The Air That’s Up There

Atmospheric scientists focus on nature's role

The atmosphere is a tough laboratory. On a bench top, chemists can devise and carry out controlled experiments. But chemistry isn’t so straightforward outdoors. Natural and humanmade chemicals from countless sources enter the air and react–again and again. Atmospheric scientists are continuously working to untangle the complicated interactions. It’s often still unclear precisely what chemicals are present in the atmosphere, how they got there, what they’re doing, or where they’re going.

LOFTY GOALS. High in the forest canopy at Finland’s Hyytiälä forest, researchers set up equipment to sample aerosols. K. Hämeri

FOREST HOME. In this main laboratory building, atmospheric scientists record measurements of gases and aerosols above the Hyytiälä forest canopy. K. Hämeri

SEASONAL SHIFT. As temperatures drop in autumn, a forest’s emissions of methanol can rise.

Complicating matters is nature–specifically, plants. Although the casual hiker doesn’t see it, trees, underbrush, and even dead leaves can emit gases that mix with pollutants above a forest canopy and form new compounds. More than 20 years ago, President Ronald Reagan took much heat for blaming killer trees for pollution. But while tree emissions alone aren’t responsible for killing anyone, they can react with humanmade compounds to make chemicals that further pollute the air or contribute to climate change.

In recent research, atmospheric scientists have been filling in holes in their basic knowledge about the ways that nature affects the chemistry of the atmosphere. The forest’s gaseous influence doesn’t stop at its boundaries–the reaction of naturally produced chemicals with humanmade pollutants can influence air quality far downwind. Moreover, many scientists say, human influences on the atmosphere can’t be fully understood unless the natural influences, such as tree emissions, are carefully calculated.

The research is fundamental, but its implications could be wide-ranging. It could help scientists understand the fate of airborne chemicals. “Are they going to stay in the atmosphere and continue to react?” asks Chris Geron of the United States Environmental Protection Agency in Research Triangle Park, N.C. “Are they going to travel far? Are they going to form some other compound that will react under different light and temperature?”

Obtaining those answers, in turn, can drive decisions about how to regulate industrial and automobile emissions. The knowledge being gathered helps scientists improve their computer models of weather and pollution patterns.

“The more you understand about the chemistry,” says Geron, “the more you can tune your models and understand what’s going on in the atmosphere.”

Such models already play various roles, from informing people of impending smog conditions to guiding multimillion-dollar decisions on how governments regulate industrial emissions. “There’s definitely a multitude of questions and areas of research that we can work in to improve the models,” Geron says.

Mystery gases

Detailed air analysis is necessary in both urban areas, where people typically think about smog, and forested regions. Although it might look pristine, forest air is often infiltrated by humanmade pollutants, such as nitrogen oxides from cars on nearby roads or distant power plants. When these pollutants mix with a variety of natural chemicals spewed by trees, they form new chemicals–including some that researchers haven’t yet identified.

By discovering exactly which pollutants are in the air, scientists can better determine where chemicals will end up, what reactions created them, and how to prevent the pollutants from forming in the first place.

Air-analysis instruments generally available today can’t simultaneously and sensitively identify all the individual nitrogen-containing chemicals. Recently, atmospheric scientist Ron Cohen and his colleagues at the University of California, Berkeley invented a new device that simultaneously measures concentrations of several classes of nitrogen-containing air pollutants.

Cohen’s instrument, described in the March Journal of Geophysical Research– Atmospheres, takes advantage of the fact that various nitrogen-based compounds break down into nitrogen dioxide at different temperatures. Air flows through a channel in the instrument, which heats it to a particular temperature that lab experiments have indicated destroys a particular class of nitrogen-containing compound, such as peroxy nitrates.

The machine measures the amount of nitrogen dioxide produced at that temperature. By taking a series of readings as the temperature rises or by setting different air-sampling channels to the breakdown temperatures for individual nitrogen-compound classes, the researchers calculate how much of each chemical class is present in the air.

