Photochemical
Smog
1
Photochemical Smog
• Background: Measurements
• Sources and Chemical Processes
➔
➔
➔
➔
•
Mobile Sources
Trends in Mobile Emissions
Photochemical Nitrogen Cycle
Hydrocarbons
Smog Concentrations Around Los
Angeles
➔ Influence of Weather and Terrain
➔ Trends
2
South Coast Air Basin Primary emissions for 2005, (Tons/day):
Source
Industrial/energy
related fuel combustion
Landfills and other
waste related activities
Cleaning and surface coatings
Oil production and
marketing
Industry
Solvent
evaporation
(mostly
consumer
products:
cleaners,
paints, etc.)
Misc.: residential
fuel
combustion,
fires, dust, cooking
Light duty cars
Light and medium
duty trucks (T1,
T2, T3)
Heavy duty gas
trucks
Diesel trucks
Motorcycles
Busses
Planes,
trains,
ships, other
NOx
31
VOCs
11
CO
43
PM2.5
7.7
% of total VOCs + NOx
2.6
1.7
6
1
0.35
0.05
0.05
57
0.01
0.06
3.5
0.36
36.7
5
0.7
2
0.17
0
20.5
144
5.4
0
4.8
0.02
1
7
67
24
158
67.5
5.6
115
116
136
95
1387
1108
3.84
3
15
13
24.7
15.3
123
0.1
2
286
1.4
27.4
285
9.23
6
5
111
43.7
52
80
824
4.7
0.04
0.5
17
18
0.4
2
24
3
2
Source
Total on-road mobile sources
Total
stationary
and area sources
Total other mobile
sources
Total
anthropogenic
NOx
570
VOCs
267
CO
2793
PM2.5
12.3
% of total VOCs + NOx
51
100
300
212
81
24
284
111
824
17.09
24
955
678
3829
110
100
4
Source of Smog Pollutants
•
Most air pollution is from combustion
➔ Internal Combustion Engines (ICE) in
transportation is one significant source
Ideally:
Fuel + O2 → CO2 + H2O
Consider the simplest hydrocarbon as a fuel:
CH4 + 2O32 → CO2 + 2H2O
In this case, the only pollutant emitted is
CO2, a greenhouse gas (more on that later).
5
➔ In reality, combustion is not perfect due to
incomplete mixing or power requirements.
➔ Less O2 may be consumed than required by
stoichiometry.
A more realistic representation:
CH4 + O2 → CO + CO2 + 2H2O + HC
HC: methane, partially broken down methane,
and some polymerized methane fragments.
Also, air is not pure O2!
6
N2 + O2 + heat → "thermal NOx"
In addition, we use complex fuels such as
gasoline, oil, and coal, all of which contain
impurities—metals, S, ash, Pb, N, and so forth.
As a result there are emissions of CO, NOx,
Pb, particulate matter, etc.
Fuel + O2 + N2 +  → CO + CO2 + H2O + HC + NOx + Pb + PM + 
7
fuel + O2 + N2 . . . → CO + CO2 + H2 O + HC + N Ox + P b + P M . . .
8
9
Air Quality Index
Air Quality
Criterion
Air Quality
Index (AQI)
Good
0–50
Moderate
51–100
Unhealthy for
Sensitive Groups
101–150
Active children and adults, and people with
respiratory disease, such as asthma, should limit
prolonged outdoor exertion
Unhealthy
151–200
Active children and adults, and people with
respiratory disease, such as asthma, should avoid
prolonged outdoor exertion; everyone else,
especially children, should limit prolonged outdoor
exertion
Very Unhealthy
(Stage-1 Alert)
201–300
Active children and adults, and people with
respiratory disease, such as asthma, should avoid all
outdoor exertion; everyone else, especially children,
should limit outdoor exertion
Description
No health impacts are expected when air quality
is in this range
Unusually sensitive people should consider limiting
prolonged outdoor exertion
10
• Hydrocarbons are not a criteria
pollutant but they are closely studied
because of their role in ozone
formation.
VOCs + NOx + sunlight → photochemical smog
• Almost all of the emission information
data gathered by government agencies
is focused on the criteria pollutants.
• Emissions in the United States have
leveled off or decreased since the
1970’s—when air pollution legislation
began.
