Cars and coal can turn trees into ‘polluters’

Sally Ng and colleagues investigated aerosol formation and chemical composition under nighttime conditions in a well-controlled laboratory setting. (Credit: Georgia Tech)

Scientists have known for years that human-made pollutants can interact with organic compounds emitted by vegetation, turning them into airborne particles.

Those particles may affect air quality, human health, and climate. However, to what extent and how exactly human-made pollutants affect aerosol formation from vegetation in the ambient environments are poorly understood.

Anthropogenic sulfate, produced mainly by coal-fired power plants, and nitrogen oxides, produced mainly by vehicle emissions, control 43 to 70 percent of the total measured organic aerosol load in the southeastern United States during summer months, according to a new study.

“This finding is good news for pollution control,” says Nga Lee (Sally) Ng, coauthor of the study and assistant professor in Georgia Tech’s School of Chemical and Biomolecular Engineering and School of Earth and Atmospheric Sciences.

“If we are able to further reduce sulfur dioxide and nitrogen oxide emissions, we will not only decrease sulfate aerosols but also organic aerosols, thus lowering the total aerosol burden in the southeast United States.”

Pollution never sleeps

According to the study, the formation of aerosols from certain tree emissions is directly controlled by the abundance of sulfate, instead of particle water or particle acidity as suggested by prior studies.

This is surprising, but it appears that in the southeastern US the particles have sufficient water and acidity to preclude them from being the controlling factors in aerosol formation. The study further shows that the nighttime aerosol chemistry is more important than previously thought.

“Aerosol chemistry does not stop at night,” Ng says. After sunset, the nitrogen oxide compounds and ozone can react with the emissions from trees to produce organic aerosols.

The findings may help improve air quality and climate simulations in regions where anthropogenic-biogenic interactions contribute substantially to aerosol formation, Ng says.

In the field and the lab

To unravel the complexity of what drives particle formation, researchers collected data in ambient field studies using advanced aerosol mass spectrometry and other techniques in order to quantify the aerosol chemical composition and its ability to take up liquid water.


The collected data reveal the contribution of organic species and sulfate to the acidity and water uptake. Additional analysis showed that sulfate alone, not its associated water uptake or acidity, was responsible for the organic aerosol levels measured.

“We have seen evidence for this possible effect in prior studies a number of years ago. Results from this study provide a coherent understanding of this interaction and allow us to put together a comprehensive picture,” adds coauthor Professor Rodney Weber.

This study also used the Georgia Tech Environmental Chamber Facility to investigate aerosol formation and chemical composition under nighttime conditions in a well-controlled laboratory setting.

The study is an outcome of one of the largest US atmospheric chemistry field projects in decades—the Southeast Atmosphere Study (SAS), which took place in Alabama’s Talladega National Forest in 2013.

The research teams used instrumentation onboard aircraft and ground sites to learn more about the region’s atmospheric chemistry. They also collected data from four sites around the Atlanta area over two years, as part of the US Environmental Protection Agency (EPA) Clean Air Research Center at Georgia.

The NSF, EPA, and NOAA supported the research. Any conclusions or opinions are those of the authors and do not necessarily represent the official views of the sponsoring agencies. The findings appear in the Proceedings of the National Academy of Sciences.

Source: Georgia Tech