free radical is OH•, others include HO2•,
CH3•, CH3O•, CH3O2•
Free radicals have unpaired electrons and have strong pairing
tendencies of electrons (i.e., to gain electrons as an oxidizing
agent)
Concentrations of free radicals in the air are usually very low
~10-7 ppm
Free radicals can take part in chain reactions in which one of
the products of each reaction is also a radical
Ultimately, one of the radicals in the chain is destroyed and the
chain reaction ends (chain-terminating reaction)
H3C• + H3C• ⌫ C2H6
The half-lives of free radicals in the air are only several minutes
28
The Formation of Free Radicals: OH•
(1) HONO + hν ⌫ OH• + NO
O3 < 315 nm O + O2
(2)
H2O2 + hν ⌫ 2OH•
(3)
O + H2O ⌫ 2OH•
(O from O3)
(4)
HO2• + NO ⌫ OH• + NO2
(HO2• from HCHO)
H2O
HNO2
<400 nm
OH•
NO
HO2•
< 370 nm H O
2 2
H
+ < 313 nmHCHO
HC •O
29
Oxidation of Hydrocarbons Initiated by OH•
• Example 1: RCH3 ⌫ RCHO
OH• + RCH3 (hydrocarbon) ⌫ RC•H2 (alkyl) + H2O
RC•H2 + O2 + M ⌫ RCH2OO • (peroxyalkyl) + M
RCH2OO• + NO ⌫ RCH2O• (alkoxyl)+ NO2
RCH2O• + O2 ⌫ RCHO (aldehyde) + HOO• (hydroperoxyl)
HOO• + NO ⌫ NO2 + OH•
Note that each step in the sequence produces a new radical.
The sum of the reactions is:
RCH3 + 2O2 + 2NO ⌫ RCHO + 2NO2 + H2O
30
Oxidation of Hydrocarbons Initiated by OH•
• Example 2: CH3CHO ⌫ PAN
– CH3CHO + OH• ⌫ CH3C•O + H2O
– CH3C•O + O2 + M ⌫ CH3C(O)OO• (acetylperoxy)
– CH3C(O)OO• + •NO2 ⌦ CH3C(O)OONO2 (PAN)
Peroxyacetic nitric anhydride and related compounds,
PANs, are the major eye irritants in a photochemical
smog. PAN is relatively stable molecule, especially at
low temperature, and therefore may be transported
over long distances by air currents.
The last equation is also the chain-terminating reaction
31
Formation of Ozone (O3)
• NO2 + hν (< 400 nm) ⌫ NO + O
• O + O2 + M ⌫ O3 + M
32
Night-Time Smog-Forming Reactions:
Role of Nitrate Radical (NO3)
• Formation of nitrate radical:
– NO2 + O3 ⌫ NO3 + O2
– NO2 + NO3 + M ⌦ N2O5 + M (energy-absorbing third body)
• Dissociation during daylight:
– NO3 + hν (λ < 700 nm) ⌫ NO + O2
– NO3 + hν (λ < 580 nm) ⌫ NO2 + O
NO3 has a lifetime only 5 seconds at noon time
• Reactions involving nitrate radical:
– Nitrate radical adds across the double bonds in alkenes
(C=C) leading to the formation of reactive radical species
that participate in smog formation (Bolzacchini et al., 1999,
ES&T, 33:461-468)
33
The Control / Prevention of
Photochemical Smog
• NOx – limited Strategy
• VOC – limited Strategy
The Control / Prevention of
Photochemical Smog
• Implementation of Clean Air Act (CAA)
• Reduce automobile emission of its precursors
– Reactive organic gases (ROG) reduction always
lead to a slowing of the ozone production process
and lower peak O3 concentration
– Studies show that NOx reductions (with constant
ROG) can lead to a speeding up of the O3
production process, and can increase or decrease
peak ozone values depending on the ROG-to-NOx
ratio.
• Chain-terminating step of smog formation:
NO2 + R• ⌫ product (e.g., PAN)
• Competition between NO2 and ROG for OH•
36
The Control / Prevention of
Photochemical Smog
• More rigid control options including:
– Bans on automobile usage during periods of
high smog potential
– Limiting traffic in certain cities to electric
vehicles or vehicles equipped to burn low
emission fuels
– Expensive overhauls of existing mass transit
systems
37
Ozone Formation Potential
• Maximum Incremental Reactivity (MIR)
MIR = Max (∂[O3]p/∂Ei)
for all VOCs/NOX
[O3]p：Peak Ozone Conc
E I ：Incremental VOCs Concentration
Low (VOCS / NOX →VOCS is critical
High (VOCS / NOX →NOX
Ozone Formation Potential
• Photochemical Ozone Creation Potentials
(POCP)
The Control of Photochemical Smog
•
•
Implementation of Clean Air Act (CAA)
Reduce automobile emission of its precursors
– Reactive organic gases (ROG) reduction always lead to a
slowing of the ozone production process and lower peak O3
concentration
– Studies show that NOx reductions (with constant ROG) can
lead to a speeding up of the O3 production process, and can
increase or decrease peak ozone values depending on the
ROG-to-NOx ratio.
• Chain-terminating step of smog formation:
NO2 + R• ⌫ product (e.g., PAN)
• Competition between NO2 and ROG for OH•
42
The Control of Photochemical Smog
• More rigid control options including:
– Bans on automobile usage during periods of
high smog potential
– Limiting traffic in certain cities to electric
vehicles or vehicles equipped to burn low
emission fuels
– Expensive overhauls of existing mass transit
systems
43
Ozone Control Strategy in Europe