“It’s a neat instrument,” comments Tom Pierce, a physical scientist with the National Oceanic and Atmospheric Administration. “I think it has a lot of potential in helping us understand the composition of the atmosphere,” says Pierce, who’s currently assigned to the EPA in Research Triangle Park, N.C.

In early experiments using the device in an urban site in Houston and over a mostly conifer forest site in the Sierra Nevada mountains, Cohen’s team discovered that many of the previously unidentified nitrogen compounds in both locations are alkyl nitrates. These compounds, like some other nitrogen pollutants, can react in the atmosphere to make ozone, the major component of smog, Cohen notes.

Researchers had believed that alkyl nitrates are minor contributors to smog since only very small amounts of the chemicals had previously been detected, says Cohen.

However, the new results suggest that alkyl nitrates are 10 times as abundant in both the urban and rural areas as scientists had previously measured, he says.

Now that he’s identified the alkyl nitrates, Cohen wants to determine what chemicals contribute to their formation in each location. In both cities and forests, alkyl nitrates are made when nitrogen oxides, which are produced by automobile tailpipes and smokestacks, react with hydrocarbon molecules. Cohen is now testing the hypothesis that in urban areas, this reaction uses mostly hydrocarbons emitted from industrial sources. In forested areas, the reaction more often uses hydrocarbons emitted by tree, he suspects.

There’s a strong chemical interaction between naturally emitted hydrocarbons and vehicle-produced nitrogen oxides, says UC-Berkeley’s Allen Goldstein, who has collaborated with Cohen.

Goldstein’s research group studies compounds coming from forests and their roles in producing ozone gas and airborne aerosols. In recent research, Goldstein and his colleagues found that much of the ozone in the air at their Sierra Nevada mountains research site is produced when humanmade nitrogen oxides react with hydrocarbons from trees.

The natural hydrocarbons are so abundant in the environment that they overshadow humanmade hydrocarbons in the ozone-producing reaction with nitrogen oxides, says Goldstein. Therefore, to reduce ozone in the Sierra Nevadas, it is more important to control humanmade nitrogen oxides than humanmade hydrocarbons because the forests will always supply plenty of hydrocarbons.

Particulate matter

Another component of atmospheric chemistry over forests is aerosols. These fine droplets, just nanometers wide initially, form naturally or through human influence as certain atmospheric gases condense. They can be found over cities, rural areas, and even oceans. Despite their small size, aerosol particles can have profound effects on both local pollution and global climate.

In smog or urban haze, aerosols directly reflect sunlight, says Colin O’Dowd of the National University of Ireland in Galway and the University of Helsinki. That’s why smoggy days appear dark. O’Dowd studies the process by which certain gases condense into aerosols.

In addition to local pollution concerns, when many types of aerosol particles become as large as 100 nanometers across, they act as nuclei for the formation of clouds, says O’Dowd. The particles’ influence on cloud cover may ultimately affect global temperatures, he adds.

Aerosols’ formation and influence is “one of the most complicated systems and least quantified in the climate models,” says O’Dowd. “So, it is a major challenge to understand how anthropogenic activities affect the aerosol influence on climate change.”

To uncover the human role in aerosol formation, researchers must figure out what part the natural environment plays. O’Dowd says that one nagging question has been: Which gases are involved in the early formation of these microscopic particles over forests?

Scientists have suspected that gaseous organic compounds from trees play a major role. But inorganic gases, such as sulfuric acid and ammonia, might also initiate particle growth. Such inorganic compounds could reach the air over a forest from a variety of sources, including the ocean and industry, says O’Dowd.

He’s currently examining whether organic gases indeed trigger the formation of atmospheric aerosols. Today, it’s practically impossible to directly establish the chemical content of aerosol particles as they’re forming, says O’Dowd. To overcome this, he and his colleagues in Finland and Germany developed an indirect way to identify the composition of small particles.

