Disproportionation redox reaction involves simultaneous oxidation and reduction of atoms of same element from one oxidation state to two different oxidation states.
+4
2NO2(g)
+2OH−⟶
+3
NO2−
+
+5
NO3−
+H2O
In this reaction, N of nitrogen dioxide is of +4 oxidation state but after the reaction, N is present in +3 and +5 oxidation state simultaneously. So, this reaction is an example of disproportionation redox reaction.