18 October 2018

A clearer path to clean air in China

Posted by llester

Formaldehyde – not sulfur dioxide — may be the key to China’s stubborn problem of wintertime air pollution

By Leah Burrows

For more than 15 years, the Chinese government has invested billions of dollars to clean up its deadly air pollution, focusing intensely on reducing emissions of sulfur dioxide from coal-burning power plants.

These efforts have succeeded in reducing sulfur dioxide emissions, but extreme pollution events are still a regular wintertime occurrence and experts estimate that more than 1 million people die per year in China from particulate air pollution.

New research published in Geophysical Research Letters shows that a key to reducing extreme wintertime air pollution may be reducing formaldehyde emissions rather than sulfur dioxide.

Beijing with haze and without haze Credit: Jonathan M. Moch/Harvard SEAS

Beijing with haze and without haze.
Credit: Jonathan M. Moch/Harvard SEAS

“We show that policies aimed at reducing formaldehyde emissions may be much more effective at reducing extreme wintertime haze than policies aimed at reducing only sulfur dioxide,” said Jonathan M. Moch, a graduate student at the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS) and first author of the paper. “Our research points towards ways that can more quickly clean up air pollution. It could help save millions of lives and guide billions of dollars of investment in air pollution reductions.”

Measurements in Beijing from days with especially high particulate air pollution, known as PM2.5, have shown a large enhancement in sulfur compounds, which have been typically interpreted as sulfate. Based on these measurements, the Chinese government has focused on reducing sulfur dioxide, the source of sulfate, to reduce air pollution.  As a result of these efforts, sulfur dioxide over eastern China has decreased significantly since 2005. The problem is, particulate air pollution hasn’t followed the same path.

Moch collaborated with SEAS graduate student Eleni Dovrou and professor Frank Keutsch. They found that the instruments used to analyze haze particles can easily misinterpret sulfur compounds as sulfate when they are, in fact, a molecule called hydroxymethane sulfonate (HMS). Hydroxymethane sulfonate is formed by the reaction of sulfur dioxide with formaldehyde in clouds or fog droplets.

Using a computer simulation, the researchers demonstrated that hydroxymethane sulfonate molecules may constitute a large portion of the sulfur compounds observed in PM2.5 in winter haze, which would help explain the persistence of extreme air pollution events despite the reduction of sulfur dioxide

“By including this overlooked chemistry in air quality models, we can explain why the number of wintertime extremely polluted days in Beijing did not improve between 2013 and January 2017 despite major success in reducing sulfur dioxide,” Moch said. “The sulfur-formaldehyde mechanism can also explain why polices seemed to suddenly reduce extreme pollution last winter. During that winter, significant restrictions on sulfur dioxide emissions brought concentrations below levels of formaldehyde for the first time and made sulfur dioxide the limiting factor for HMS production.”

The primary sources of formaldehyde emissions in eastern China are vehicles and major industrial facilities such as chemical and oil refineries. The researchers recommend that policymakers focus efforts on reducing emissions from these sources to reduce extreme haze in the Beijing area.

Next, the team aims to directly measure and quantify hydroxymethane sulfonate in Beijing haze using modified observation systems. The team will also implement the sulfur-formaldehyde chemistry within an atmospheric chemistry model to quantify the potential importance of the sulfur-formaldehyde chemistry that creates hydroxymethane sulfonate across all of China.

“Our work suggests a key role for this overlooked chemical pathway during episodes of extreme pollution in Beijing,” said Loretta J. Mickley, senior research fellow in SEAS.

—Leah Burrows is a communications officer at Harvard’s John A. Paulson School of Engineering and Science. This post originally appeared as a press release on the Harvard website.