Concentration of hypochlorite ion in bleach

[Daaft]

I have to do an investigation about why hypochlorite ion concentration decreases over times of air exposure. I've done basically everything I need, but I can't find the answer as to WHY it decreases. I think it may have to do with an equilibrium between chlorine gas and OCl- but I don't know the exact equation... Can anyone help me out? ANY information is appreciated.

EDIT: I have a feeling this is it:

O2 + Cl2 <---> 2OCl

How does this look?