(EVT 474)LECTURE 3 - Urban Air Pollution

8/2/2019 (EVT 474)LECTURE 3 - Urban Air Pollution

1/63

LECTURE 3

URBAN AIR

POLLUTION


2/63

What is urbanization?

Urbanization is a process of relativegrowth in a countrys urban populationaccompanied by an even faster increasein the economic, political and cultural

importance cities relative to rural area.

2


3/63

Urban air

3


4/63

Urban air Air pollution was first perceived as a local problem in

urban industrialized areas.

Taller smoke-stacks for industries and power plantswere a ready solution.

However, taller stacks merely transported the problemelsewhere and soon regional problems such as acid rainwere recognized.

For example, in Scandinavia, the acidification of lakes

was found to result largely from industrial emissions ofsulfur dioxide from tall stacks located in centralEuropean countries such as Germany.

4


5/63

Urban air

Most recently, global problems such as climate change andstratospheric ozone depletion have been widely publicized.

Obviously, laws and regulations to correct and control local,regional and global environmental threats require progressively

more cooperation; starting from city ordinances, through statelaws (or, in Europe, individual country statutes) and finallyrequiring full participation in international agreements.

Unlike forest and mineral resources, the atmosphere is truly ashared resource that respects no man-made boundaries.

5


6/63

POLLUTANTS FORMATION INCOMBUSTION PROCESSES

Basically under three mechanisms: Thermal

Fuel

Prompt

Thermal Produced by the reaction of atmospheric oxygen and nitrogen at

elevated temperatures.

The reactions are described as follows:N2 + O = NO + N

N + O2 = NO + O

N + OH = NO + H

6

NOX

FORMATION


7/63

Fuel NOx

From the reaction of the organically boundnitrogen in the fuel with oxygen.

The process is complex but can be simply

expressed as follows:

Fuel-N thermal N-intermediates

decomposition (HCN, NH , N, CN)

N-intermediates + O2oxidation NOx

7

NOX

FORMATION


8/63

Prompt NOx

Formed by the reaction of hydrocarbon radicals withatmospheric nitrogen to produce HCN, hence NOx via a complexseries of gas phase reactions.

The process can be simply expressed as follows:

CH + N2 HCN + N

N + O2 NO + O

HCN + OH CN + H2O

CN + O2 NO + CO

8

NOX

FORMATION


9/63

The combustion of methane gas(CH4 + 2O2 CO2 + 2H2O) produces an undesirable product, namely the "global warming

gas" carbon dioxide.

Carbon monoxide is oxidized to CO2CO + H- + O2 CO2 + HO2

-

9

CO

X

FORM

ATION


10/63

SO2 is almost exclusively formed during the combustion

of fossil fuels with relevant sulfur content (coal)CH3-SH +3O2 SO2 + CO2 + 2H2O

During the combustion and in exhaust channels, the SO2

can be oxidized to SO3

2SO2 + O2 2SO3

10

SO

X

FORM

ATION


11/63

Mobile Sources

Emissions from an individual car or passenger truck aregenerally low, relative to smokestack plumes that many peopleassociate with air pollution.

But in numerous cities across the country, the personalautomobile is the single greatest pollution category, as

emissions from millions of vehicles on the road add up.

Driving a private car is probably a typical citizens mostpolluting daily activity. The power to move a car comes from

burning fuel in an engine.

Pollution from cars comes from by-products of thiscombustion process (exhaust) and from evaporation of the fuelitself.

11


12/63

What are the emissions of concern that come from cars

and trucks?HYDROCARBONS

HCs emissions result when fuel molecules in the engine do not burnor burn only partially. HCs react in the presence of NOx and sunlightto form ground-level ozone, a major component of smog.

- Ozone irritates the eyes, damages the lungs, andaggravates respiratory problems. It is our most widespreadurban air pollution problem.

VOCs are the most commonly tracked HCs and are emitted from thetailpipe, by the evaporation of fuel and refuelling.

- About 31 percent of anthropogenic VOC emissions are fromcars and trucks. An exhaust HCs are also toxic, with thepotential to cause cancer.

