Harvard scientists say the Chinese government might need to look at a new suspect in the deadly air pollution that clouds Beijing.

The government has spent billions of dollars to clean up the country’s notorious air pollution, focusing on reducing emissions of sulfur dioxide from coal-burning power plants, researchers said. Estimates say more than a million people a year die in the country from particulate air pollution.

But extreme pollution events in winter in the Beijing area have been a regular occurrence, researchers said. In a study published in the journal Geophysical Research Letters, they assert that a key to reducing the events may be cutting emissions of a previously overlooked chemical: formaldehyde.

“Our research points towards ways that can more quickly clean up air pollution. It could help save millions of lives,” Jonathan M. Moch, a graduate student at the Harvard John A. Paulson School of Engineering and Applied Sciences and first author of the paper, said in a statement.

The research was a collaboration between Harvard University, Tsinghua University, and the Harbin Institute of Technology.

The main sources of formaldehyde emissions in eastern China are vehicles and industrial facilities such as chemical and oil refineries, researchers said.

Researchers said instruments measuring the air in Beijing on days with high particulate air pollution picked up sulfate, which can be created by sulfur dioxide. Hence the Chinese government’s focus on cutting sulfur dioxide.

But the researchers said the instruments may have confused sulfate with hydroxymethane sulfonate, which is formed by the reaction of sulfur dioxide with formaldehyde.

The confusion may explain why between 2013 and January 2017 the government had major success in reducing sulfur dioxide, but the number of extremely polluted days in Beijing did not decline, researchers said. The reason was a mystery.

The mystery deepened when last winter there was a reduction. The researchers believe they have the answer: The level of sulfur dioxide may have finally been driven down to the point where there was not enough to react with the available formaldehyde to form hydroxymethane sulfonate.

Moch said that, while sulfur dioxide reduction efforts should not be stopped, Chinese authorities “went a very long way to get to this result. We think there could have been a shorter way if they had gone straight at formaldehyde.”

“Looking at these things synergistically is important to more quickly reduce air pollution,” he said in a telephone interview. “We should look at the two together.”