Module 9

1. Module 9 Photochemistry

2. MCEN 4131/5131 Preliminaries • Exams back, will go over on Thursday, • FCQs? • Thanks for coming to the Pizza Party! • Final here in this classroom, Tues 10:30

3. MCEN 4131/5131 Review Module 8 Educational Objectives • General circulation patterns • Coriolis force • Stability and vertical mixing • Temperature gradient in atmosphere • Lapse rate • Temperature inversions

4. MCEN 4131/5131 LearningObjectivesfor Today Module 9 Educational Objectives • Photochemical smog • How it forms, key ingredients • Photolysis • Photolytic cycle • Role of reactive organics • Control of smog • EKMA model

5. MCEN 4131/5131 LearningObjectives Clicker Question? Photochemical smog Photolysis Reactive organics Control EKMA model • What are the three essential ingredients for creating photochemical smog? • Methane, reactive organics, oxygen, sun • Nitric acid, reactive organics, high RH • Nitrogen oxides, sun, reactive organics • High T, carbon dioxide, nitrogen oxides

6. MCEN 4131/5131 LearningObjectives Health effects Photochemical smog Photolysis Reactive organics Control EKMA model • Reduce lung function • irritate the respiratory system • aggravate asthma • inflame and damage the lining of the lung This 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.

7. MCEN 4131/5131 LearningObjectives Typical Smog Episode Photochemical smog Photolysis Reactive organics Control EKMA model

8. MCEN 4131/5131 LearningObjectives Photolysis Photochemical smog Photolysis Reactive organics Control EKMA model • 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 + hn --> NO + O* (kmax = 0.5 min-1) • h = Planck’s constant (6.63 x 10-34 J s) • n = frequency of photon (=c/l, where c = 3 x 108 m/s) • Dissociation only occurs if photon possesses enough energy to break molecular bonds, l < 430 nm for NO2

9. MCEN 4131/5131 1 NO2 + h --> O(3P)*+ NO 2 Main source of ozone in troposphere O(3P)* + O2 + M --> O3 + M 3 O3 + NO --> NO2 + O2 LearningObjectives Primary photolytic Cycle Photochemical smog Photolysis Reactive organics Control EKMA model • 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

10. MCEN 4131/5131 LearningObjectives Ozone is higher than photolytic reactions predict! Photochemical smog Photolysis Reactive organics Control EKMA model • 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?

11. MCEN 4131/5131 O2 & M O3 O h NO2 NO RO2 LearningObjectives Role of Reactive Organics Photochemical smog Photolysis Reactive organics Control EKMA model • 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 Reactive organic RO2 provides a shortcut to NO2 production that does not consume ozone - so ozone accumulates

12. MCEN 4131/5131 LearningObjectives Control of Smog Photochemical smog Photolysis Reactive organics Control EKMA model • Question: Is smog a ? • Secondary air pollutant • 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

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