China has made tremendous progress in emission control for on-road vehicles (light duty vehicles, buses, and heavy-duty trucks) in recent decades. The vehicle population in China grew nearly sevenfold, from 16 million to 108 million, between 2000 and 2012, and this rapid pace will continue for the foreseeable future.

In many regions of the world, older, high-emitting vehicles account for a small percentage of the overall vehicle fleet but a disproportionately large share of total emissions. It is estimated that these vehicles may be responsible for more than 50% of particulate matter (PM) and black carbon (BC) emissions by 2020 (Yan et al., 2011).