Arrhenius Acid:
Arrhenius Acid is any specie which when dissolved in water produces H⁺ (proton) Ions.
Nitrous Acid:
Nitrous Acid also acts as Arrhenius Acid when dissolved in water.
HNO₂ + H₂O → NO₂⁻ + H₃O⁺
Where,
H₃O⁺ ⇆ H⁺ + H₂O
As HNO₂ produces H⁺ Ions in water, so it is an Arrhenius Acid.