Understanding Stratospheric Ozone Chemistry: Mechanisms and Impacts

1. Chem. 253 – 2/5 Lecture

2. Announcements I • I have updated the homework set on the web site • Now has solutions for non-collected subset 1.1 problems • Also has subset 1.2 (for next week) • Last Week’s Group Assignment • Trial, not graded • This Week’s Group Assignment • on stratospheric chemistry (O only, mostly covered last time)

3. Announcements II • Today’s Lecture Topics • More on O only chemistry (putting reactions in context and covering spatial variations) • Catalytic destruction of ozone – Mechanism I • Catalytic destruction of ozone - reservoir species and Mechanism II • The ozone hole

4. Stratospheric ChemistryO only Chemistry • Factors Affecting Ozone Formation • Full set of Chapman Mechanism reactions: O2 + hn→ 2O O + O2 + M → O3 + M O3 + hn→ O2 + O O + O3→ 2O2 • Short l UV needed to photolyze O2 • see next slide • Pressure: O3 formation is slower at higher altitudes (lower O2 and M) • Short UV also shortens O3 lifetime at higher altitudes rxn 4 kinetic equation: -d[O3]/dt = k[O][O3]

5. Stratospheric ChemistryO only Chemistry • Factors Affecting Ozone Formation • Short l UV needed to photolyze O2 • more prevalent at higher altitudes (less removal from absorption above) • and at lower latitudes (due to transmission through more atmosphere at higher latitudes) Earth longer pathlength surface at high latitudes surface in tropics

6. Stratospheric ChemistryO only Chemistry • Ozone Production Rate Wayne, Chemistry of Atmospheres, 2nd Ed., p. 124

7. Stratospheric ChemistryO only Chemistry N • However, despite production occurring mostly at high altitudes/ low latitudes, concentrations are higher at high latitudes/low altitudes • This is due to transport O3 Concentration Plot Wayne - Chemistry of Atmospheres, 2nd Ed., p. 119

8. Stratospheric ChemistryOzone Loss • Ozone Destruction – O only chemistry • Reaction 4 (O + O3→ 2O2) is only real loss (ozone photolysis recycles odd O) • Ozone Destruction – observations • models predict higher than observed concentrations • Catalytic Mechanisms for loss • a catalytic mechanism should result in the same net reaction as Chapman Rxn 4, but can involve another species

9. Stratospheric ChemistryOzone Loss – Mechanism I • Mechanism I rxn 1: X + O3→ O2 + XO rxn 2: XO + O → O2 + X net rxn: O + O3→ 2O2 same as Chapman #4 • X = Catalytic Species • X = Cl, NO, OH • sources: from troposphere (will go into in more detail)

10. Stratospheric ChemistryOzone Loss – Mechanism I • X = NO • NO source: mainly N2O (both natural and anthropogenic) • N2O (stable in troposphere, so can survive slow transport to stratosphere) • reaction in stratosphere: N2O + O* → NO +O2 • Catalytic Cycle: rxn 1: NO + O3 → O2 + NO2 rxn 2: NO2 + O → O2 + NO • Complications: NO2 + hn→ NO + O (“null” cycle reaction when teamed with rxn 1)

11. Stratospheric ChemistryOzone Loss – Mechanism I • X = Cl, Br • Cl sources: • natural sources: CH3Cl (a fraction is transported to stratosphere) • anthropogenic sources: CFCs • very stable in troposphere (zero losses there) • C-Cl bonds photolyzed in stratosphere • Br sources: halons (for extinguishing fires) + CH3Br (fumigant) • Both halogen reactions are very fast

12. Stratospheric ChemistryOzone Loss – Mechanism I • X = OH • H sources: • tropospheric H2O (actually very little transport – tropopause is a cold trap) • CH4 + O* → OH + CH3 (CH3 produces more H radicals – OH and HO2)

13. Stratospheric ChemistryOzone Loss – Mechanism I • Mechanism I resulted in over prediction of O3 loss, particularly from Cl cycle • Reason: missed reservoir species • In 1970s, predictions changed frequently O3 Loss Rate Plot Wayne - Chemistry of Atmospheres, 2nd Ed., p. 158