Next figs: http://www.epa.gov/ttn/chief/trends/trends98/chapter3.pdf
11
12
13
14
15
16
17
18
19
20
But many Americans still live in places where air
quality standards are not met.
21
PM10 (particles < 10 µm)
PM2.5 (particles < 2.5 µm)
22
Air pollution in the United States has been well studied in the last 30
years, and great strides have been made in protecting people. On a
global scale, US cities and Los Angeles are not the worst.
23
LA Times - 9/9/04
24
OZONE FORMATION
Ozone is one of
the important air
pollution
problems in Los
Angeles.
Let’s consider
the chemistry of
ozone formation,
and the
connection of
emissions and
meteorology.
25
Ozone in Smog
• Ozone is not directly emitted into the
atmosphere, but is produced by a
series of reactions involving:
➔ Oxides of nitrogen (NOx)
➔ Hydrocarbons (HC or VOCs/ROGs)
➔ Sunlight (hν)
26
NOx (NO and NO2) atmospheric reactions
produce ozone
( )
NO2 + hν → NO + O 3 P
( )
O P + O2 + M → O 3 + M
3
where M is some other substance
(usually N2 or O2)
Ozone is also consumed when it reacts with NO
O3 + NO → NO2 + O2
The actual O3 concentration for this set of
reactions depends on the relative rate of each
reaction.
27
The observations of the temporal variation of NO,
NO2, and O3 are consistent with this mechanism.
( )
NO2 + hν → NO + O 3 P
( )
O P + O2 + M → O 3 + M
3
O3 + NO → NO2 + O2
“Null Cycle”
But, we cannot explain the observed ozone
concentrations with this set of reactions—
they only make a few ppb of ozone.
28
Add Hydrocarbons, stir...
• More reactions—this time with free
radicals!
• Alkanes: simple organic molecules
CH4, C3H8, etc.
➔ When a hydrogen is removed, an “alkyl”
radical is formed
➔ methyl (CH3•), ethyl (C2H5•), propyl (C3H7•)
29
• Another important radical: OH• This
really wants to be water again and it
will rip a hydrogen atom right out of
your Grandma’s lungs, if its convenient.
• How OH forms:
( )
O3 + hν → O 1D + O2
( )
O 1D + H2O → 2OH•
30
Hydrocarbon Chemistry
• General hydrocarbon compound
designated as: RH
RH + OH → R• + H2O
R• + O2 → RO2 •
RO2 • + NO → RO• + NO2
• No ozone formed yet; however,
• Photodissociation of NO2 forms O3, and
reaction with NO destroys O3.
• Hydrocarbons convert NO to NO2 without
using up ozone
31
Completing the set of hydrocarbon reactions:
RH + OH → R• + H2O
R• + O2 → RO2 •
RO2 • + NO → RO• + NO2
RO• + O2 → HO2 + R′CHO
HO2 + NO → NO2 + OH
32
hν
NO2 → NO + O3
HO2
CO + OH →  → HO2
The HO2 radical that
converts NO back to NO2 is
converted in the process to
OH, which then is available
to react with another
molecule of CO to make
more HO2.
Overall effect of HC reactions: converts NO to
NO2
➔ Net production of O3
33
34
This is a classic example of a chain reaction.
The net effect is the OH• is regenerated, and
NO is oxidized to NO2, which then forms O3.
The reactions would run away were it not for
the chain termination reaction:
OH + NO2 → HNO3
Nitric acid is very soluble in water and is
responsible for most of the acidity in rainwater
in LA, and in much of the West.
Also,
HNO3,g + NH3,g → NH4NO3,s
➡ Leads to particle formation in Riverside
35
HN O (g) + N H (g) → N H N O (s)
3
3
4
3
• If the hydrocarbon
has
> 1 C atom,
multiple atoms of NO can be converted
to NO2 as the hydrocarbon is oxidized to
CO2.
• The other product (R´CHO) is an
aldehyde (in some cases it is a ketone
or other oxygenated or nitrated organic
➔ Some of these products are quite toxic;
many are not
36
• Aldehydes like formaldehyde and
acrolein are “air toxics”
➔ They are often more reactive than their
parent hydrocarbons, so they tend to
speed up the formation of smog.
