What gasses discolor brass in air ?
The presence of the hydrogen sulphide (H_2S) gas in air can produce a discoloration of brass since this gas interacts with the copper alloys and brass is a zinc-copper alloy.
Since H_2S is corrosive to some metals such that brass, bronze, cast iron, steel types, lead, under moist or wet conditions, the contact with these materials should be avoided. It is recommended to consider the concentrations of H_2S and exposure conditions of materials that are in contact to hydrogen sulphide.
For instance, dry H_2S is not corrosive to nickel and nickel-based alloys, at room temperature, while dry H_2S is not corrosive to high nickel cast iron, at high temperatures.