NOx emission from road transport
	Emission from vehicles was one of the major sources of NOx emission. It contributed to 22% of the total emission in 2007. The tightening of vehicle emission standards progressively from Euro I in 1995 to Euro IV in 2006 have apparent effects on NOx reduction. For instance, NOx emission from a Euro IV vehicle was about 60% lower than a Euro I vehicle. The various incentive schemes launched in 2007 to encourage owners to early phase-out their pre-Euro and Euro I commercial vehicles or to switch to new environmental friendly petrol private cars also helped reduce the emissions.