BEIJING - After an unusually smoggy January, Beijingers were happy to see a blue sky on Friday, but it wasn't enough to distract them from the debate over what is causing the putrid air.

The latest round of smog started from Tuesday and covered 1.3 million square kilometers across the country's central and eastern regions.

Xue Manzi, a popular Chinese microblogger who lives in Beijing, said Wednesday that low-quality fuel threatens people's lives and called for immediate action to curb air pollution.

China has no country-wide standard on sulphur content in gasoline, and such standards currently vary by region: 10 parts per million (ppm) or below in Beijing; 50 ppm or below in developed provinces and municipalities like Shanghai and Jiangsu; and 150 ppm or below elsewhere.

Meanwhile, the standards in the United States and Europe are set at 30 ppm and 10 ppm, respectively -- well below China's average.

Chenwen.com, a social networking site for car owners and dealers, conducted sample surveys on the sulphur content of fuel from gas stations in east and suburban Beijing on January 24. According to the survey, nearly one quarter of the gasoline tested failed to meet the city's 10 ppm standard.

The results, which were unveiled on Thursday, prompted calls for action over vehicle emissions and fuel quality, which allegedly contribute to the lingering smog.

Fu Chengyu, chairman of Sinopec Group, said that enterprises involved in oil refinement are responsible for the heavy smog, but he also defended fuel quality and placed blame on the country's low gasoline standards.

On Sina Weibo, China's Twitter-like microblogging platform, netizens quickly jumped on Fu's remarks, largely with censure and sarcasm.

"Yangshiweiping," the microblog account for commentary from China Central Television, the country's state broadcaster, questioned whether Sinopec was excusing itself or apologizing for delays in improving its oil products and the company's weak sense of social responsibility.

Yue Xin, a member of the committee that sets fuel standards, wrote that over 70 percent of the committee's 37 members work for oil companies and its secretariat is located in the Sinopec headquarters.

"Suibianbushuohua" compared Sinopec to Sanlu for using the country's regulations to distract the public. Sanlu became the center of a notorious food safety incident in 2008, when its melamine-tainted protein powder caused the deaths of at least six infants and sickened 300,000 others.

Sinopec responded on Friday, saying its 12 affiliated enterprises will finish the construction of desulphurizers and put them into use by the end of 2013, as well as provide fuel with sulphur content of 50 ppm or below across the nation from 2014.

While some netizens expressed faith in Sinopec's ability to upgrade its equipment and improve its products, they were also worried that the company could pass the costs of these efforts onto consumers.

Others, however, gave Sinopec the benefit of the doubt, calling it a massive state-owned conglomeration under great pressure to coordinate its business around the country. They continued to hold climate change responsible for the lingering smog.