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

1976

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1997

2000

2003

Sta

ge 1

SM

OG

ALE

RT

1-hr

, 120

ppb

8-hr

85

ppb

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Day

s E

xcee

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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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