1. Module 9
Photochemistry

2. Preliminaries Exams back, will go over on Thursday, FCQs?
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. Module 8 Educational Objectives
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. Module 9 Educational Objectives
MCEN 4131/5131
Learning Objectives for 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
Learning Objectives
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. Health effects Reduce lung function irritate the respiratory system
MCEN 4131/5131
Learning Objectives
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. Typical Smog Episode Learning Objectives MCEN 4131/5131
Photochemical smog
Photolysis
Reactive organics
Control
EKMA model

8. Photolysis Energy for reaction comes from an absorbed photon
MCEN 4131/5131
Learning Objectives
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 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. Primary photolytic Cycle
MCEN 4131/5131
Learning Objectives
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
NO2 + h --> O(3P)*+ NO
O(3P)* + O2 + M --> O M
O3 + NO --> NO2 + O2 1 2 3
Main source of ozone in troposphere

10. Ozone is higher than photolytic reactions predict!
MCEN 4131/5131
Learning Objectives
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. Role of Reactive Organics
MCEN 4131/5131
Learning Objectives
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 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

12. Control of Smog Question: Is smog a ?
MCEN 4131/5131
Learning Objectives
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

13. MCEN 4131/5131

14. MCEN 4131/5131

15. MCEN 4131/5131