Vehicle exhaust emissions have become the main contributor to worsening air pollution in big Chinese cities as the country undergoes a surge in car sales, the national environmental watchdog said on Thursday.

The Ministry of Environmental Protection released its 2010 report on pollution caused by China's vehicle exhaust emissions - the first of its kind - on its official website on Thursday, in which it blamed the car sales bonanza for the devastating air quality in most cities.

Since 2009, China has been the largest auto market in the world.

Its annual output and sales reached 13.79 million and 13.64 million in 2009, with respective year-on-year rises of 48 percent and 46 percent, the ministry said.

The number of vehicles in China rose 9.3 percent to 170 million in 2009, which was 25 times that in 1980, the ministry said.

The increasing number of vehicles is having a severe impact on the environment and threatening the urban air quality, according to the ministry's report.

Out of 113 big and medium-sized cities across the country, more than 30 percent failed to meet the national air standard, the report said.

"The air pollution in a number of cities, especially big and medium-sized cities, has featured vehicle exhaust emissions and some coal, making it more difficult to curb air pollution," said an unidentified official of the ministry who was quoted in the report.

The report said air pollution problems, ranging from acid rain, haze and photochemical smog, became more frequent in some regions.

"Some regions even suffered hazy weather for more than 200 days in 2009," it said.

"All the problems are closely related to vehicle exhaust emissions, including nitrogen oxide and other small particles."

The total volume of vehicle exhaust emissions reached more than 51 million tons in 2009, including more than 40 million tons of carbon monoxide, nearly 5 million tons of hydrocarbons, about 6 million tons of nitrogen oxide and 590,000 tons of fine particles, according to the ministry's statistics.

Although vehicle ownership has increased sharply in China since 2000, the report said the rise in the volume of vehicle exhaust emissions is being stemmed by the introduction of policies to control vehicle gas emissions.

The governments in Beijing, Tianjin and Hebei have completed the renovation of 1,976 petrol stations, 48 oil storage tank facilities and 1,265 oil tankers to help recycle gas emissions, the report said.

Some cities in the Pearl River Delta and Yangtze River Delta have also launched programs to address vehicle exhaust emissions, it said.

The Ministry of Environmental Protection will further strengthen the supervision and control of vehicle exhaust emissions to alleviate their impact on the environment, the report said.

A proposal of the 12th Five-Year Plan (2011-2015), which was adopted in October, said the automobile industry in China will focus on the research and development of new energy vehicles, such as battery-powered electric cars and plug-in hybrid vehicles.