Match the reaction in Column I with its role in Column II. Column I: (P) $\mathrm{O}{}_2\stackrel{\mathrm{h}\nu }{\to }2\mathrm{O}$ followed by $\mathrm{O}+\mathrm{O}{}_2\to \mathrm{O}{}_3$; (Q) $\mathrm{ClONO}{}_2+\mathrm{HCl}\to \mathrm{Cl}{}_2+\mathrm{HNO}{}_3$ on cloud particles; (R) $\mathrm{ClO}+\mathrm{NO}{}_2\to \mathrm{ClONO}{}_2$; (S) $\mathrm{Cl}+\mathrm{O}{}_3\to \mathrm{ClO}+\mathrm{O}{}_2$. Column II: (1) formation of a chlorine reservoir species, (2) formation of stratospheric ozone, (3) conversion of reservoir into active chlorine, (4) chain propagation that consumes ozone.

1. AP-2, Q-3, R-1, S-4
2. BP-2, Q-1, R-3, S-4
3. CP-4, Q-3, R-1, S-2
4. DP-2, Q-3, R-4, S-1