NATS 101 Lecture 2 Atmospheric Composition and Vertical Structure

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

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

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

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

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

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

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

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

(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

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

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

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

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

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

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

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

Pressure Decreases Exponentially with Height Logarithmic Decrease For each 16 km increase in altitude, pressure drops by factor of 10. 48 km - 1 mb 32 km - 10 mb 16 km - 100 mb 0 km - 1000 mb 1 mb 48 km 10 mb 32 km 100 mb 16 km Ahrens, Fig. 1.5 Lecture 2-Nats 101

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

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

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

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

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

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

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