University Park, Pa. -- Something is missing in the analysis of emissions of volatile organic compounds from a Michigan forest, and, according to a team of atmospheric scientists, what they do not know can have a large impact on air pollution in areas in and near forests.
"Organic compounds emitted by some trees play a role in ozone and aerosol production in the lower atmosphere," says William H. Brune, professor of meteorology and head of Penn State's department of meteorology. "It appears that, at least in wooded areas, we have been underestimating the amounts of these chemicals produced."
The researchers were looking at the production of hydroxyl radical in the atmosphere. Volatile organic compounds like isoprene react with the hydroxyl radical resulting in the production of ozone and other chemicals. There has been some discrepancy between the actual measurement of hydroxyl radicals in the atmosphere and what the models predict.
"We developed a device that can measure the reactivity of hydroxyl radical in the air," says Brune. "Then we decided to measure everything in the atmosphere that reacts with the hydroxyl radical." The researchers reported the results of this experiment in the April 30 issue of Science.
The researchers, who included Brune; Piero Di Carlo, a postdoctoral fellow from the Universita di L'Aquila in Italy who worked with Brune; Monica Martinez and Hartwig Harder, former post doctoral fellows, Robert Lesher, research assistant in electrical engineering, and Xinrong Ren, postdoctoral fellow, all at Penn State, measured the hydroxyl radical and the other chemicals in a forest in Northern Michigan at the University of Michigan Biological Station. The researchers took measurements from a tower above the canopy of a mixed transition forest of northern hardwood, aspen and white pine.
Over a two-year period, the researchers also collected a wide array of data on the area, including temperatures, chemical constituents and where the air masses came from.
"When we measured an air mass coming from Detroit, we found all the urban pollutants one would expect, but we also found that the levels of chemicals were not dependent upon local temperature profiles," says Brune. "The urban readings were flat with respect to temperature change."
But the researchers found that chemicals in air masses that came from the forested area did change with a change in temperature. Forest-generated emissions change a lot with temperature.
Isoprene is the forest-generated chemical with the largest piece of the emission pie, but it is only produced during daylight. Other chemicals, particularly larger terpenes, are produced around the clock and are also temperature sensitive.
"We think we measured all major components of the hydroxyl radical reactants, but there is something still unaccounted for," says Brune. "We know that something we cannot identify is reacting with the hydroxyl radicals and we know it is temperature dependent and not light dependent. We just do not know what it is."
This missing substance is not trivial. It makes up a fourth to a third of the reactants, with the hydroxyl radical in the forest, which is significant. Because it is temperature sensitive, and is seen in both clean and polluted air at the forest site, it is forest-generated, not city-generated.
"We followed up the Michigan experiment with one in Houston," says Brune. "There we saw all the expected compounds from cars, oil refining, asphalt and burger joints, but the hydroxyl reactivity was not temperature dependent. We could account for all the hydroxyl reactivity." In New York City and Nashville as well as Houston, no temperature dependency was found.
Because hydroxyl radicals play an important part in the production of tropospheric ozone and are key in many of the chemical reactions that break down pollutants in the atmosphere, understanding exactly how much hydroxyl radical is actually in the atmosphere is important for measuring and predicting pollution levels. If researchers are consistently underestimating the volatile organic compounds generated by forests, their analysis of the environment, especially in cities surrounded by forests, will be incorrect. Applying environmental regulations and assessing pollution levels becomes very difficult with incomplete information.
The researchers note that, "New, more specific and more sensitive detection techniques will likely be required to determine the identity of the biologically produced volatile organic compounds responsible for the missing hydroxyl radical reactivity and the reason they have not yet been detected."
This work was part of the PROPHET 1998 and PROPHET 2000 campaigns and was funded by the National Science Foundation, the National Oceanographic and Atmospheric Agency and the U.S. Environmental Protection Agency.
Other researchers involved in the project were Troy Thornberry, Coleen Campbell and Mary Anne Carroll, University of Michigan; Valerie Young, Ohio University; Paul B. Shepson, Purdue University; Daniel Riemer, University of Miami, and Eric Apel, NCAR.