1. NATS 101 Lecture 2 Atmospheric Composition and Vertical Structure

2. Atmospheric Composition Permanent Gases
N2 and O2 are most abundant gases
Percentages hold constant up to 80 km
Ar, Ne, He, and Xe are chemically inert
N2 and O2 are chemically active, removed & returned
Ahrens, Table 1.1, 3rd Ed.
Lecture 2-Nats 101

3. Atmospheric Composition Important Trace Gases
Ahrens, Table 1.1, 3rd ed.
Lecture 2-Nats 101

4. CO2 Trend
Keeler Curve from Hawaii Obs Some gases can vary by season and can vary over many years
CO increases in spring decreases in fall
Ahrens, Fig. 1.3, 3th Ed.
Lecture 2-Nats 101

5. H2O Vapor Variability Precipitable Water (mm)
Some gases can vary spatially and daily
Lecture 2-Nats 101

6. Two Important Concepts
Let’s introduce two new concepts...
Density
Pressure
Lecture 2-Nats 101

7. What is Density? Density () = Mass (M) per unit Volume (V)  = M/V
 = Greek letter “rho”
Typical Units: kg/m3, gm/cm3
Mass =
# molecules  molecular weight (gm/mole)
Avogadro number (6.023x1023 molecules/mole)
Lecture 2-Nats 101

8. Density Change Density () changes by altering either
a) # molecules in a constant volume
b) volume occupied by the same # molecules a b
Lecture 2-Nats 101

9. What is Pressure? Pressure (p) = Force (F) per unit Area (A)
Typical Units: pounds per square inch (psi), millibars (mb), inches Hg
Average pressure at sea-level:
14.7 psi
1013 mb
29.92 in. Hg
Lecture 2-Nats 101

10. (Note that pressure acts in all directions!)
Can be thought of as weight of air above you.
(Note that pressure acts in all directions!)
So as elevation increases, pressure decreases.
Top
Higher elevation
Less air above
Lower pressure
Lower elevation
More air above
Higher pressure
Bottom
Lecture 2-Nats 101

11. Density and Pressure Variation
Key Points
Both decrease rapidly with height
Air is compressible, i.e. its density varies
Ahrens, Fig. 1.5
Lecture 2-Nats 101

12. Why rapid change with height?
Consider a spring with 10 kg bricks on top of it
The spring compresses a little more with each addition of a brick. The spring is compressible.
10 kg
Lecture 2-Nats 101

13. Why rapid change with height?
Now consider several 10 kg springs piled on top of each other.
Topmost spring compresses the least!
Bottom spring compresses the most!
The total mass above you decreases rapidly w/height.
 mass
 mass
 mass
 mass
Lecture 2-Nats 101

14. Why rapid change with height?
Finally, consider piled-up parcels of air, each with the same # molecules.
The bottom parcel is squished the most.
Its density is the highest.
Density decreases most rapidly at bottom.
Lecture 2-Nats 101

15. Why rapid change with height?
Each parcel has the same mass (i.e. same number of molecules), so the height of a parcel represents the same change in pressure p.
Thus, pressure must decrease most rapidly near the bottom. p p p p
Lecture 2-Nats 101

16. Water versus Air
Pressure variation in water acts more like bricks, close to incompressible, instead of like springs.
Top
Air:
Lower density, Gradual drop
Higher density
Rapid decrease
Top
Water:
Constant drop
Bottom
Bottom
Lecture 2-Nats 101

17. A Thinning Atmosphere Top Lower density, Gradual drop Higher density
Bottom
Top
Lower density, Gradual drop
Higher density
Rapid decrease
NASA photo gallery
Lecture 2-Nats 101

18. Pressure Decreases Exponentially with Height
Logarithmic Decrease
For each 16 km increase in altitude, pressure drops by factor of 10.
48 km - 1 mb km - 10 mb km mb km mb
1 mb
48 km
10 mb
32 km
100 mb
16 km
Ahrens, Fig. 1.5
Lecture 2-Nats 101

19. Equation for Pressure Variation
We can Quantify Pressure Change with Height
Lecture 2-Nats 101

20. What is Pressure at 2.8 km? (Summit of Mt. Lemmon)
Use Equation for Pressure Change
Lecture 2-Nats 101

21. What is Pressure at Tucson?
Use Equation for Pressure Change
Let’s get cocky…
How about Denver? Z=1,600 m
How about Mt. Everest? Z=8,700 m
You try these examples at home for practice
Lecture 2-Nats 101

22. Temperature (T) Profile
More complex than pressure or density
Layers based on the Environmental Lapse Rate (ELR), the rate at which temperature decreases with height.
inversion
isothermal
6.5oC/km
Lecture 2-Nats 101
Ahrens, Fig. 1.7

23. Higher Atmosphere Molecular Composition
Homosphere- gases are well mixed. Below 80 km. Emphasis of Course.
Heterosphere- gases separate by molecular weight, with heaviest near bottom. Lighter gases (H, He) escape.
Ahrens, Fig. 1.8
Lecture 2-Nats 101

24. Summary Many gases make up air N2 and O2 account for ~99%
Trace gases: CO2, H2O, O3, etc.
Some are very important…more later
Pressure and Density
Decrease rapidly with height
Temperature
Complex vertical structure
Lecture 2-Nats 101

25. Reading Assignment Ahrens Pages 13-22; 427-428 (Appendix C)
Problems 1.17, 1.18, 1.20
(1.17  Chapter 1, Question 17)
Don’t Forget the 4”x6” Index Cards
Lecture 2-Nats 101