I saw a dragon flying in the air, I am at its approximate height, can I track in the air using the scent ability while flying?
A creature with the Track feat and the scent ability can follow tracks by smell, making a Wisdom check to find or follow a track. The typical DC for a fresh trail is 10. The DC increases or decreases depending on how strong the quarry’s odor is, the number of creatures, and the age of the trail. For each hour that the trail is cold, the DC increases by 2. The ability otherwise follows the rules for the Track feat. Creatures tracking by scent ignore the effects of surface conditions and poor visibility.
The only thing you might need is the feat Track, but only if the check's DC is greater than 10. The check shouldn't be greater than 10 because a creature bigger than Medium is way easier track. The feat Track lists following Survival check modifiers:
SURVIVAL CONDITION DC MODIFIER Every three creatures in the group being tracked -1 Size of creature or creatures being tracked Fine +8 Diminutive +4 Tiny +2 Small +1 Medium +0 Large -1 Huge -2 Gargantuan -4 Colossal -8 (For a group of mixed sizes, apply only the modifier for the largest size category.) Every 24 hours since the trail was made +1 Every hour of rain since the trail was made +1 Fresh snow cover since the trail was made +10 Poor visibility -2 Overcast or moonless night +6 Moonlight +3 Fog or precipitation +3 Tracked party hides trail (and moves at half speed) +5
However, thanks to scent, most environmental conditions are ignored, but some (fog, for example) could still make tracking the creature harder.