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A New Reason To Control Emissions

Study finds that reducing pollution might remove aerosols previously thought to be part of the natural background

by Michael Torrice
May 17, 2010 | A version of this story appeared in Volume 88, Issue 20

Mapping Aerosols
Credit: Environ. Sci. Technol.
The share of biogenic secondary organic aerosols formed from pollutants generated by humans varies across the U.S. (Blue is 0% and red is 100%.)
Credit: Environ. Sci. Technol.
The share of biogenic secondary organic aerosols formed from pollutants generated by humans varies across the U.S. (Blue is 0% and red is 100%.)

In the atmosphere, volatile hydrocarbons released by trees and other plants react to form hazardous particles called biogenic secondary organic aerosols. These aerosols can irritate people’s airways and scatter sunlight, altering a region’s climate.

Because the aerosol particles’ origins are natural, policymakers have considered them uncontrollable. But new simulations by scientists at the Environmental Protection Agency could change this long-held belief and increase pressure to tighten aerosol regulations. The simulations specifically suggest that reducing pollution generated by humans could decrease aerosols that come from plants, thus helping to manage these hazardous particles.

Atmospheric chemists have known for a decade that anthropogenic emissions provide ingredients to convert plants’ volatile organic compounds into aerosols. For example, nitrogen oxides emitted from the smokestacks of power plants can promote oxidation of tree hydrocarbons. Aerosols form when these oxidized compounds condense onto particulate matter, such as water droplets or carbon particles spewed from cars and trucks.

But no one had tried to quantify the influence of anthropogenic emissions on the formation of biogenic aerosols, says Annmarie G. Carlton, a research physical scientist at EPA’s National Exposure Research Laboratory, in Research Triangle Park, N.C. The study led by Carlton, however, does just that (Environ. Sci. Technol. 2010, 44, 3376).

To estimate the anthropogenic contribution to biogenic secondary organic aerosols, Carlton and her colleagues simulated three weeks of air quality over the continental U.S. They began by feeding EPA data on average daily emission levels of plant hydrocarbons, nitrogen oxides, and other anthropogenic and biogenic compounds into chemical models of aerosol formation. After modeling the actual air quality over the three-week period, the researchers simulated alternative scenarios with lower levels of emissions generated by humans.

Carlton and her team report that when they removed all anthropogenic emissions from their simulations, the average biogenic aerosol levels fell by 50% in the eastern half of the U.S., where levels of volatile plant compounds and pollution are greatest. “Half of the biogenic secondary organic aerosol that forms in the atmosphere only forms when there is enough anthropogenic pollution around,” Carlton says.

For the western half of the U.S., the drop in average biogenic aerosol levels was more variable. When the scientists removed anthropogenic emissions, aerosols in most of this region dropped by 20%, but the drop in Washington state reached 50%, Carlton says.

The simulations also pointed to so-called carbonaceous particulate matter, such as the carbon particles that cars and trucks emit, as the most influential pollutant type. This result surprised the researchers because they expected oxidant sources such as nitrogen oxides to be the more prominent pollutant. But the contributions of particulate matter make sense, Carl­ton says: “When we remove that particulate matter from the atmosphere, there is less stuff to condense onto.”

Atmospheric chemist Allen Goldstein of the University of California, Berkeley, says the results show that the term “biogenic secondary organic aerosol” needs rethinking. “As soon as the biogenic compounds react with anthropogenic pollutants, we have to ask if the label ‘biogenic’ continues to be appropriate,” he explains. “It’s biogenic carbon, but not biogenic aerosol.”

Carlton and other researchers note that the simulations could impact air-quality policies. One of the pollution types that EPA regulates is particulate matter smaller than 2.5 μm, which includes secondary organic aerosols. Policymakers have seen biogenic particulate matter—which falls into this size category—as untouchable, says atmospheric chemist Jose-Luis Jimenez of the University of Colorado, Boulder. “What this study shows is that that’s not true,” he says, “and that those aerosols should be part of those targets.”


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