12


13/63

What are the emissions of concern that come

from cars and trucks?

NITROGEN OXIDES (NOx)

Under the high pressure and temperature conditions in anengine, nitrogen and oxygen atoms in the air react to form

various nitrogen oxides, collectively known as NOx.

Nitrogen oxides, like hydrocarbons, are precursors to theformation of ozone. They also contribute to the formation of acidrain.

About 30 percent of NOx emissions are from cars and trucks.

13


14/63



CARBON MONOXIDE

Carbon monoxide (CO) is a product of incomplete combustionand occurs when carbon in the fuel is partially oxidized rather

than fully oxidized to carbon dioxide (CO2 ).

Exposure to carbon monoxide reduces the flow of oxygen in thebloodstream and is particularly dangerous to persons with heartdisease.

About 60 percent of CO emissions are from cars and trucks.

14


15/63



PARTICULATE MATTER

Particulate Matter (PM) includes both fine and coarse particles.- Fine particles (PM2.5) result from fuel combustion including cars

and trucks.- Coarse particles (PM10) generally result from vehicles traveling

on unpaved roads with a small amount caused by combustion.- These particles can accumulate in the respiratory system and

are associated with numerous health effects.

Exposure to coarse particles is primarily associated with the aggravation ofrespiratory conditions, such as asthma.

Fine particles are most closely associated with heart and lung disease,increased respiratory symptoms and disease, decreased lung function, andeven premature death.

About 25 percent of anthropogenic PM10 emissions are from cars and trucks.

15

16


16/63



GREENHOUSE GASES

Greenhouse gas emissions are primarily carbon dioxide (CO2

),which is a product of fuel combustion.

CO2 does not directly impair human health, but it is a"greenhouse gas" that traps the earth's heat and contributes tothe potential for global warming.

About 26 percent of anthropogenic greenhouse gas emissions arefrom cars and trucks.

16

17


17/63

EMISSION INVENTORY

a detailed listing of pollutant emissions,and their calculated emission rate

estimates, as identified from specificsources

17

Accounts for the mixture of emissions and

predicts future air quality.

Air quality is the result of a mixture of pollutantsadded by many different sources.

18


18/63

What pollutants are in the emission

inventory?

Includes data about :

Total organic compounds Reactive organic compounds Oxides of nitrogen Particulate matter Carbon monoxides Sulphur dioxide

Ammonia Lead

18

19


19/63

EMISSION FACTOR

Representative value that attempts to relatethe quantity of a pollutant released to the

atmosphere with an activity associated withthe release of that pollutant.

19

Usually expresses as weight of pollutantunit weight, vol, distance, duration of activity

Example : kg of particulate emitted / megagram of coal burned

20


20/63

The general equation for emissions estimation is

E = A x EF x (1 ER/100)

Where :

E = emissions A = activity rate EF = emission factor ER = overall emission reduction efficiency, %

20

21


21/63

Emission inventory & emission factor

Fundamental tools for air quality management

Emission estimates are important for developing emissioncontrol strategies, determining applicability of permitting and

control programs, to figure out the effects of sources andappropriate mitigation strategies.

The inventories used for ambient dispersion modeling andanalysis, control strategy development, and in screeningsources for compliance investigations.

may also use in some permitting applications, such as inapplicability determinations and in establishing operatingpermit fees.

21


22/63

CONTROL OF POLLUTION FROMAUTOMOBILES

Important points requiring control:Evaporation loss from fuel

tank

Evaporation of HCs from

carburetor

Emission of unburned gasand partially oxidized HCs

from crankcase NOx.

HCs, and CO in theexhaust


23/63

CONTROL OF THE POTENTIAL EMISSIONPOINTS

Evaporation from the gas tankcan be eliminated by use of gastank caps that prevent vaporescaping

Losses from carburetors can bereduced by using activatedcarbon canisters that adsorbvapors emitted when the engineis turned off and hot gasoline inthe carburetor vaporizes.

The vapors are purged fromthe canister by air when the caris restarted and burned in theengine.

