It is the ram air intake for the air conditioning system. There are two of them, one on each side.
Here is an image from the ATR-72 FCOM:
ATR-72 Air conditioning System, image from 737ng.co.uk
Details from the same document:
The air conditioning system is supplied by air processed through two packs which regulate air flow and temperature. The two packs are installed in the main landing gear fairings and operate automatically and independently.
The left pack supplies the cabin and cockpit air conditioned; the right pack supplies only the cabin air conditioned.
Also, as @Jonathan Walters points out, the intake is a good example of NACA inlet (or NACA scoop) and are found in a number of aircraft, for example Boeing 737.
A ram air inlet for reducing the heat of the Primary and Secondary Heat Exhangers caused by the bleed air coming from the PACK Valve (Primary XCGR) and from ACM Compressor part (Secondary XCGR). It is usually ventilated by 2 means:
- RAM AIR if the speed is > 150kts or
- Ground cooling turbofan (operated by bleed air coming from Pneumatic system) if the speed is < 150 kts.
This ram air is exhausted outboard via a ram air outlet near the Main Landing Gears
- Note: The operation of the Ground cooling turbofan is electrically controlled by MFC and pneumatically operated by bleed air from pneumatic source (ENGINE).
Hope this helps..
ATR 72-500 Maintenance Instructor