14. Break for Group Activity

15. Stratospheric ChemistryOzone Loss – Reservoir Species • Example of reservoir species ClO + NO → ClONO2 • this removes both Cl and NO from catalytic loss cycles • like many reservoir species, ClONO2 can be “reactivated”: • ClONO2 + hn→ ClO + NO • Other reservoir species: HCl, HBr, HNO3 • Inclusion of reservoir species tends to reduce predicted O3 loss rates (much of mechanism I X species in inactive forms) • However, reactivation results in quick loss of ozone

16. Stratospheric ChemistryOzone Loss – Mechanism II • Mechanism I reactions expects greater loss at high altitudes, while observations showed loss at lower altitudes • Mechanism II losses X + O3 → XO + O2 X’ + O3 → X’O + O2 (note: X or X’ must be Cl) XO + X’O → XOOX’ → (or → →) X + X’ + O2 Net Reaction: 2O3 → 3O2 • Mechanism II does not involve O in reaction • This is favored at lower altitudes/ lower temperatures

17. Stratospheric ChemistryOzone Holes • In the early 1980s, the main O3 loss expected was through gas phase mechanism I – not focused on low altitude/high latitudes • Ozone loss was observed through ground based and satellite measurements • Ground Based measurement is Dobson Unit (equivalent thickness if a column is reduced to pure O3 at ground P, std T) • Ground based measurements showed large loss in Antarctic Spring, but not observed in initial satellite measurements (very low concentrations were removed as “not believeable”)

18. Stratospheric ChemistryOzone Holes • Dobson unit O3 measures a column content, so it gives an indication for UV blockage • More UV is blocked when Dobson Units are high – but also depends on latitude • Loss of ozone in Antarctic was not expected • initial investigation was over cause (dynamic vs. chemical) • special conditions occurs over Antarctica • polar vortex isolating stratospheric air from low latitude air • polar stratospheric clouds (PSCs) provide a surface for heterogeneous reactions

19. Stratospheric ChemistryOzone Holes • Since ozone loss occurred in spring, reactivation of reservoir species was thought to play a role • PSC particles were found to help with reactivation reactions: ClONO2 (g) + H2O(aq) → HOCl(aq) + HNO3(aq) and HCl(g) → H+ + Cl- and Cl- + HOCl(aq) → Cl2 (g) + OH

20. Stratospheric ChemistryOzone Holes • Upon the end of winter, sunlight converts unstable Cl species, HOCl and Cl2 into reactive Cl • Measured ClO concentrations were found to be very high • Also, NOx remains locked in PSC particles • Ozone holes are transient and end when the vortex ends, warming air and releasing HNO3 from PSC particles allowing ClONO2 to reform

21. Stratospheric ChemistryCauses and Effects • While there are natural sources of catalytic species, for each type (of X), anthropogenic sources are significant • Sources – NOx species • Anthropogenic N2O sources: • fertilizer use • nylon production • Air Transport (combustion produces NOx and planes can flow near or in the stratosphere) • However, NOx also forms ClONO2 (not all bad)

22. Stratospheric ChemistryCauses and Effects • Sources – HOx species • Anthropogenic CH4 sources (rice farming, livestock, natural gas production) • Besides affecting HOx catalytic reactions, H2O concentration and PSC occurrence is affected

23. Stratospheric ChemistryCauses and Effects • Sources – Halogen species • The greatest source of Cl is from CFCs • Effects from this are dropping due to Montreal Protocol • Br containing species are much greater at causing ozone loss because HBr and BrONO2 are poor reservoir species • Br comes from halons (similar to CFCs but Br containing and used for fire extinguishers) and CH3Br (fumigant)

24. Stratospheric ChemistryCauses and Effects • Effects – main worry is UV • Higher UV flux leads to: worse sunburns, greater incidence of skin cancer, and cataracts • Ozone hole occurrence is in a low human density region (plus bad weather can limit worse problems), but high UV can affect other life forms and be transported to midlatitudes