The toxicity of emission from

exhaust can be reduced byapplying the catalytic converter.

Crankcase emissions have beeneliminated by recycling crankcasegases into the intake manifold and

the installation of the positivecrankcase ventilation valve (PCV).


24/63

Exhaust Emissions

60% of the HCs and almost all of the NOx

, CO, and lead comefrom the exhaust.

The quantity of emissions changes with the operatingconditions of the vehicle.


25/63

When the car is accelerating the combustion is efficient (lowCO and HC), but high amounts of NOx are produced.

When the car is decelerating there are low amounts of NOxproduced but high amounts of HCs due to partially burnedfuel.

This makes it difficult to determine how much pollution aparticular engine design produces. The EPA has developed astandard test to make this determination. The test includes a

cold start, cruising with a simulated load, and a hot start.


26/63

EXHAUST EMISSION

CONTROL TECHNIQUES

Tuning the engine to burn fuelefficiently

Installation of catalytic reactors Engine modifications


27/63

Engine Tuning

A well tuned engine is the first line of defense forcontrolling automobile emissions


28/63

CATALYTIC CONVERTERS

Oxidize CO and HCs to CO2 and H2O

Most common catalyst - platinum

Problems:Fouled by some gasoline additives like lead

(this is why lead has been eliminated fromgasoline)Sulfur in gasoline converted to particulateSO3


29/63

Limiting Vehicle Emissions

Catalytic Converter

Platinum-Rhodium catalyst

Accomplishes the following reactions

Conversion of Nitric Oxide to original reactants

2 NO (g) N2(g) + O2(g)

Conversion of carbon containing gases to CO2 and H2O(in other words, completing the oxidation!)

2 CO (g) + O2(g) 2 CO2(g)

hydrocarbons + O2(g) CO2(g) + H2O (g)

Pt-Rh catalyst

Pt-Rh catalystPt-Rh catalyst


30/63

REDESIGN OF INTERNALCOMBUSTION ENGINES

Fuel injectorCylinder configuration


31/63

Smog

There are two types of smog: Industrial

Photochemical

These pictures are of Los Angeles on a clear dayand on a smoggy day


32/63

Two Types of SmogLondon Los Angles

Time

Pollutants

Fuels

Season

Temperature

Humidity

Sunlight

O3 conc.

Time of event

VisibilityToxicity

1873

PM, SO2, H2SO4

Coal, fuel oils

Winter

Low (


33/63

INDUSTRIAL SMOG

33


34/63

Industrial Smog

Occurs from oil or coal combustion

Combustion products contain particulates(soot, fly ash) with absorbed SO2

SO2 is a main ingredient in industrial smog Promotes aerosol formation

Characterized by high humidity and lowtemperatures.

In aerosol droplets: 2SO2(g) + O2(g) ------>2 SO3 (g)

SO3(g) + H2O (l) --- H2SO4 (aq)


35/63

Industrial smog

Sources of SO2 Sulfur containing compounds in coal and oil

Non-ferrous smelters Smelters convert metal sores to free metals

- Example: Nickel sulfide ores- 2NiS(s) + 3O2(g) - 2 NiO (s) + 2SO2 (g)

- Instead of releasing SO2 in form of aerosoldroplets:- 2SO2(g) +O2(g) --- 2SO3 (g)- SO3(g) +H2O (l)H2SO4(aq)

- SO2 (g) can be captured, catalytically convertedto SO3 (g) and reacted with water to formconcentrated sulfuric acid


36/63

Primary and Secondary Pollutants

Primary Pollutants: Pollutants emitted directly into abiogeochemical reservoir, such as the atmosphere

In the case of an industrial smog, SO2 is the primarypollutant

In aerosol droplets: 2SO2(g) + O2(g) -- 2 SO3 (g)

SO3 (g) + H2O (l) --

H2SO4 (aq)

