Next Two Weeks—Stratospheric Chemistry

1. Next Two Weeks—Stratospheric Chemistry READING: Chapter 10 of text • Mid-latitude Ozone Chemistry (and depletion) • Polar Ozone Destruction (the Ozone Hole)

2. Final Projects See web site for more details The general idea To allow you to explore in greater detail a topic area that we covered (or did not cover) in class. The topic must deal with atmospheric chemistry or composition. A manuscript which is between 10 - 15 double spaced pages This length does not include references or figures. • Typical Formats • Critical Literature Review • NSF-like Proposal • Short Research Project

3. Ozone and Pop-Culture Day of the Animals (1977)A hole in the ozone layer drives forest animals berserk in Northern California, and a group of vacationing hikers is caught in the fray… See clips on You Tube The Naked Gun II "A love affair is like the ozone layer," says Lieut. Frank Drebin. "You only miss it when it's gone." The “Ozone Awards”, Ozone “air purifiers” Ozone Spa Therapy …

4. Stratospheric O3: Overview Most O3 (90%) in stratosphere. Remaining 10% in troposphere. “Ozone layer” (15-30 km)is 3000 – 5000 ppb of O3. Surface O3 ~ 40 – 100 ppb.

5. Stratospheric Ozone: Overview ~3,000 ppb <100 ppb • sustains terrestrial life by absorbing UV radiation • impacts the atmospheric vertical temperature profile • important role in Earth’s overall radiative balance

6. Stratospheric Ozone: Motivating Questions • What are the natural production and loss mechanisms for stratospheric O3? • What is the effect of anthropogenic emissions on stratospheric ozone, both past and future? • What are the mechanisms responsible for the drastic ozone decreases in the Antarctic? • What role, if any, does the stratospheric aerosol layer play in the gas-phase chemistry of the stratosphere?

7. Stratospheric Ozone—a brief history 1840’s Ozone first discovered and measured in air by Schonbein who named it based on its smell. 1880-1900’s: Hartley postulates the existence of a layer above the troposphere, where ozone is responsible for the absorption of solar UV between 200 and 300 nm. 1913: Fabry and Buisson used UV measurements to estimate that if brought down to the surface at STP, O3 would form a layer ~ 3 mm thick. 1920-25: Dobson first shows that T(z) in stratosphere not constant but increases with z and implicates O3 absorption. Makes first extensive set of O3 column measurements with his spectrophotometer 1930: Chapman proposed that O3 continually produced in a cycle initiated by O2 photolysis.

8. Chapman Mechanism Net O3 formation slow slow Net O3 loss Fast Fast O2 hv O Odd Oxygen Chemical Familly Ox = O3 + O hv O2 O3 Mass balance for [O]: small small

9. Odd Oxygen Family: Ox [Ox] = [O] + [O3]  [O3] Mass balance for [Ox]: • [O3] controlled by slow net production and loss via O2 + hv (R1) and O + O3 (R4) NOT by fast production and loss of O3 from O + O2 (R2) and O3 + hv (R3)

10. Questions 1. The original Chapman mechanism included a fifth reaction: O + O + M  O2 + M What would be the effect of this reaction on ozone? Is it more important in the lower or in the upper stratosphere? 2. [O]~109 molec/cm3 in the upper stratosphere and ~ 106 in the lower stratosphere, if the lifetime of O3 w.r.t. transport away from equator is ~ month, is it fair to assume O3 is in a chemical steady state throughout the stratosphere? kO+O3 (US) ~ 4x10-15 cm3/molec/s; kO+O3(LS) ~ 3x10-16 cm3/molec/

11. Stratospheric OH Profile Wennberg, et al Science 1994

12. Hydrogen oxide (HOx) radical family Initiation: H2O + O(1D)  2OH Propagation through cycling of HOx radical family (example): OH + O3 HO2 + O2 HO2+ O3 OH + 2O2 Net: 2O3  3O2 Termination (example): OH + HO2H2O + O2 HOx = H + OH + HO2 From troposphere and CH4 oxidation HOx is a catalyst for O3 loss but not the only one…

13. Nitrogen oxide (NOx) radical family Initiation N2O + O(1D) 2NO Propagation NO + O3  NO2 + O2NO + O3  NO2 + O2 NO2 + h NO + ONO2 + O  NO + O2 O + O2 + M  O3 + M Null cycleNetO3 + O  2O2 Termination Recycling NO2 + OH + M  HNO3 + M HNO3 + h NO2 + OH NO3 + NO2 + M  N2O5 + M N2O5 + hNO2 + NO3 NOx = NO + NO2

14. Cycling of NOx and NOy

15. What have we learned about NOy? • Production: N2O + O(1D) - well understood natural source • Loss: via transport from stratosphere to troposphere. Residence time for air in stratosphere is 1-2 years. Loss rate well constrained • Cycling: O3 loss related to NOx/NOy ratio. Under most conditions s.s. between NOy species is a good approximation NOx catalytic cycle reconciled Chapman theory with observations…1995 Nobel Prize

16. Stratospheric O3 Production and Loss Constrained from 1980s Space Shuttle Observations Gas-phase chemistry only • NOx is most important Ox sink throughout middle stratosphere • Fully accounts for missing O3 sink in Chapman Mechanism

17. Human Influence on Stratospheric NOx Year IPCC SYR Figure 2-1

18. Questions 1. Of the ozone loss mechanisms we have examined so far, can any operate atnight? 2. There are several variations possible for NOx and HOx catalyzed O3 destruction cycles. Given that NO3 photolysis can proceed via one of two channels a) NO3 + hv NO2 + O b) NO3 + hv NO + O2 Come up with at least one additional catalytic cycle for NOx. 3. A minor oxidation pathway for NO is HO2 + NO -> OH + NO2 What is the net effect of this reaction on ozone?

19. Lovelock’s First Measurements of CFCs Latitude CCl3F (pptv)

20. Roland and Molina What is the fate of this, non-toxic CFC?

21. Stratospheric Distribution of CFC-12 CF2Cl2

22. Chlorine radical family (ClOx=Cl + ClO) Initiation: CF2Cl2 + hv  CF2Cl + Cl Propagation: Cl + O3  ClO + O2 ClO + O  Cl + O2 Net: O3 + O  2O2 Termination: Recycling: Cl + CH4 HCl + CH3 HCl + OH  Cl + H2O ClO + NO2 + M  ClNO3 + M ClNO3 + hv  ClO + NO2 Cl not the only halogen…Br chemistry is important too O3 loss rate:

23. Cycling of ClOx and Cly

24. Questions 1. Can any of the catalytic cycles we’ve studied explain why severe ozone depletion occurs over the South Pole, just after the sun rises there?

25. Stratospheric O3 Production and Loss Constrained from 1980s Space Shuttle Observations Gas-phase chemistry only • NOx is most important Ox sink throughout middle stratosphere • Fully accounts for missing O3 sink in Chapman Mechanism

26. Early Warning Signs

27. Atmospheric Trend of CFC-11 CFC production Is banned Montreal protocol

28. Ozone Column Trend, 60oS-60oN [WMO, 1998] Stabilization of chlorine? Chlorine Or dynamics (transport)? Pinatubo

29. Recent Rise in HCFC’s (CFC Replacements)

30. Coupling Between HOx, NOx, and ClOx Cycles What is the effect of increasing stratospheric NOx on the rate of ClOx-catalyzed ozone loss? Give an example of how HOx and NOx are coupled. How might an increase in OH affect ClOx cycles?

31. SPADE Mission

32. Observations of HOx, NOx, and ClOx Couplings lead to non-linear responses to increases in NOx Wennberg, et alScience 1994