He Hong, a leading scientist on the team, said nitrogen oxides are a major source of PM2.5 and ozone, which indicates that motor vehicle emissions play a key role in the formation of smog and haze.

Diesel vehicles, which account for fewer than 18 percent of all those on China's roads, emit almost 70 percent of all nitrogen oxides and 99 percent of particulate matter, according to the 2012 Annual Report on Motor Vehicle Pollution Control, released by the Environmental Protection Ministry.

Measures have been taken to ease the situation. Phase 4 of the emission standards for heavy-duty diesel vehicles came into effect on July 1. In line with the new standard, nitrogen oxide emissions will be reduced by 30 percent, and emissions of particulate matter by 80 percent.

Phase 4 of the emission standards for light-duty diesel vehicles was to have been implemented at the same time, but has been postponed. "There is no new implementation date for some regions," said Tang Dagang, director general of the Vehicle Emission Control Center at the Environmental Protection Ministry. "The reason is that supplies of diesel suitable for new vehicles are insufficient."

This is not the first time the new standard for light-duty vehicles has been postponed. Phase 4 of the emission standards for heavy-duty diesel vehicles was originally scheduled to come into force on Jan 1, 2011, but implementation was postponed for a year, before being deferred again until July 1 this year.

Meanwhile, the new standard for light-duty diesel vehicles was set to come into force on July 1, 2011, having already been postponed for two years.

"The key difference between heavy- and light-duty diesel vehicles under phase 4 of the emissions standards is that heavy-duty vehicles don't have catalytic converters," said Yue Xin, an associate researcher at the Chinese Research Academy of Environmental Sciences.

He said the catalytic converters for light-duty diesel vehicles are more sensitive to the concentration of sulfur in the fuel and the sulfur content of 350 mcg per gram under the current standard is too high for this type of vehicle.

"If the vehicles are loaded with this diesel, drivers may not feel anything unusual when driving, but the catalytic converters will become ineffective, leading to more pollutants in the exhaust fumes, causing the vehicles to fail their annual check," said Yue.

The sulfur content allowed in diesel fuel was reduced to 350 mcg per gram nationwide on July 1. Before that, the figure was 2,000 mcg per gram.

"About 95 percent of all diesel vehicles on the roads have long been loaded with 2,000 microgram-per-gram diesel, which is only supposed to be used for vehicles such as tractors and bulldozers, but drivers can easily find it at gas stations," said Yue.

He said diesel with a sulfur content below 50 mcg per gram, suitable for phase 4 of the emissions standards for light-duty diesel vehicles, will not be available nationwide until Jan 1, 2015.