Photochemical Smog
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
Source NOx VOCs CO PM2.5 % of total VOCs + NOx
Industrial/energyrelated fuel com-bustion
31 11 43 7.7 2.6
Landfills and otherwaste related activ-ities
1.7 6 1 0.35 0.05
Cleaning and sur-face coatings
0.05 57 0.01 0.06 3.5
Oil production andmarketing
0.36 36.7 5 0.7 2
Industry 0.17 20.5 5.4 4.8 1Solvent evapo-ration (mostlyconsumer prod-ucts: cleaners,paints, etc.)
0 144 0 0.02 7
Misc.: residentialfuel combustion,fires, dust, cooking
67 24 158 67.5 5.6
Light duty cars 115 136 1387 3.84 15Light and mediumduty trucks (T1,T2, T3)
116 95 1108 3 13
Heavy duty gastrucks
24.7 15.3 123 0.1 2
Diesel trucks 286 9.23 43.7 4.7 18Motorcycles 1.4 6 52 0.04 0.4Busses 27.4 5 80 0.5 2Planes, trains,ships, other
285 111 824 17 24
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South Coast Air Basin Primary emissions for 2005, (Tons/day):
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Source NOx VOCs CO PM2.5 % of total VOCs + NOx
Total on-road mo-bile sources
570 267 2793 12.3 51
Total stationaryand area sources
100 300 212 81 24
Total other mobilesources
284 111 824 17.09 24
Total anthro-pogenic
955 678 3829 110 100
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• Most air pollution is from combustion
➔ Internal Combustion Engines (ICE) in
transportation is one significant source
Source of Smog Pollutants
Ideally:
Consider the simplest hydrocarbon as a fuel:
In this case, the only pollutant emitted is
CO2, a greenhouse gas (more on that later).
Fuel + O2 → CO2 + H2O
CH4 + 2O2 → CO2 + 2H2O
➔ 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:
HC: methane, partially broken down methane,
and some polymerized methane fragments.
Also, air is not pure O2!
CH4 + O2 → CO + CO2 + 2H2O + HC
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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.
N2 + O2 + heat → "thermal NOx"
Fuel + O2 + N2 + → CO + CO2 + H2O + HC + NOx + Pb + PM +
fuel + O2 + N2 . . .! CO + CO2 + H2O + HC + NOx + Pb + PM . . .
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Air Quality Criterion
Air Quality Index (AQI) Description
Good 0–50 No health impacts are expected when air quality is in this range
Moderate 51–100 Unusually sensitive people should consider limiting prolonged outdoor exertion
Unhealthy for Sensitive Groups 101–150
Active children and adults, and people with respiratory disease, such as asthma, should limit prolonged outdoor exertion
Unhealthy 151–200Active 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
Air Quality Index
• Hydrocarbons are not a criteria pollutant but they are closely studied because of their role in ozone
formation.
• 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
VOCs + NOx + sunlight → photochemical smog
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But many Americans still live in places where air quality standards are not met.
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PM10 (particles < 10 µm) PM2.5 (particles < 2.5 µm)
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.
LA Times - 9/9/04
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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.
• 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)
Ozone in Smog
NOx (NO and NO2) atmospheric reactions
produce ozone
Ozone is also consumed when it reacts with NO
The actual O3 concentration for this set of
reactions depends on the relative rate of each
reaction.
where M is some other substance
(usually N2 or O2)
NO2 + hν → NO + O 3P( )O 3P( ) + O2 + M → O3 + M
O3 + NO → NO2 + O2
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But, we cannot explain the observed ozone
concentrations with this set of reactions—
they only make a few ppb of ozone.
The observations of the temporal variation of NO,
NO2, and O3 are consistent with this mechanism.
“Null Cycle”
NO2 + hν → NO + O 3P( )O 3P( ) + O2 + M → O3 + M
O3 + NO → NO2 + O2
• 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•)
Add Hydrocarbons, stir...
• 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•
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• General hydrocarbon compound designated as: RH
Hydrocarbon Chemistry
• 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
RH + OH → R• + H2OR• + O2 → RO2•
RO2• + NO → RO• + NO2
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
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
NO2 →hν
NO + O3
HO2
CO + OH → → HO2
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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 O
3.
The reactions would run away were it not for
the chain termination reaction:
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,
➡ Leads to particle formation in Riverside
OH + NO2 → HNO3
HNO3,g + NH3,g → NH4NO3,s
• 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
HNO3 (g) + NH3 (g)! NH4NO3 (s)
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• 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 HCHOAcetaldehyde CH3CHOAcrolein CH2CHCOH
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 10Brain alpha wave rhythm and autonomous nervous
system changes
4.0 1 Unbearable
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
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• 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.
• 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.
B
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Air Quality Criterion
Air Quality Index (AQI) Description
Good 0–50 No health impacts are expected when air quality is in this range
Moderate 51–100 Unusually sensitive people should consider limiting prolonged outdoor exertion
Unhealthy for Sensitive Groups 101–150
Active children and adults, and people with respiratory disease, such as asthma, should limit prolonged outdoor exertion
Unhealthy 151–200Active 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
Air Quality Index
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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
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1950’s Now Population 4.8 Million 6 MillionVehicles 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/m3 183 μg/m3
70% Reduction of most pollutants!!!
How have we done?
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
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1997
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2003
Sta
ge 1
SM
OG
ALE
RT
1-hr
, 120
ppb
8-hr
85
ppb
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s E
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Aerosol Particles
Aerosol Particles
• Sizes of Particles
• Roles of Aerosols: Cloud Formation
• Sources of Particles
• Particle Removal Processes
➔ Diffusion
➔ Gravitational Settling (Sedimentation)
• Respirability of Particles
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µm
Mass Concentrations and Composition of
Tropospheric Aerosols
Percent CompositionRegion Mass
(µg/m3)C(elem) C(org) NH+
4 NO!3 SO2!
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Remote 4.8 0.3 11 7 3 22Non-urban Continental 15 5 24 11 4 37Urban 32 9 31 8 6 28Rubidoux, CA (1986 an-nual average)
87 3 18 6 20 6
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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%.
• 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).
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➔ 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%.
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.
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.
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• Gravitational Settling—speed of removal is proportional to D2; 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:
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)
mg =π6d 3ρg = 3πηvd
v =d 2ρg18η
ExampleFind 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)
0.1 µm = 10–7 m:
v =d 2ρg18η
ρg18η
=106 g
m3( ) 9.80 ms2( )
18 0.0172 gms( ) = 31653747 1
ms
v = 10–14m2( ) 31653747 1ms( ) = 3.17 × 10−7 m
s
⇒100 m
3.17 × 10−7 ms= 3.15 × 108 s = 10.0 yr
1 µm = 10–6 m:
10 µm = 10–5 m:
v = 10–12m2( ) 31653747 1ms( ) = 3.17 × 10−5 m
s
⇒100 m
3.17 × 10−5 ms= 3.16 × 106 s = 36.6 days
v = 10–10m2( ) 31653747 1ms( ) = 3.17 × 10−3 m
s
⇒100 m
3.17 × 10−3 ms= 3.16 × 104 s = 8.78 hr
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Small particles (less than 1 micron)
undergo Brownian diffusion. The equation
governing this process is given by:
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.
k = Boltzmann constantT = temperature (K)d = particle diameter (m)η = viscosity of air (0.0172 g/ms)D = diffusion coefficientC = Cunningham correction factor
D =kTC3πηd
This means that the particles that are between
~0.05 and 1 µm radius have the longest lifetime
in the air.
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NucleationMode
AccumulationMode
CoarseMode
• 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.
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