With the increasing number of automotive vehicles, the country is confronted with an increasingly urgent task to ramp up governance on mobile engine emissions, especially as diesel-powered vehicles have become an even larger contributor to air pollutants, according to a report from the Ministry of Ecology and Environment.

With a year-on-year increase of 6.4 percent, the number of automotive vehicles across the country reached 348 million in 2019, including 3.8 million new energy vehicles, the report published on Monday said.

"China has topped the world in production and sales of automotive vehicles for 11 straight years," it noted.

Per the report, diesel-powered vehicles continued to dominate the emissions of some of the major air pollutants from automobiles in 2019, as they did in previous years. While their contributions to particulate matter remained unchanged, their contributions to oxynitride soared from about 71 percent in 2018 to 88.9 percent.

Sunlight and high temperatures could trigger chemical reactions of oxynitride and form ozone, a health-endangering gas that was the only major air pollutant to see its density increase in the country in 2019 as overall air quality continues to improve.

While the ozone layer helps shield the Earth from harmful ultraviolet radiation, it could damage the lungs and result in shortness of breath even if people breathe in a relatively low amount of it.

The 337 major Chinese cities reported a year-on-year increase of 6.5 percent in average ozone density in 2019.

After years of efforts in air pollution control, "there has been no marked decrease in oxynitride emissions from mobile engines and industrial furnaces," Liu Bingjiang, head of air quality management at the ministry, said in a recent news conference.

Per the report, road transportation dominated by diesel-powered vehicles transported 73 percent of the country's freight and 73.9 percent of passengers in 2019. The country, however, is confronted with challenges in clean diesel management as unveiled in a 2019 campaign in Beijing and another 30 nearby cities.

A total of 561 illegal gas stations were discovered by inspectors dispatched by the central government－from the ministries of ecology and environment, commerce, public security and the State Administration for Market Regulation－after 905 were shut down by local authorities in an earlier stage of the campaign, according to the report.

It said 339 of the illegal gas stations discovered by the central government inspection teams had no license or approval at all. The others either failed to provide some of the necessary licenses and approvals or the ones they had were out of date.

The report noted the challenges authorities face in cracking down on illegal gas stations as many of them were "hidden and mobile".

"Some of the stations hid in residential buildings, parking lots and freight yards," the report said. "Some are hidden underground. Some are disguised as rescue vehicles, water carts and vans for moving services."

Meanwhile, 4.5 percent of diesel samples collected from 11,769 gas stations in the 31 cities proved to be substandard.

On average, the sulfur content in the substandard diesel was 25 times higher than the national standard.

The most extreme example was found in a sample from a gas station in Cangzhou, Hebei province, which had a sulfur content 902 times higher than the national standard.

Liu Youbin, spokesman of the Ministry of Ecology and Environment, said the construction of special railway lines for major bulk cargo transportation enterprises will be one of the key air pollution control projects in the Beijing-Tianjin-Hebei cluster as the region endeavors to further improve its air quality this year.