Formaldehyde HCHO
Acetaldehyde CH3CHO
Acrolein
CH2CHCOH
37
Formaldehyde
concentration (ppmv)
Exposure time
(minutes)
Health Effects
0.5
5
Eye irritation
0.6
1
Odor threshold
0.08
1
Cerebral cortex affected
0.2
1
Eye, nose, and throat
irritation
0.8
10
Brain alpha wave rhythm
and autonomous nervous
system changes
4.0
1
Unbearable
38
Principle Factors in Smog Formation
• Source of primary pollutants
➔ NOx from vehicles (cars, buses, trucks)
and industry. NOx is essentially all from
anthropogenic sources
➔ Hydrocarbons from vehicles, industry,
residential sources and …..trees
39
• Timing of emissions
➔ Emissions during the morning are more
effective than evening emissions in
generating smog
➔ Simultaneous emissions magnify smog.
• Distribution of sources
➔ More distributed emissions are less likely
to result in high pollutant levels
➔ Densely concentrated urban areas focus
emissions.
40
• Prevailing meteorology
➔ To get smog, need high temperatures,
sunlight, and a temperature inversion.
• Regional topography
➔ Mountain barriers, valleys or lowlands,
plateaus affect the flow if air and
dispersion of pollutants
➔ Land/sea breezes and marine boundary
layers influence pollutant formation.
41
B
42
43
44
45
46
47
48
49
Air Quality Index
Air Quality
Criterion
Air Quality
Index (AQI)
Good
0–50
Moderate
51–100
Unhealthy for
Sensitive Groups
101–150
Active children and adults, and people with
respiratory disease, such as asthma, should limit
prolonged outdoor exertion
Unhealthy
151–200
Active children and adults, and people with
respiratory disease, such as asthma, should avoid
prolonged outdoor exertion; everyone else,
especially children, should limit prolonged outdoor
exertion
Very Unhealthy
(Stage-1 Alert)
201–300
Active children and adults, and people with
respiratory disease, such as asthma, should avoid all
outdoor exertion; everyone else, especially children,
should limit outdoor exertion
Description
No health impacts are expected when air quality
is in this range
Unusually sensitive people should consider limiting
prolonged outdoor exertion
50
51
52
53
http://www.arb.ca.gov/ch/chapis1/chapis1.htm
Check out this mapping utility that shows you
what pollutants are being emitted and where
they are coming from
54
How have we done?
1950’s
Now
Population
4.8 Million
6 Million
Vehicles
2.3 Million
11 Million
Peak Ozone Levels 0.68 ppm
0.21 ppm
Peak CO Levels
33 ppm
10 ppm
Peak NO2 Levels
0.69 ppm
0.21 ppm
Peak PM10 Levels
649 μg/m
3
183 μg/m
3
70% Reduction of most pollutants!!!
55
Schedule for meeting Federal Standards:
CO: 2000—Actually met in 2003
PM10: 2006
Ozone (1-hr): 2010 but we will not meet it
PM2.5—2018 but it is unlikely we will meet it
Ozone (8-hr)—2018 but it is unlikely we will
meet it
56
200
150
100
50
2003
Stage 1 SMOG ALERT
1-hr, 120 ppb
8-hr 85 ppb
2000
1994
1997
1988
1982
1991
Year
1985
1976
0
1979
Days Exceeding
Standard
250
57
Aerosol Particles
58
Aerosol Particles
• Sizes of Particles
• Roles of Aerosols: Cloud Formation
• Sources of Particles
• Particle Removal Processes
➔ Diffusion
➔ Gravitational Settling (Sedimentation)
• Respirability of Particles
59
60
61
µm
Mass Concentrations and Composition of
Tropospheric Aerosols
Region
Mass
(µg/m3 )
Remote
4.8
Non-urban Continental 15
Urban
32
Rubidoux, CA (1986 an- 87
nual average)
C(elem)
Percent Composition
C(org)
N H4+
N O3−
SO42−
0.3
5
9
3
11
24
31
18
22
37
28
6
7
11
8
6
3
4
6
20
62
Aerosols and Cloud Formation
• Aerosols are an essential player in the
atmosphere, since without them,
clouds would form only rarely if at all.