The research took advantage of a trait of small, developing aerosols, those just 3 to 10 nm wide. Their ultimate size depends on their chemical composition; materials that are more soluble in the atmosphere grow faster and bigger.

In the lab, O’Dowd and his colleagues compared the growth of various aerosol particles that they suspect were forming in the forest. The researchers introduced particles, between 3 and 5 nm in diameter, into a chamber of butanol gas. An instrument then counted and measured the aerosol that the particles produced. The researchers demonstrated, for instance, that 5-nm organic pinic acid and cis-pinonic acid particles grow into much larger aerosols than 5-nm inorganic ammonium sulfate particles do.

With this information in hand, the researchers used the butanol chamber to identify the aerosol particles over the Hyytiälä forest in Finland. O’Dowd and his colleagues conclude in the April 4 Nature that the particles they found in the forest air started out from organic gases similar to pinic acid or cis-pinonic acid.

Scientists know that these acids are the oxidation products of compounds called terpenes, which are produced by the forest canopy. Human activities that increase air concentrations of oxidizers, such as ozone, can thus boost the concentration of aerosols over forests, the researchers note. O’Dowd and his colleagues are also investigating aerosol particles over oceans.

Seasonal emissions

It’s well known among researchers that trees differ in the amount and types of chemicals they emit. Now, researchers are examining whether forests’ natural emissions change with the seasons.

In Michigan, some researchers have perched gas detectors high in the forest canopy to better detect the ups and downs of trees’ chemical output. Alex Guenther of the National Center for Atmospheric Research in Boulder, Colo., and his colleagues knew that freezing leaves in the laboratory makes them release methanol, acetaldehyde, and other gases. Last fall, the team set out to determine whether trees in the Michigan forest likewise increase their emissions of such gases when the temperature drops.

Their preliminary results suggest that forest emissions of methanol and acetaldehyde double during the first week of cold weather. These compounds are among the chemicals that can react with nitrogen oxides to eventually form ozone, says Guenther. Further, they can affect climate by slowing the breakdown of so-called greenhouse gases, such as methane, that hold heat in the atmosphere, he says.

Guenther returned to the Michigan forest in May when a caterpillar outbreak was expected. Lab studies suggest that wounded leaves, such as those that caterpillars have gnawed, increase their emissions of hexenol and hexenal, he says.

In other work, Goldstein and his colleagues measured forests’ emissions of yet another compound, ethanol, and found that they increase exponentially as temperature rises. Few studies have measured forests’ ethanol emissions, he notes, so fluxes in the natural, background concentrations of the gas haven’t been documented. Yet, the United States is considering requiring refiners to substitute ethanol for the automobile-fuel additive methyl tert-butyl ether, or MTBE, which is known to contaminate groundwater (SN: 4/8/00, p. 229). If this switch occurs, scientists will need data on natural ethanol fluxes to gauge any impact of ethanol-infused fuel on the atmosphere.

Currently, natural emissions of ethanol dominate over humanmade sources in the Sierra Nevadas, Goldstein says. “But that might not be true after switching fuel [additives],” he says.

Nobody should alter natural vegetation to try to control emissions, says Guenther. However, anyone doing artificial landscaping and tree farming might benefit from what scientists are learning about trees’ roles in climate change. People should consider what types of trees they plant, Guenther suggests, and in urban environments, choose species that produce only small amounts of natural gases.

For example, large poplar plantations in Oregon “actually are changing the chemistry of that region,” he says, because they have much higher emission rates of organic gases than many other trees do.

Meanwhile, the work of atmospheric scientists is continuing to fill in the gaps in understanding natural emissions. And as that happens, computer models should get better at forecasting local pollution and global climate.

In 5 years, Pierce speculates, computer models that account for both urban and rural atmospheric chemistry might produce pollution advisories that resemble weather forecasts. Scientists would provide pollution forecasts nationwide based on meteorological conditions, background concentrations of natural atmospheric chemicals, and the natural and humanmade pollutants expected. “It would be a weather forecast, except it would have chemistry,” he says.

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