If it's 2.5 in a billion, it;s gonna be less in a million - so you need to move the point to the left. How far you move it will depend on whether your billion is an English one (1 million million) or (most likely) an American one (1000 million).
well assuming we are using 10E9 as a billion then it's 1000 times more than a million so for parts per million divide by 1000, ie move the DP 3 places to the left so 2.5 parts per billion = 0.0025 parts per million. The Geezer is wise today