Stratospheric Chlorine Reservoir Identification

In the stratosphere, catalytically inactive molecules containing chlorine are known as reservoir species. These molecules temporarily store reactive chlorine, preventing it from participating immediately in ozone depletion cycles. They can later release active chlorine species under certain conditions.

Analysis of Chlorine Species

Let's examine the given options:

• HOCl (Hypochlorous acid): This molecule reacts with hydroxyl radicals (OH), participating in the catalytic destruction of ozone. It is not considered a primary reservoir species.
• ClO (Chlorine monoxide): This is a highly reactive intermediate species that plays a direct role in the catalytic cycle of ozone destruction. It is the active form of chlorine.
• ClONO$_2$ (Chlorine nitrate): This molecule is formed from the reaction of ClO with nitrogen dioxide (NO$_2$). It is relatively stable and acts as a major sink for reactive chlorine, making it a key reservoir species.
• Cl$_2$ (Chlorine molecule): While it contains chlorine, gaseous chlorine (Cl$_2$) photolyzes relatively easily in the stratosphere, releasing chlorine atoms. It is less significant as a long-term reservoir compared to ClONO$_2$.

Conclusion

Based on the analysis, ClONO$_2$ is the catalytically inactive (reservoir) molecule containing chlorine in the stratosphere.