BEIJING — Scientists have found a new culprit contributing to China’s notorious wintertime smog, and controlling it could help sustain the significant improvements in air quality that Beijing and other northeastern cities experienced last winter, according to research published on Thursday.

Scientists from Harvard and two Chinese universities reported that emissions of formaldehyde — principally from vehicles and chemical and oil refineries — played a larger role than previously understood in producing the thick, toxic pollution that chokes much of the country each winter.

In a report published in the journal Geophysical Research Letters, the scientists argued that “a large portion” of the sulfur in the haze was the result of a chemical reaction between formaldehyde and sulfur dioxide released by burning coal.

China’s efforts to reduce the haze have focused lately on reducing sulfur dioxide emissions. While those efforts have shown some success, with drastic declines in pollutants recorded last winter, the research suggests that China could improve air quality still more by directing efforts — and resources — toward reducing emissions of formaldehyde in vehicles and industrial refining.