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Trees and Air Pollution

Forests amit volatile organic compoundsEPA researchers have discovered that controlling man-made sources of air pollution will have the added benefit of also reducing air pollution formed from compounds released from trees and plants.

Trees and plants release more than just oxygen into the atmosphere as a result of photosynthesis: They also release a variety of gases that contribute to air pollution. In fact, the planet's vegetation accounts for about two-thirds of the pollutants known as volatile organic compounds (VOCs) emitted globally.

In the study, published in the May issue of Environmental Science &Technology, EPA researchers quantified for the first time how emissions from vehicles, industry and power plants interact with natural emissions from vegetation to change the composition and make-up of chemicals in the air.for the worse.

The implications of the study are considerable. "If we can control the man-made sources of emissions, we can indirectly affect the formation of these naturally derived atmospheric pollutant particles," says Dan Costa, National Program Director for Clean Air Research at EPA.

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FIGURE 1. The total amount of secondary organic aerosols (SOAs) formed in the atmosphere solely from the emissions of trees and other foliage, wildfires or prescribed burns during a period of 18 days between Aug. 18 - Sept. 4, 2003. The blue color indicates the lowest emissions and red the highest.

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FIGURE 2. The total amount of SOAs formed when trees and foliage interact with pollutants emitted from cars and industry. This figure reflects the same time period as Figure 1.

Using computerized air quality modeling, investigators conducted simulations of natural and human-related pollution in the United States. When scientists took man-made pollutants out of the simulation, there was a 50 percent drop in pollutants from trees and plants in the Eastern United States.  These pollutants, known as secondary organic aerosols (SOAs) are produced by sunlight when VOCs from trees, plants, cars or industrial emissions interact with other airborne chemicals. SOAs are important for the formation of two regulated air pollutants, particulate matter and nitrogen oxide, a greenhouse gas.

"This study suggests that roughly half of the "natural" SOA in the eastern U.S. forms only when there is enough man-made pollution around to form it," says EPA scientist and lead author Annmarie Carlton.

"This model can guide us in developing strategies that can control our atmospheric chemistry," says Costa.  Over the last 40 years, similar EPA research efforts to develop a better understanding of clean air science have led directly to policies widely credited with better air quality, which in turn reduces hospitalizations, worsening levels of asthma, cardiac events and even deaths.

The research can also guide EPA to develop more refined and focused regulations and strategies for decreasing pollution effectively and efficiently. "This is just a first step in gaining an understanding of the complexity of the atmosphere such that we might intervene," says Costa.

The research may also have implications for future air pollution management strategies as well. Scientists predict climate change will stimulate the growth of trees and plants and extend growing seasons, resulting in even more emissions from natural sources. By controlling man-made emissions, the impact of emissions from trees and plants may be reduced.

Learn more:


Carlton, A.G., R.W. Pinder, P.V. Bhave and G.A. Pouliot. "To What Extent Can Biogenic SOA be Controlled?", Environmental Science & Technology 44:3376-3380 (2010).

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