1. IV. Ozone Hole Observations Heterogeneous chemistry Decadal ozone loss

2. Vertical [O 3 ] Profile at One Site

3. Column Densities of O 3 1 D.U. = 2.7 x 10 16 molecules cm -2

4. Color-Coded Map of Column O 3

5. Chlorine Chemistry Critical …

6. …But Meteorology Also Critical Polar Vortex isolates air mass (no NO x gets in from outside) Very cold (195 K, clouds form)

7. Sequence of Events Late Fall/Early Winter - not much light - radicals all in reservoirs Mid Winter - gets cold, clouds form - reactions on clouds sequester NO x not ClO x reservoirs: ClONO 2 (g) + HCl (surf) → HOCl (g, surf) + HONO 2 (surf) HOCl + HCl (surf) → Cl 2 (g) + HOH (surf)

8. Sequence of Events Early Spring – gas phase chemistry HOCl + h → OH + Cl Cl 2 + h → 2 Cl 2 {Cl + O 3 → ClO +O 2 } ClO + ClO → ClOOCl ClOOCl + h → 2 Cl + O 2 (net) 2 O 3 → 3O 2

9. Sequence of Events Late Spring – heat breaks up vortex HONO 2 evaporates NO x from outside move in ClO + NO 2 → ClONO 2

10. Key Points Rapid, localized, structured Meteorology and Chemistry Timeline (Winter) - Cold enough for new/big particles - NO x reservoirs bind to particles - ClO x reservoirs stay as gas Timeline (Spring) - light starts run-away chemistry - eventual warming normalizes chemistry