Air quality in northern China has improved as a result of government policies leading up to the 2008 Beijing Olympics, researchers from Harvard and Tsinghua University say.

Over the past six years, the ratio of carbon dioxide to carbon monoxide—known as “combustion efficiency”—in the air flows from Beijing has nearly doubled, meaning that China’s polluters have become more efficient, according to a study published last month in Atmospheric Chemistry and Physics.

Carbon dioxide is generally a product of efficient combustion, while carbon monoxide tends to come from inefficient combustion.

The study’s findings signal that China, the world’s largest carbon dioxide emitter, has replaced older inefficient carbon dioxide sources with cleaner technology, according to J. William Munger, a senior research fellow at the Harvard School of Engineering and Applied Sciences and co-author of the paper.

The station observed a notable decrease in carbon dioxide emissions and an increase in combustion efficiency as a result of government efforts to improve air quality in Beijing for athletes and tourists during the games, Munger said.

“We can see observationally what the impact of the air quality controls have been, and we can see that in fact there has been a reduction of carbon monoxide levels in the Beijing air,” he said.

The study provides a new kind of quantitative evidence of energy trends in China that is independent of government or industry statistics, said Chris P. Nielsen, a co-author of the paper.

The researchers collected data at a research station in Miyun, a rural area north of Beijing. By analyzing weather and wind patterns, they identified and measured pollutant levels carried in the air from Beijing in the south and differentiated it from cleaner air flowing from the north, Munger said.

Seasonal trends also affected their data collection as plants tend to absorb increased amounts of carbon dioxide during the spring and summer months, which the researchers said they accounted for in their analysis.

Since 2004, the research station has measured ozone, carbon dioxide, and carbon monoxide levels. Recently, researchers have added new instruments that measure other pollutants like nitrogen oxides, sulfur dioxide, mercury, and black carbon, according to Munger.

“As you add more and more capability, you can do more and more interesting things comparing the data and looking at interactions between [different gas levels],” said Nielsen.

The research was conducted under the auspices of the Harvard China Project, which studies air pollution and greenhouse gases in China and their economic and health impacts.

This article has been revised to reflect the following correction:

CORRECTION: October 19, 2010

An earlier version of the Oct. 15 news article "Beijing Improves Air Quality, Harvard Study Finds" incorrectly stated that the study's data represents the first atmospheric measurements of air quality in China. In fact, the data is the first independent evidence that examines combustion efficiency, not specifically air quality.