Murky haze continued to shroud large swathes of China on Tuesday, prompting choking residents to point the finger at the country's top oil firms for allegedly producing substandard gasoline and diesel, an accusation denied by the oil giants. Analysts have proposed an energy structure adjustment as the ultimate solution.

A total of 1.3 million square kilometers of the country was enveloped by dense haze, covering most parts of northern and eastern China, according to the Ministry of Environmental Protection, which graded the air quality of Beijing, Tianjin, Shijiazhuang in Hebei and Jinan in Shandong as level 6, indicating "serious pollution."

The National Meteorological Center (NMC) on Tuesday issued a yellow warning for the haze, implying moderately smoggy weather.

The Beijing Environmental Protection Bureau on Tuesday noted that unfavorable meteorological conditions, namely a lack of wind, were a direct cause of the heavy haze, which has hit north China for at least the fourth time this winter.

Agitated by the deteriorating smog, many picked out the country's two major oil giants - State-owned China National Petroleum Corporation and China Petrochemical Corporation (Sinopec) as the main "culprits" for the pollution after reading reports analyzing how the oil Chinese automobiles consume has badly polluted the air.

According to a diagram on sohu.com, the standard of China's petroleum is greatly inferior to that of the US and Europe. It pointed out that China is the world's largest buyer of "bad-quality" crude and the gasoline has a high content of sulfur due to insufficient investment in refining technology. 

The report soon triggered a massive online uproar after being forwarded by some Weibo celebrities. Some even blamed the authorities and media for intentionally dodging this serious problem while analyzing the causes behind the smog.

"The smog was caused by diverse reasons. Oil is just one of the factors contributing to the issue," Lü Dapeng, a spokesperson for Sinopec, told the Global Times Tuesday, adding that starting from May 2012, the firm has been providing the capital with oil products equivalent to Euro V standards.

"It's the strictest standard in the world, the sulfur content of which is less than 10 ppm," said Lü. "We are also concerned about the air quality and have been striving to make improvements."

Han Xiaoping, an energy industry analyst, told the Global Times that the burning of coal in winter, and not vehicle emissions, was the main contributor to air pollution.

"Last year, the consumption of coal increased by over 20 million tons in Beijing compared with that in 2011, as compared to the 250 million tons nationwide," said Han, noting that by contrast, the consumption of oil grew by 30 million tons across the nation. He suggested that the restructuring of energy consumption would be an effective way of solving the problem, such as using natural gas and nuclear power as alternatives for coal.

The public also blamed the oil manufacturers' tardiness in upgrading product quality.

"We have invested more than 200 billion yuan ($32 billion) in upgrading related facilities during the past years, and the efforts are proceeding in different cities," said Lü. He said since the upgrading of oil quality calls for large amounts of investment, he hoped the government would unveil some policy stimulus measures, such as preferential tax policies.

The public discontent over the pollution has also alarmed the government.

During recent meetings to solicit opinions for his last government work report, Premier Wen Jiabao said that the recent dense haze had affected people's lives and health, and vowed to raise hopes among the public by accelerating the adjustment of the industrial structure and promoting energy conservation and emission reduction.