This study aims to:

• Investigate whether the air quality in European cities can be improved by fitting retrofit systems to older vehicles, passenger cars and light commercial vehicles.

• Find technical solutions for the retrofitting of such vehicles with additional hardware and/or software.

• Test available retrofit exhaust after-treatment systems, either already on the market, under development or prototypes.

• Verify the possibility of vehicle emissions improvement with retrofits.

• Propose ways forward to establish the legal boundaries for suppliers of the retrofit systems and the vehicle owners.

• Investigate the possibility of an EU retrofit program for passenger cars and light commercial vehicles.