(37)In view of the nature and extent of the modifications which should be made to Directive 2001/81/EC, that Directive should be replaced to enhance legal certainty, clarity, transparency and legislative simplification. In order to ensure continuity in improving air quality, Member States should comply with the national emission ceilings set out in Directive 2001/81/EC until the new national emission reduction commitments laid down in this Directive become applicable in 2020.