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This Week

This Week. READING: Chapter 7-8 of text Announcements Problem Set 2 due Tuesday Oct 16. NO CLASS Tu OR WED. Atmospheric Composition and Climate. Solar and Terrestrial Radiation Earth’s Energy Balance (Simple Climate Models!) The Greenhouse Effect Climate Forcings

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This Week

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  1. This Week READING: Chapter 7-8 of text Announcements Problem Set 2 due Tuesday Oct 16. NO CLASS Tu OR WED. Atmospheric Composition and Climate • Solar and Terrestrial Radiation • Earth’s Energy Balance (Simple Climate Models!) • The Greenhouse Effect • Climate Forcings • Aerosols, Clouds and the Planetary Albedo

  2. Recent and Past Climate Change

  3. Sun and Earth as Black Bodies max ~ 0.5 microns max ~ 10 microns

  4. Solar Radiation Spectrum: Blackbody 5800 K

  5. Solar Radiation vs. Altitude

  6. Kirchoff’s Law For any object: …very useful! Emissivity (,T) = Absorptivity

  7. Radiative Equilibrium For the Earth Solar flux at Earth’s location = = 1370 W m-2 Radiative Balance: Terrestrial Flux Out = Solar Flux Absorbed TE = 255 K TE4 = Fs(1-A)/4 rs DS-E Solar flux intercepted and absorbed by Earth, distributed over its surface area = Fs(1-A)/4

  8. Greenhouse Effect f absorption of outgoing terrestrial radiation by the atmosphere

  9. Greenhouse Model Radiative Balance for Earth + Atmosphere: (1-f)Tsurf4 Fs(1 – A)/4 = (1-f)Tsurf4 + fTatm4 fTatm4 fTatm4 Radiative Balance for Atmospheric Layer: Fs(1 – A)/4 fTsurf4 = 2fTatm4 Tsurf = (2)1/4Tatm Tsurf4 Atmospheric Layer Tatm Absorptivity = f Earth’s Surface Tsurf

  10. Terrestrial Radiation Spectrum From Space composite of several blackbody radiation spectra corresponding to different temperatures surface troposphere Scene over Niger valley, N Africa top of stratosphere

  11. Effect of Greenhouse Gas Addition Example of a GG absorbing at 11 mm 1. 1. Initial state 2. Add to atmosphere a GG absorbing at 11 mm; emission at 11 mm decreases (we don’t see the surface anymore at that l, but the atmosphere) 2. 3. 3. At new steady state, total emission integrated over all l’s must be conserved e Emission at other l’s must increase e The Earth must heat!

  12. Question • Does increasing CO2 cause a warming or cooling of the stratosphere? Why? • Early in Earth’s history, the sun was likely ~30% less intense than now. Supposing the greenhouse effect was the same, what would the average temperature have been? • There is evidence for at least two globalglaciation events in Earth’s history (“Snowball Earth”). Provide a mechanism using your climate model and C-cycle knowledge to explain how Earth might have emerged from this snowball climate state?

  13. Scattering of Radiation by Aerosol By scattering solar radiation, aerosols increase the Earth’s albedo • Scattering efficiency is maximum when particle diameter = l • particles in 0.1-1 mm size range are efficient scatterers of solar radiation

  14. Typical U.S. Aerosol Size Distributions Fresh urban Aged urban rural remote Warneck [1999]

  15. Aerosols Tend to Increase Earth’s Albedo F = - FsA/4 F ~ 0.9 W/m2 from direct effect of aerosol Smoke particles from biomass burning in Southeast Asia appear as white haze modis.gsfc.nasa.gov

  16. Global Climate Forcings Since 1750 To  F IPCC [2001]

  17. Questions 1. What is the SIGN of the radiative forcing caused byan increase in the solar constant? 2. CFC-12 absorbs in the atmospheric window (8-13 microns) and has an atmospheric lifetime of ~ 100yrs. Which would be more effective in terms of reducing anthropogenic contributions to global warming over the next hundred years, reducing CFC 12 emissions by 10 kg, or CO2 emissions by 10,000 kg?

  18. Global Warming Potential (GWP) The GWP measures the integrated radiative forcing over a time horizon t from the injection of 1 kg of a species X at time to, relative to CO2:

  19. Earth’s Energy Balance IPCC 2001

  20. Chemical Kinetics (Reaction Rates) Rate of reaction at any time, t, is the slope of the tangent to curve describing change in concentration with time A + B C + D t2 t1 Concentration molec cm-3 time Rates can change w/time because reactant concentrations can change w/time. Note this is just the concept of mass balance d[A]/dt = d[B]/dt = -d[C]/dt = -d[D]/dt (by mass conservation)

  21. Rate Expressions for Gas-phase Reactions Unimolecular: A B First order process Lifetime = 1/k; k has units of s-1 Examples - decomposition: N2O5 NO3 + NO2 photolysis: O3 + hv  O2 + O Bimolecular: A + B C kII, bimolecular rate constant, has units of cm3 molec-1 s-1 Example- OH + CH4 H2O + CH3 Special cases: 1. B=A, rate law becomes 2nd Order in [A] 2. [B]>>[A] rate law becomes pseudo-first order in [A] Termolecular: A + B + M C + M M is total air number density AKA: Pressure dependent bimolecular reactions

  22. Questions • Which of the following are examples of first order reactions? • a. Photolysis of stratospheric gases • b. Dry deposition of gases to Earth’s surface • c. Uptake of CO2 by plants • Atmospheric hydrogen peroxide is produced by the self reaction of HO2: HO2 + HO2 H2O2 + O2 • Write an expression for the loss rate of HO2 and for the production rate of H2O2. • Is this a first-order loss process?

  23. Question • If the rate constant for HO2 + HO2 H2O2 + O2 is 1x10-12 cm3 molec-1 s-1, what is the HO2 lifetime?

  24. Energy Requirements Affect Rates Reaction rate constants are often functions of Temperature due to energy requirements AB* T2 Ea2 Potential Energy Ea1 A+B T1 C+D Reaction Progress Energy barriers are common: higher T gives higher energy collisions, increasing the probability of a reaction

  25. Termolecular (Pressure Dependent) Reactions 1. A + B AB* k1 2. AB* A + B k2 3. AB* + M  C + M* k3 4. M*  M + heat k4 A bimolecular reaction which requires activated complex to be stabilized by collisions with surrounding gas molecules “M” [M] is TOTAL AIR NUMBER DENSITY Assume lifetime of AB* very short, reacts as soon as its formed (quasi steady state approximation):

  26. Termolecular Rate Constants: Examples T=250 K kClO+ClO and kO+O2 have been scaled

  27. Questions • What was the important assumption we made in deriving the rate constant for a termolecular reaction? • Does [AB*] change with time?

  28. Quasi Steady State of Intermediate Approach to Equilibrium kforward A+BC + D A+BC kreverse t1 [C](t) to equilibrium Concentration molec cm-3 Concentration molec cm-3 [AB*](t) [A](t) time time At equilibrium (forward rate = reverse rate)

  29. OH is produced in the atmosphere by the reaction of an energetically “hot” oxygen atom (we’ll talk about why its “hot” later) with H2O H2O + O* 2OH • What is the rate expression for the loss of O* by this reactive process? • What is the rate expression for the production of OH by this reactive process? • Typically [O*] is << 1x106 molecules/cm3, while [H2O] in the troposphere can be ~ 1x1015 molecules/cm3. If the bimolecular rate constant for the above reaction is 1x10-11 cm3 molec-1 s-1, what is a typical lifetime for [O*] w.r.t this reaction in the troposphere?

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