Planetary Atmospheres

Transcription

1 Planetary Atmospheres Structure Composition Meteorology Clouds Photochemistry Atmospheric Escape EAS 4803/ CP 20:1

2 Cloud formation Saturated Vapor Pressure: Maximum amount of water vapor partial pressure P liquid solid C B A Deposition Sublimation T tr 0 C gas T EAS 4803/ CP 20:2

3 Cloud formation The phase change of water has an effect on the temperature structure of the atmosphere as well: Altitude Temperature Gradient Reduced further from Latent Heat of Fusion Ice Wet Adiabat* Dry Adiabat Liquid T Cloud Base *Thermal gradient reduced due to Latent Heat of Condensation EAS 4803/ CP 20:3

4 Clouds Clouds modify the surface/atmsopheric temperature of a planet in the following ways Decrease amount of incoming sunlight due to Albedo (reflectivity) 2. Heat immediate environment by absorbing solar radiation, thus changing the lapse rate 3. Blocking outgoing IR radiation can lead to green house warming near the surface 4. Reducing the lapse rate via latent heat release during cloud formation EAS 4803/ CP 20:4

5 Cloud formation Wet Adiabatic Lapse Rate: P = C L e "L S / ( R gas T ) c v dt = "PdV " L S dw S c P dt = 1 # dp " L Sdw S dt dz = g P c P + L S dw S /dt Saturation Vapor Pressure L s is Latent Heat w s is the mass of water vapor that condenses out per gram of air EAS 4803/ CP 20:5

6 Planetary Atmospheres Structure Composition Meteorology Clouds Photochemistry Atmospheric Escape EAS 4803/ CP 20:6

7 Photochemistry We can characterize chemical reactions in the atmosphere in the following way: 1. Photolysis: Molecular breakup directly driven by solar radiation (also referred to as photodissociation) 2. Photoionization: Reactions that result in the ionization of atoms and molecules 3. Recombination: Direct/indirect reversing of the photolysis and photoionization reactions 4. Dissociative Recombination: Reversing the process of photoionization 5. Charge Exchange: Direct electron exchange between a close passing ion and neutral 6. Atom-Ion Interchange: Interaction between an ion and atom that results in compositional alteration of the ion. EAS 4803/ CP 20:7

8 Photolysis Oxygen in the Earth atmosphere processed by photons: (1) O 2 + h" #O + O d O [ ] dt Where J i (z) is the reaction rate for a reaction i as a function of altitude, and [atom or molecule] indicates the number per unit volume J i = 2[ O 2 ]J ( 1 z) z ' ( ) = " x# F # e $% # z for λ < 175nm Production rate of O ( ) / µ & d# Since the number of photons decreases exponentially with depth penetrated into the atmosphere, and O 2 increases as you approach the surface, production of O increases with altitude. EAS 4803/ CP 20:8

9 Recombination Direct two body recombination reverses photolysis: O + O "O 2 + h# (2) However, this reaction is slow, and dominated by three body processes: O + O +M "O 2 + M (3) Where the reaction rates can be written: [ ] dt [ ] dt d O 2 d O 2 = O [ ] 2 k r2 O 2 + O +M "O 3 + M = [ O] 2 [ M]k r3 EAS 4803/ CP 20:9 (4) k ri is the reaction rate dependent on the collision rate (thus T) of the molecules

10 Photochemical Equilibrium Assuming that the system is in photochemical equilibrium (I.e. no net building up or depleting of constituent atoms/molecules), these reaction rates can be used to derive altitude distributions for each species. k r2 " k gk = # x $ o % 2x10-10 k r3 = 2R k 2 $ r2 % k r2 o T 300 Assuming the gas-kinetic rate of collision, which the upper limit of the reaction rate for direct recombination (where every collision sticks ), it s not immediately apparent why the three body production rate dominates at Earth s near-surface atmospheric pressure/density. **White board derivation** EAS 4803/ CP 20:10

11 Photoionization Oxygen and Nitrogen in the Earth atmosphere ionized by photons: However, these products are efficiently processed via charge exchange and atom-ion interchange to yield mostly NO + and O 2 + O 2 + h" #O e - N 2 + h" #N e - O + h" #O + + e - N 2 + +O 2 "N 2 +O 2 + O + +O 2 "O O N O "NO + + N EAS 4803/ CP 20:11

12 Ion Loss: Recombination Dissociative Recombination: O e - "O +O NO + + e - "N + O Radiative Recombination is much less efficient: O + +e - "O + h# Hence rapid processes like charge exchange and atom-ion interchange quickly replace the produced ions with dominant ions that can undergo dissociative recombination EAS 4803/ CP 20:12