How anthropogenic emissions interact with organic compounds emitted by trees

MIT researchers are conducting fieldwork this summer in the Talladega National Forest in Alabama as part of the National Science Foundation’s Southeast Atmosphere Study (SAS), an umbrella study comprising five projects undertaken by scientists from 30 research institutions. At the field site in a clearing in the forest, Jesse Kroll — who is an associate professor in the departments of civil and environmental engineering and chemical engineering — postdoctoral associate Eben Cross and graduate student Jon Franklin are looking at the chemical reactions between anthropogenic air pollution and the organic compounds emitted by trees.

The goal of the SAS is to learn more about the processes that control biosphere-atmosphere interactions affecting regional climate and air quality in the southeastern United States, one of the few places in the world that has cooled during the last century. Organic compounds emitted into the atmosphere as gases react with other compounds to form particles of organic aerosol, which can continue to evolve through oxidation into secondary aerosols of large compounds. The dense deciduous trees in the Southeast emit organic compounds that form high concentrations of these secondary organic aerosols. Scientists think enhanced secondary organic aerosol formation may be responsible for the cooling effect in that part of the country.

But how emissions from anthropogenic activity, such as the nitric oxides in vehicle emissions, influence the amount and properties of this secondary organic aerosol is unknown. Any such effect likely would be particularly strong in the southeastern U.S., because of the substantial amount of anthropogenic activity and the very high emissions of biogenic volatile organic compounds from the forest.

“If nitric oxides or some other anthropogenic pollutant enhance biogenic secondary organic compound formation, that could explain the cooling trend in the Southeast,”  Kroll says. “We're going after this same topic in the laboratory, but the only way to really be sure we’re realistically capturing the conditions of the atmosphere is to make measurements in the field.”

To understand and be able to predict organic aerosol, scientists need to understand the life cycle of all organic species in the atmosphere — emissions, chemical transformations and deposition — in both the gas and particle phases. But instruments typically miss two important classes of organic compounds, intermediate-volatility and semi-volatile species, which are volatile enough to be in the gas phase, but “sticky” enough to attach to surfaces, Kroll says.

To capture these compounds in their measurements, Kroll and his team developed a new mass spectrometric instrument that can measure these nebulous species of low-volatility organic compounds. He says the key is to measure the compounds as an ensemble, as opposed to trying to quantify all individual species. They deployed their new mass spectrometer in one of the 12 research trailers located at the field site.

“Having a real-time measurement of intermediate and semi-volatile gas phase organics in the atmosphere is somewhat uncharted territory in atmospheric chemistry, but with our instrument we think that we can obtain quantitative estimates of the amount and chemical characteristics of these important species,” Cross says. “The regional summertime haze throughout the Southeastern U.S. may be a result of the combined influence of biogenic and anthropogenic emissions. Measuring intermediate and semi-volatile gas phase organics in a forested site downwind of urban areas will help us unravel the atmospheric chemistry in that haze.”

Other projects in the Southeast Atmosphere Study involve two aircraft that are sampling air chemistry from the Mississippi River to the Atlantic Ocean, and from the Ohio River Valley to the Gulf of Mexico; ground instruments to measure low-level winds, moisture and temperature; and instrumented towers taking measurements within and above the forest canopy. The study is supported by the National Science Foundation, the U.S. Environmental Protection Agency and the U.S. National Oceanic and Atmospheric Administration.