An arrhenius acid increases the concentration of H3O+ (or H+ they're the same for the sake of acids and bases) in an aqueous solution.
HNO3(aq) (known as nitric acid) is made up of two principle particles. A nitrate ion NO3-, and a hydrogen ion H+, which is what determines that it will be an acid, due to this hydrogen ion. The equation will be:
HNO3(aq) + H2O(l) --> H3O+(aq) + NO3-(aq)
Because HNO3(aq) increases H3O+ concentration in the solution by donating its H+ ion to the water, it acts as an arrhenius acid.
