Module 9Module 9

Photochemistry

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PreliminariesPreliminaries• Exams back, will go over on Thursday, • FCQs?• Thanks for coming to the Pizza Party!• Final here in this classroom, Tues 10:30

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Review

Module 8 Educational Module 8 Educational ObjectivesObjectives

• General circulation patterns– Coriolis force

• Stability and vertical mixing– Temperature gradient in

atmosphere

• Lapse rate• Temperature inversions

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LearningObjectivesfor Today Module 9 Educational Module 9 Educational

ObjectivesObjectives

• Photochemical smog– How it forms, key ingredients

• Photolysis– Photolytic cycle

• Role of reactive organics• Control of smog• EKMA model

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Clicker Question?Clicker Question?

• What are the three essential ingredients for creating photochemical smog?

a. Methane, reactive organics, oxygen, sun

b. Nitric acid, reactive organics, high RHc. Nitrogen oxides, sun, reactive

organicsd. High T, carbon dioxide, nitrogen

oxides

LearningObjectives

Photochemical smogPhotolysisReactive organicsControlEKMA model

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Health effectsHealth effects

• Reduce lung function• irritate the respiratory

system• aggravate asthma• inflame and damage the linin

g of the lungThis photo shows a healthy lung air way (top) and an inflamed lung air way (bottom). Ozone can inflame the lung's lining, and repeated episodes of inflammation may cause permanent changes in the lung.

LearningObjectives

Photochemical smogPhotolysisReactive organicsControlEKMA model

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Typical Smog EpisodeTypical Smog Episode

QuickTime™ and aTIFF (Uncompressed) decompressor

are needed to see this picture.

LearningObjectives

Photochemical smogPhotolysisReactive organicsControlEKMA model

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PhotolysisPhotolysis

• Energy for reaction comes from an absorbed photon– Only certain species photolyze– In troposphere, photolysis is initiated for light

of 280–730 nm wavelengths

• Example: NO2 dissociation– NO2 + h --> NO + O* (kmax = 0.5 min-1)– h = Planck’s constant (6.63 x 10-34 J s) – = frequency of photon (=c/, where c = 3 x

108 m/s)– Dissociation only occurs if photon possesses

enough energy to break molecular bonds, < 430 nm for NO2

LearningObjectives

Photochemical smogPhotolysisReactive organicsControlEKMA model

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Primary photolytic Primary photolytic CycleCycle

• Group Question: Which of the following is part of the photolytic cycle?

– a. NO + hv --> N + O

– b. O + NO --> NO2

– c. O3 + NO --> NO2 + O2

NO2 + h --> O(3P)*+ NO

O(3P)* + O2 + M --> O3 + M

O3 + NO --> NO2 + O2

1

2

3

Main source of ozone in troposphere

LearningObjectives

Photochemical smogPhotolysisReactive organicsControlEKMA model

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Ozone is higher than Ozone is higher than photolytic reactions photolytic reactions

predict!predict!• Assuming photostationary state

for O and O3, then we can derive equation for O3 formation:

• When NO2 ~ NO, ozone is predicted by above to be around 20 ppb

• But in reality it is much higher by as much as an order of magnitude - WHY?

O3[ ]=k1 NO2[ ]k3 NO[ ]

LearningObjectives

Photochemical smogPhotolysisReactive organicsControlEKMA model

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Role of Reactive Role of Reactive OrganicsOrganics

• Hydrocarbons are source of radicals that convert NO to NO2 – Increase rate at which O3 is produced by

increasing NO2

– Decrease rate at which O3 is scavenged by decreasing NO

O3

NO2

O

NO

O2& M

h

RO2

Reactive organic RO2 provides a shortcut to NO2 production that does not consume ozone - so ozone accumulates

LearningObjectives

Photochemical smogPhotolysisReactive organicsControlEKMA model

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Control of SmogControl of Smog

• Question: Is smog a ?a. Secondary air pollutant b. Primary air pollutant

• Must control emissions of precursors to control smog: HCs, NOx– source of HCs: solvents, cars, lighter fluid,

industrial sources– source of NOx: cars, all combustion, power

plants, etc

• System is highly nonlinear• Analyze system using EPA’s Empirical

Kinetic Modeling Approach (EKMA)– Relates changes in HCs and NOx emissions to

changes in maximum ozone

LearningObjectives

Photochemical smogPhotolysisReactive organicsControlEKMA model

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QuickTime™ and aTIFF (LZW) decompressor

are needed to see this picture.

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QuickTime™ and aTIFF (LZW) decompressor

are needed to see this picture.

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QuickTime™ and aTIFF (LZW) decompressor

are needed to see this picture.