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8/2/2019 (EVT 474)LECTURE 3 - Urban Air Pollution
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LECTURE 3
URBAN AIR
POLLUTION
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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.
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Urban air
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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.
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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.
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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
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NOX
FORMATION
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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
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NOX
FORMATION
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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
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NOX
FORMATION
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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
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CO
X
FORM
ATION
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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
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SO
X
FORM
ATION
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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.
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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.
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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.
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What are the emissions of concern that come
from cars and trucks?
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.
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What are the emissions of concern that come
from cars and trucks?
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.
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What are the emissions of concern that come
from cars and trucks?
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.
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EMISSION INVENTORY
a detailed listing of pollutant emissions,and their calculated emission rate
estimates, as identified from specificsources
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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.
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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
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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.
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Usually expresses as weight of pollutantunit weight, vol, distance, duration of activity
Example : kg of particulate emitted / megagram of coal burned
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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, %
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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.
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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
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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).
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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.
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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.
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EXHAUST EMISSION
CONTROL TECHNIQUES
Tuning the engine to burn fuelefficiently
Installation of catalytic reactors Engine modifications
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Engine Tuning
A well tuned engine is the first line of defense forcontrolling automobile emissions
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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
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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
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REDESIGN OF INTERNALCOMBUSTION ENGINES
Fuel injectorCylinder configuration
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Smog
There are two types of smog: Industrial
Photochemical
These pictures are of Los Angeles on a clear dayand on a smoggy day
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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 (
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INDUSTRIAL SMOG
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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)
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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
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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
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Industrial smog
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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
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FORMATION OFPHOTOCHEMICAL
SMOG
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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
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Photochemical smog
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MAIN COMPONENTS OFPHOTOCHEMICAL SMOG FORMATION.
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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.
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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
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Photochemical Smog
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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.
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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.?
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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
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Photochemical Smog
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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
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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)
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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)
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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)
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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
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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)
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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
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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
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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
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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.
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FORMATION OF THERMAL INVERSION
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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
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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.