Secondary Pollutants: Pollutants formed inbiogeochemical reservoir by subsequent chemicalreactions. In this case, sulfur trioxide and sulfuric acid are

secondary pollutants

37


37/63

Industrial smog


38/63

Photochemical Smog

Photochemical smog: the primary pollutants ofnitrogen oxides (NOx) and gaseous hydrocarbons

interact in the presence of sunlight, oxygen, andwater vapor to form a hazy cloud, which is acollection of secondary pollutants

39


39/63

FORMATION OFPHOTOCHEMICAL

SMOG


40/63

Nature of Photochemical Smog

Species Polluted Area

(g/m3)Unpolluted Air

(g/m3)CO

NOHC (excluding CH4)

O3PANs

10,000-30,000

100-400600-3,000

50-150

50-250


41/63

Photochemical smog


42/63

MAIN COMPONENTS OFPHOTOCHEMICAL SMOG FORMATION.

43


43/63

PhotochemicalSmog

Photochemical smog is a type of air pollution producedwhen sunlight acts upon motor vehicle exhaust gasesto form harmful substances such as ozone (O3),aldehydes and peroxyacetylnitrate (PAN).

Ozone causes breathing difficulties, headaches, fatigueand can aggravate respiratory problems.

The PAN (CH3CO-OO-NO2) in photochemical smogcan irritate the eyes, causing them to water and sting.


44/63

3 Ingredients Required for the Formation

of Photochemical Smog

UV light (temperatures >18 C ) Hydrocarbons (VOCs)

Nitrogen oxidesPhotochemical 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


45/63

Photochemical Smog

46


46/63

Photochemical Smog

Early morning traffic increases the emissions of both nitrogenoxides and VOCs as people drive to work.

Later in the morning, traffic dies down and the nitrogen oxides

and volatile organic compounds begin to be react, formingnitrogen dioxide, increasing its concentration.

As the sunlight becomes more intense later in the day, nitrogendioxide is broken down and its by-products form, increasingconcentrations of ozone.

At the same time, some of the nitrogen dioxide can react with thevolatile organic compounds to produce toxic chemicals such asPAN.

As the sun goes down, the production of ozone is halted. The ozonethat remains in the atmosphere is then consumed by several

different reactions.

47


47/63

Photochemical Smog

Why does the atmospheric concentration of non-methane hydrocarbons peak at 8:00 a.m., and thendecrease throughout the day?

Why does the atmospheric concentration of aldehydespeak at 1:00 p.m., and then decrease throughout theafternoon?

Why does the atmospheric concentration of oxidants(PAN) peak at 3:00 p.m.?


48/63

During fuel combustion:

Formation of Nitric Oxide:N2(g) +O2(g) -2NO(g)

Nitric oxide reacts rapidly with atmosphericoxygen: Formation of Nitrogen oxide:

2NO(g) +O2(g) -2NO2(g)

Nitric Oxide (NO) and Nitrogen Dioxide (NO2)are both free radicals:

Possess unpaired electron Unpaired electron makes them reactive

Not every atom has a complete octet

49


49/63

Photochemical Smog

50


50/63

First recognized as a problem in the 1940sin Los Angeles, CA

Problem in other cities: Mexico city, Brazil, New Delhi, India;

Beijing

Since 1950s automobile is the leading

contributor to air pollution globally

Photochemical Smog

51


51/63

Ozone Production

Motor vehicles produce exhaust gases containingoxides of nitrogen such as nitrogen dioxide (NO2)and nitric oxide (NO).

At the high temperatures of the car's combustionchamber (cylinder), nitrogen and oxygen from the

air react to form nitric oxide (NO):

N2(g) + O2(g) -----> 2NO(g)

Some of the nitric oxide (NO) reacts with oxygen to

form nitrogen dioxide (NO2):

2NO(g) + O2(g) -----> 2NO2(g)

52


52/63

The mixture of nitric oxide (NO) and

nitrogen dioxide (NO2) is sometimesreferred to as NO

x.