➔ In principal, water droplets could form if
the air is just above saturation with water
vapor, i.e., the relative humidity is 100 + δ
%.
1
• But the entropy, or random motion of
the molecules is larger than the very
small energy that holds together two
water molecules (dipole-dipole
interactions).
63
➔ In order to form a stable cluster, many
water molecules must get together—
enough to form a quasi-liquid
➔ The likelihood of a hundred or so water
molecules colliding at once is nil, so water
vapor can be highly supersaturated in an
absolutely clean gas (500% or more).
• If, on the other hand, water molecules
collide with a particle (all except a very
hydrophobic one), then it will begin to
form a quasi-liquid layer, and then grow
to a cloud droplet.
• There are enough aerosols that
supersaturation rarely exceeds 1%.
64
Sources of Particulates
• Abrasion and grinding: Grinding,
abrasion or crushing produces road
dust and construction dusts.
➔ “Fugitive” dusts
• Combustion: As hot gasses cool, some
have low enough volatility to
condense and form particles.
• Photochemistry: VOCs, NOx, and SO2
are oxidized to compounds that are
sufficiently non-volatile that they
condense out onto particles.
65
Aerosol Removal Processes
• Diffusion—Brownian motion of
molecules also applies to particles.
This is inversely proportional to D or
D2; the smaller the particle the larger
the removal rate.
➔ Diffusion is completely negligible for
particles larger than ~10 µm, and it is 106
times faster for 0.001 µm particle than a
2 µm particle.
66
of
• Gravitational Settling—speed
2
removal is proportional to D ; therefore
more important for larger particles, and
completely negligible for 0.01 µm
particles.
• The rate at which particles settle due
to gravity is derived by balancing the
gravitational pull with the frictional
drag force provided by air:
π 3
d ρg = 3πηvd
6
mg =
m = mass of particle (g)
g = gravitational acceleration (9.80 m/s2)
d = particle diameter (m)
ρ = particle density (g/m3)
η = viscosity of air (0.0172 g/ms)
v = settling velocity (m/s)
d 2 ρg
18η
v =
67
Example
Find the settling velocity of a 0.1, 1 and 10
µm diameter particle. How long will it take
each of them to travel 100 meters?
(Assume a density of 1 g/cm3)
v =
(
)(
)
106 mg3 9.80 sm2
ρg
1
=
= 31653747
g
18η
ms
18 0.0172 ms
d 2 ρg
18η
0.1 µm = 10–7 m:
(
(
)
)
1
v = 10 –14 m2 (31653747 ms
) = 3.17 × 10−7 m s
100 m
⇒
3.17 × 10 −7
m
s
= 3.15 × 10 8 s = 10.0 yr
68
(
)
1
v = 10 –12 m2 (31653747 ms
) = 3.17 × 10−5 m s
1 µm = 10–6 m:
100 m
⇒
3.17 × 10 −5
(
m
s
= 3.16 × 106 s = 36.6 days
)
1
v = 10 –10 m2 (31653747 ms
) = 3.17 × 10−3 m s
10 µm =
10–5
m:
⇒
100 m
3.17 × 10 −3
m
s
= 3.16 × 10 4 s = 8.78 hr
69
Small particles (less than 1 micron)
undergo Brownian diffusion. The equation
governing this process is given by:
D =
kTC
3πηd
k = Boltzmann constant
T = temperature (K)
d = particle diameter (m)
η = viscosity of air (0.0172 g/ms)
D = diffusion coefficient
C = Cunningham correction factor
C: an empirical correction factor that takes
care of the transition between when particles
“see” a continuous fluid and when they
experience the life of a billiard ball. The
mean free path of air @ 1 atm, room temp is
0.07 µm.
70
71
This means that the particles that are between
~0.05 and 1 µm radius have the longest lifetime
in the air.
72
Nucleation
Mode
Accumulation
Mode
Coarse
Mode
73
• They also can penetrate into your lungs
most effectively (actually ~ 2–4 µm
particles).
• They also happen to scatter light very
effectively and destroy visibility (0.5 µm
particles)!
• EPA contends that the new PM 2.5
standard will save at least 15,000
excess deaths/year in the US.
74
75
76