Start by checking the pollen filter then if that's ok and nothing obvious to your eyes else where as mentioned above then have the vehicle scanned with VCDS to check and test the 1K0907659 Quality Sensor.
AQS detects polluted air and automatically puts the climatronic on to recycle mode to stop the polluted air entering the cabin and maintaining passenger comfort. Once the air cleans up it reverts back to external air which cause that fault.
Finally, leaving the window open "slightly" for a few minutes greatly assist if you've got carpet mats which are damp!