What happens when nitrogen oxide reacts with rainwater?

The Nitrogen dioxide dissolves in atmospheric moisture in the presence of oxygen of the air and forms nitric acid which is washed down by the rain and combines with the salt present on the surface of the earth. This Rain is known as Acid Rain.

The ecological effects of acid rain are seen in aquatic environments, such as streams, lakes, and marshes where it can be harmful to fish and other wildlife. As it flows through the soil, acidic rainwater can leach aluminium from soil clay particles and then flow into streams and lakes

NO_X + H₂O --> HNO₃ (Nitric Acid).