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

Recent and Past Climate Change

Sun and Earth as Black Bodies max ~ 0.5 microns max ~ 10 microns

Solar Radiation Spectrum: Blackbody 5800 K

Solar Radiation vs. Altitude

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

Radiative Equilibrium For the Earth D S-E rsrs Solar flux at Earth’s location = = 1370 W m -2 Solar flux intercepted and absorbed by Earth, distributed over its surface area = F s (1-A)/4 Radiative Balance: Terrestrial Flux Out = Solar Flux Absorbed  T E 4 = F s (1-A)/4 T E = 255 K

Greenhouse Effect f absorption of outgoing terrestrial radiation by the atmosphere

Greenhouse Model Atmospheric Layer T atm Absorptivity = f Earth’s Surface T surf F s (1 – A)/4  T surf 4 (1-f)  T surf 4 f  T atm 4 f  T surf 4 = 2f  T atm 4 T surf = (2) 1/4 T atm Radiative Balance for Atmospheric Layer: F s (1 – A)/4 = (1-f)  T surf 4 + f  T atm 4 Radiative Balance for Earth + Atmosphere:

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

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

Question 1.Does increasing CO 2 cause a warming or cooling of the stratosphere? Why? 2.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? 3.There is evidence for at least two global glaciation 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?

Scattering of Radiation by Aerosol By scattering solar radiation, aerosols increase the Earth’s albedo Scattering efficiency is maximum when particle diameter =  particles in  m size range are efficient scatterers of solar radiation

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

modis.gsfc.nasa.gov Smoke particles from biomass burning in Southeast Asia appear as white haze  F = - F s  A/4  F ~ 0.9 W/m 2 from direct effect of aerosol Aerosols Tend to Increase Earth’s Albedo

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

Questions 1. What is the SIGN of the radiative forcing caused by an 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?

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 t o, relative to CO 2 : GasLifetime (years) GWP for time horizon 20 years 100 years 500 years CO 2 ~ CH N2ON2O CFC-12 (CF 2 Cl 2 ) HFC-134a (CH 2 FCF 3 ) SF

IPCC 2001 Earth’s Energy Balance

A + B  C + D Concentration molec cm -3 time Rate of reaction at any time, t, is the slope of the tangent to curve describing change in concentration with 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) t1t1 t2t2 Chemical Kinetics (Reaction Rates)

Unimolecular: A B Bimolecular: A + B C Termolecular: A + B + M C + M Lifetime = 1/k; k has units of s -1 Special cases: 1. B=A, rate law becomes 2 nd Order in [A] 2. [B]>>[A] rate law becomes pseudo-first order in [A] M is total air number density AKA: Pressure dependent bimolecular reactions Examples - decomposition: N 2 O 5  NO 3 + NO 2 photolysis: O 3 + hv  O 2 + O k II, bimolecular rate constant, has units of cm 3 molec -1 s -1 Example- OH + CH 4  H 2 O + CH 3 First order process Rate Expressions for Gas-phase Reactions

Questions 1.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 CO 2 by plants 2.Atmospheric hydrogen peroxide is produced by the self reaction of HO 2 :HO 2 + HO 2  H 2 O 2 + O 2 a.Write an expression for the loss rate of HO 2 and for the production rate of H 2 O 2. b.Is this a first-order loss process?

Question If the rate constant for HO 2 + HO 2  H 2 O 2 + O 2 is 1x cm 3 molec -1 s -1, what is the HO 2 lifetime?

AB* Potential Energy Reaction Progress T1T1 C+D Reaction rate constants are often functions of Temperature due to energy requirements E a1 E a2 T2T2 A+B Energy barriers are common: higher T gives higher energy collisions, increasing the probability of a reaction Energy Requirements Affect Rates

1. A + B  AB* k1k1 2. AB*  A + B k2k2 3. AB* + M  C + M* k3k3 4. M*  M + heat k4k4 Assume lifetime of AB* very short, reacts as soon as its formed (quasi steady state approximation): A bimolecular reaction which requires activated complex to be stabilized by collisions with surrounding gas molecules “M” [M] is TOTAL AIR NUMBER DENSITY Termolecular (Pressure Dependent) Reactions

T=250 K k ClO+ClO and k O+O2 have been scaled Termolecular Rate Constants: Examples

1.What was the important assumption we made in deriving the rate constant for a termolecular reaction? 2.Does [AB*] change with time? Questions

Concentration molec cm -3 time t1t1 [AB*] (t) Concentration molec cm -3 time At equilibrium (forward rate = reverse rate) to equilibrium A+B C + D k forward k reverse A+B C Approach to EquilibriumQuasi Steady State of Intermediate [C] (t) [A] (t)

H 2 O + O *  2OH OH is produced in the atmosphere by the reaction of an energetically “hot” oxygen atom (we’ll talk about why its “hot” later) with H 2 O 1.What is the rate expression for the loss of O * by this reactive process? 2.What is the rate expression for the production of OH by this reactive process? 3.Typically [O * ] is << 1x10 6 molecules/cm 3, while [H 2 O] in the troposphere can be ~ 1x10 15 molecules/cm 3. If the bimolecular rate constant for the above reaction is 1x cm 3 molec -1 s -1, what is a typical lifetime for [O * ] w.r.t this reaction in the troposphere?