Module 9

1. Module 9 Atmospheric Stability

2. MCEN 4131/5131 Preliminaries • I will be gone next week, Mon-Thur • Tonight is design night, 7:30ish, meet in classroom • Next tues Tan and Nick will be in class to help you with your Projects - they are graduate students who took class

3. MCEN 4131/5131 Review Module 7 Educational Objectives • Increased use of cars worldwide has altered the field of air pollution control • The air ER is the actual air/fuel ratio divided by the stoichiometric air/fuel ratio. • For gasoline, the AFR is 14.7 • fuel rich for ER < 1 • major pollutant emissions are CO, HCs • fuel lean for ER > 1 • major pollutant emissions are NOx especially near ER = 1 • The IC engine does not have complete combustion because of the temperature distribution within the cylinder, and the walls are cooler, quenching reactions • Add-on technologies that control emissions are the catalytic converter and the carbon canister

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

5. MCEN 4131/5131 LearningObjectives Circulation of the Atmosphere Circulation patterns Vertical mixing Lapse rate Temperature inversions • Global circulation patterns due to • nonuniform heating of earth’s surface • Buoyancy (warm air rises) • Coriolis effect • Nonuniform heating of earth’s surface • Greatest heating at equator • Air rises at equators, subsides at poles • Because of earth’s rotation, this pattern is broken up

6. MCEN 4131/5131 Geostrophic layer 300-500 m Planetary boundarylayer Ekman layer 50-100 m Surface layer LearningObjectives Wind profiles in lower atmosphere Circulation patterns Vertical mixing Lapse rate Temperature inversions • geostrophic layer • inviscid (viscous effects are negligible) • Wind profile determined by pressure gradient and coriolis effect • planetary boundary layer • Effect of earth’s surface is important • Important in pollutant transport • surface layer • Wind profile determined by surface drag and temperature gradient and pressure gradient • Ekman layer • Wind profile determined by surface drag, pressure gradient and Coriolis

7. MCEN 4131/5131 Clicker Question • This force results from the earth’s rotation and deflects air movement to the right in the N. hemisphere • Friction force • Coriolis force • Rotational atmospheric force • Centrifugal force

8. MCEN 4131/5131 LearningObjectives Coriolis Forces Circulation patterns Vertical mixing Lapse rate Temperature inversions • Influences circulation in the geostrophic layer • Think of wind blowing toward south in northern hemisphere • Surface velocity of earth increases toward equator • From earth, wind gains a velocity toward west

9. MCEN 4131/5131 LearningObjectives Coriolis Cont’d Circulation patterns Vertical mixing Lapse rate Temperature inversions W E N Equator E W N rotation Earth from the side Earth from above

10. MCEN 4131/5131 LearningObjectives Ekman Spiral Circulation patterns Vertical mixing Lapse rate Temperature inversions • refers to winds near a horizontal boundary in which the flow direction rotates as one moves away from the boundary • Happens within planetary boundary layer • Consequences: top of plumes can move in directions as much as 50 degrees from the bottom of the plume

11. MCEN 4131/5131 Typically u1 is measured at z1 = 10 m. LearningObjectives Clicker Question? Circulation patterns Vertical mixing Lapse rate Temperature inversions • The relationship between wind velocity and height in the atmosphere are described by which function? • Exponential • Logarithmic • Power • Linear

12. MCEN 4131/5131 LearningObjectives Temperature structure of the lower atmosphere Circulation patterns Vertical mixing Lapse rate Temperature inversions • Affects stability of troposphere • Controls vertical air movement • Disperses near-surface emissions • Troposphere: T decreases with height • Warm air is less dense than cool air • Warm air under cool air results in vertical mixing

13. MCEN 4131/5131 LearningObjectives Temperature of Atmosphere Circulation patterns Vertical mixing Lapse rate Temperature inversions • In the troposphere normally the temperature decreases as you go up in altitude • Rate is on average 0.65 degrees C per 100 meters (called a lapse rate) • This decrease in temperature helps to mix the air, dispersing pollutants

14. MCEN 4131/5131 LearningObjectives Lapse Rate Circulation patterns Vertical mixing Lapse rate Temperature inversions • Consider stationary mass of air governed by pressure forces and gravity (ignore viscous effects) • Large distortable volume • Slowly exchanges heat and mass with surroundings • Pressure equilibrates rapidly • no energy is added or removed • Hydrostatics: -(dP/dz) = (MWag/RT)P • Solve for dT/dz

15. MCEN 4131/5131 LearningObjectives Adiabatic lapse rate Circulation patterns Vertical mixing Lapse rate Temperature inversions • Rate at which temperature of dry air changes with height in the atmosphere due to adiabatic expansion or compression

16. MCEN 4131/5131 LearningObjectives Group clicker question Circulation patterns Vertical mixing Lapse rate Temperature inversions • If the lapse rate is equal to the dry adiabatic lapse rate, the stability condition is: • Unstable • Neutral • Stable • And what if the lapse rate is less than gd?

17. MCEN 4131/5131 LearningObjectives Atmospheric Stability Circulation patterns Vertical mixing Lapse rate Temperature inversions • Stable • buoyancy returns a parcel of air to its original position after it has been displaced upward or downward • Atmospheric lapse rate < adiabatic lapse rate • Atmosphere cools less rapidly with height than parcel • Vertical mixing suppressed • Unstable • buoyancy increases the displacement of the parcel of air that has moved upward or downward • adiabatic lapse rate < atmospheric • Atmosphere cools more rapidly with height than parcel • Vertical mixing is promoted • Neutral • the lapse rate is equal to the dry adiabatic lapse rate, parcel of air stays where it has been displaced • Adiabatic = atmospheric lapse rate

18. MCEN 4131/5131 LearningObjectives Pasquill stability class Circulation patterns Vertical mixing Lapse rate Temperature inversions • Would like to predict atmospheric lapse rate from readily observable properties • Pasquill (1961) introduced notion of stability class • Based on 3 characteristics • Intensity of solar radiation • Near-surface wind speed • Extent of nighttime cloud cover • Relationship of stability class to lapse rate

19. MCEN 4131/5131 LearningObjectives Stability Classes Circulation patterns Vertical mixing Lapse rate Temperature inversions Stability class Lapse rate (C/100 m) A (extremely unstable) < -1.9 B (moderately unstable) -1.9 to -1.7 C (slightly unstable) -1.7 to -1.5 D (neutral) -1.5 to -0.5 E (slightly stable) -0.5 to 1.5 F (moderately stable) > 1.5

20. MCEN 4131/5131 Temperatureprofile as a function of height LearningObjectives Temperature Inversions Circulation patterns Vertical mixing Lapse rate Temperature inversions • When there is cold air near the ground, and a layer of warmer air above

21. MCEN 4131/5131 LearningObjectives Circulation patterns Vertical mixing Lapse rate Temperature inversions

22. MCEN 4131/5131 LearningObjectives Circulation patterns Vertical mixing Lapse rate Temperature inversions • When there is cold air near the ground, and a layer of warmer air above • Clicker Question? Which of the following Inversions plays the most important role in cause smog problems? • Subsidence • Frontal • Radiation And what about for wood-burning in the winter?