Air roots? Soil roots? One and the same. The roots of orchids that are under the media have their velamen opened up so as to allow moisture to easily penetrate the velamen. It is more humid within the pot under the media and there is less moisture loss and so the velamen is more open. The roots above the media has a less open structure so that there is less moisture loss. As it ages it becomes more open. They aren't different type roots just more or less open.