When the nitrogen dioxide (NO2)concentration is well above clean air levels

and there is plenty of sunlight, then anoxygen atom splits off from the nitrogendioxide molecule:

NO2(g)sunlight-->NO(g)+O(g)

53


53/63

This oxygen atom (O) can react with oxygenmolecules (O2) in the air to form ozone (O3):

O + O2 -----> O3 Nitric oxide can remove ozone by reacting

with it to form nitrogen dioxide (NO2) andoxygen (O2):

NO(g) + O3(g) -----> NO2(g) + O2(g)

54


54/63

OZONE PRODUCTION

- ratio of NO2 : NO is > 3, the formation ofozone is the dominant reaction.

- ratio NO2 : NO < 0.3, the nitric oxide reactiondestroys the ozone at about the same rate as it

is formed (ozone concentration below harmfullevels)

The reaction of HCs (unburned petrol) with

nitric oxide and oxygen produce nitrogendioxide, also in the presence of sunlight,increasing the ratio of NO2 : NO

55


55/63

Peroxyacetylnitrate Production

Nitrogen dioxide (NO2), oxygen (O

2) and

hydrocarbons (unburned petrol) react inthe presence of sunlight to produceperoxyacetylnitrate (CH3CO-OO-NO2):

NO2(g) + O2(g) + HCs + sunlight ---> CH3CO-OO-NO2(g)

(PAN)


56/63

Nitrogen Dioxide is corrosive and reacts rapidly with watervapor

Formation of nitrous acid and nitric acid2NO2(g) + H2O(g) -HNO2(g) +HNO3(g)

Nitrogen dioxide also forms ozone Sunlight necessary to supply enough energy to break one of the

nitrogen-oxygen bondsNO2NO+O

O+O2O3 Atomic oxygen also generates the hydroxyl radical by reactingwith atmospheric water vapor O(g) + H2O (g) 2OH(g)

Ozone (oxidant) concentration doesnt increase until most of NOis converted to NO2. This is due to the following competing

atmospheric chemical reaction: NO+O3NO2 +O2


57/63

VOLATILE ORGANIC COMPOUND (VOC)

The presence of hydrocarbons and hydrocarbon-likeorganic compounds called volatile organic compounds(VOCs) correspond to the Non-Methane hydrocarbons

Sources of VOCs: Anthropogenic sources:

Gasoline pumps

Cold starts leading to incomplete combustion

Natural sources:

Trees, plants

Most reactive VOCs have double bonds Benzene, toluene

58


58/63

Several meteorological factors can influence theformation of photochemical smog. These conditionsinclude:

Precipitation can alleviate photochemical smog as the pollutantsare washed out of the atmosphere with the rainfall.

Winds can blow photochemical smog away replacing it with freshair. However, problems may arise in distant areas that receive thepollution.

Topographyis important factor influencing how severe a smogevent can become. Communities situated in valleys are moresusceptible to photochemical smog because:

- hills and mountains surrounding them tend to reduce the air flow,

allowing for pollutant concentrations to rise.

- valleys are sensitive to photochemical smog because relatively

strong temperature inversions

59


59/63

60


60/63

Temperature inversions

- Normally, during the day the air near the surface is heated and as it

warms it rises, carrying the pollutants with it to higher elevations.

- However, if a temperature inversion develops, pollutants can be

trapped near the Earth's surface.

- Temperature inversions cause the reduction of atmospheric mixingand therefore reduce the vertical dispersion of pollutants.

- Inversions can last from a few days to several weeks.

61


61/63

FORMATION OF THERMAL INVERSION

62


62/63

REACTION CYCLES INVOLVING ORGANIC OXIDATION

OF NO2 WITH OZONE BUILDUP

OzoneO3

OxygenAtom

O

NitrogenDioxide

NO2

NitricOxideNO

O2 + O O3

O3 + NO NO2 + O2

NO2 + h NO +ONO + RO2 NO2 +

RO

+O2

+Light

63


63/63

Photochemical Smog - conclusion

Initiated by sunlight

1st step photochemical

cleavage of NO2 NOand O

Yellow brown haze reducesvisibility

Irritating substances

(respiratory tract and eyewatering)

Four conditions NOx,sunlight, HC temperaturesabove 18 degree Celsius.

Documents

(EVT 474)LECTURE 3 - Urban Air Pollution