$SO _{2}$ in presence of moisture acts as a bleaching agent. This is due to the reducing nature of $SO _{2}$. But bleaching action is only temporary.
$SO _{2}+2 H _{2} O \rightarrow H _{2} SO _{4}+2[ H ]$
Similarily, $Cl _{2}$ in presence of moisture give out nascent, oxygen and cause bleaching.
$Cl _{2}+ H _{2} O \rightarrow 2 HCl +[ O ]$
here bleaching action is permanent.