According to statistics from the Traffic Management Bureau of the Ministry of Public Security, as of the end of 2011, the total number of motor vehicles in China was 225 million. If the length of a car is calculated as 5 meters, these vehicles can circle the Earth's equator 28 times with their heads and tails connected. It's not an exaggeration to say it's the ocean of cars. A car is a PM2.5 generator.

What is the amount of emissions from so many motor vehicles? The 2011 China Motor Vehicle Pollution Prevention and Control Annual Report showed that in 2010, motor vehicles in China emitted 52.268 million tons of pollutants. If a train had 30 carriages, it could fill 29000 trains. 

What is the relationship between these emissions and PM2.5? Taking the PM10 share rate of motor vehicle pollutants in Beijing as an example: gasoline sedans at 5.53%, light gasoline vehicles at 4.67%, medium gasoline vehicles at 4.64%, heavy gasoline vehicles at 11.72%, heavy diesel vehicles at 72.14%, and motorcycles at 1.30%. It can be seen that diesel vehicles account for the vast majority of PM10 pollutants emitted by motor vehicles. Compared with cities with developed transportation, this value is relatively low. Generally speaking, diesel vehicles account for over 80% of PM10 emissions from motor vehicles, while PM2.5 accounts for over 90% of PM10 emissions. That is to say, the emissions from diesel vehicles contribute the most to the total PM2.5 emissions from motor vehicles. 

According to customs data from the National Bureau of Statistics, the apparent consumption of diesel in China was 167.17 million tons and the consumption of gasoline was 76.8 million tons in 2011. The consumption of diesel is more than twice that of gasoline, and the particulate matter emitted by diesel engines is more than 50 times that of gasoline engines. Overall, diesel vehicles contribute over 100 times more PM to air pollution than gasoline vehicles. 

The main reason for the generation of particulate matter in diesel engine emissions is due to uneven fuel combustion, incomplete combustion of some fuels, and oxidation and cracking under high temperature and oxygen deficient conditions. 

The composition of diesel engine particulate matter includes two major parts. One of them is mainly insoluble particles composed of carbon particulate matter (dry carbon smoke DS), sulfates, and ash particles; The other component mainly comes from soluble particles composed of incompletely burned fuel, engine oil and its intermediate products or suspended particles (aerosols).