Yeah, atmosphere only HDRI's are usually just cropped, so the ground isn't there. So the south pole of the HDRI is actually the horizon of the photo they took.
To do this in TG you could do as Kevin said with a crop.
You could also put your atmosphere height limits higher, and put your camera higher, so the "haze ground" is lower in the 360 image.
Update: With that crop image, you can still make it a equirectangular image if you make sure the crop is a 2:1 ratio, and than in Photoshop you can make the bottom equirectangular by using the polar coordinates (rectangular to polar), and then using the clone tool or something to fix the hard seam. If there is no hard seam, the image needs to be roated vertically. Once you have the seam fixed you do Polar Coordinates again, and this time use Polar to Rectangular. The image will now have the bottom repeatable.