The picturesque hazes of Tennessee’s Smoky Mountains appear when volatile organic chemicals released by trees react with other gases in the atmosphere. And every time a raindrop falls into the ocean, microscopic droplets of salt water splatter upward into the atmosphere. Both mountain haze and fine ocean spray are nature’s aerosols. People’s penchant for burning fossil fuels is now creating a rival source of aerosol formation.
Liquid droplets and solid particles small enough to remain suspended in air can scatter light in ways that make for hazy mountain vistas, stunning sunrises and sunsets, and urban smog. Scientists, however, are finding that these aerosols also play critical global-scale roles in climate and atmospheric chemistry. The researchers have been examining lab-made and natural aerosols in an effort to discover how their constituent particles form, what effects they have on the atmosphere, and whether something needs to be done about them.
In a fog
Clouds, smoke, soot, smog, sea spray, sneezes, suspended powders, and plumes of sulfuric acid droplets from active volcanoes–aerosols are everywhere.
Although microscopic, aerosol grains or droplets of a certain size scatter light back into space. This produces a cooling effect that counteracts global warming. That’s why climatologists are so keen on measuring the phenomenon.
Aerosols play a big role in atmospheric chemistry, too. Droplets of liquid aerosols often serve as tiny wafting beakers in which dissolved substances interact. Also, solid particles suspended in the atmosphere can react with surrounding gases or act as catalytic sites where other substances can conveniently intermingle and react.
Now, scientists at the University of North Carolina (UNC) at Chapel Hill suggest why conditions in some areas generate much higher aerosol concentrations than are found elsewhere.
One city that pumps out a lot of pollution is Houston. Scientists noticed that on days when the atmosphere there contained particularly high concentrations of organic chemicals and sulfur dioxide, the air over oil refineries was especially thick with haze, says Myoseon Jang, an environmental scientist at UNC. When sulfur dioxide–which reacts with water vapor in the air to form sulfuric acid–wasn’t abundant over Houston, the aerosols weren’t so prevalent.
To study this phenomenon, Jang and her colleagues conducted laboratory tests in which they spewed extremely small particles into Teflon-coated chambers, some with air volumes equivalent to that of a medium-size room. The particles served as seeds around which aerosol droplets could grow. Then, the researchers introduced various chemicals that form when hydrocarbon vapors undergo reactions with other gases in the atmosphere.
In experiments in which the particles had been coated with small amounts of sulfuric acid–about the same concentrations found in diesel soot–the mixture produced up to 10 times as much aerosol as was generated in chambers holding uncoated particles. The sulfuric acid apparently serves as a catalyst and boosts the mist-creating reactions, says Jang. She and the team report their findings in the Oct. 25 Science.
“This finding opens up a new area of research,” says Edward O. Edney, an atmospheric chemist with the Environmental Protection Agency in Research Triangle Park, N.C. Jang’s team is the first to propose that aerosols result from acid-catalyzed reactions, Edney notes. “If it can happen in a lab, it may happen in the atmosphere,” he says.
Jang and other chemists employ both the traditional enclosed vessels and newer, flow-through chambers. By fine-tuning the temperature, humidity, and other aspects of a mixture of flowing gases in such chambers, researchers can analyze how reactions might progress under a variety of weather conditions or at different times of day.
“We don’t know enough about how [aerosols] form,” says James G. Hudson of the Desert Research Institute in Reno, Nev. For example, there are thousands of chemicals that can be produced by such reactions, he says.
Edney agrees: “The atmosphere is chemically complicated.” The challenge, he notes, is in sorting through that complexity and striking a balance between simulations that are simple enough to be manageable but realistic enough to be useful.
Jang and her colleagues are now using their test results to develop mathematical models of aerosol production. If these formulas accurately portray actual atmospheric conditions, scientists could predict, for example, variations in aerosol production resulting from changes in emissions of hydrocarbons and other organic chemicals from cars or industrial sources.
Although substantial amounts of aerosols stem from the organic substances emitted by human activity, the largest source of aerosols is the oceans. Scientists with the Geneva-based Intergovernmental Panel on Climate Change estimate that in the year 2000, as many as 3.3 billion metric tons of salt spray entered the atmosphere.
Those saline droplets form in a variety of ways. Breaking waves toss them skyward, and drops of rain or bursting bubbles splash them into the air. Particle diameters range from 100 nanometers to several micrometers, but the predominant size is 1 to 2 m across, says Murray V. Johnston, an atmospheric chemist at the University of Delaware in Newark. All of these dimensions are smaller than a typical biological cell. If they stay airborne, the droplets’ chemical composition almost immediately begins to evolve.
As the droplet’s water evaporates into the surrounding atmosphere, the tiny globule’s salt concentration goes up, says Johnston. When the airborne droplet becomes saturated, the dissolved salt begins to crystallize. The air may contain nitric acid vapor formed when nitrogen oxides from cars and other sources combined with water vapor. That nitric acid can react with the sodium chloride in a droplet to form sodium nitrate and hydrochloric acid vapor. The acid vapor then reacts with any ozone that’s available and destroys it. In some situations–for example, if hydrocarbon vapors are present–the acid vapor may instead create ozone.
In either case, says Johnston, the nitrate that remains in the droplet eventually ends up fertilizing the ocean or the land onto which it falls, perhaps as much as 160 kilometers inland. That can be bad or good, leading to either harmful algal blooms or greener fields.
In some circumstances, the droplets flung into the air carry more than just salt water. Scraps of dead marine microorganisms may hitch a ride heavenward on the spray.
Researchers have long suspected that ocean-spawned aerosols could be coated with organic chemicals, but they had only indirect evidence to support the idea. The long-chain, carbon-based molecules that make up cell membranes have one end that attracts water and one end that repels it. That repulsion could drive the molecules to the surface of the ocean, where they would form a thin layer–a biological oil slick. Droplets splashed from the surface could carry a coating of this slick.
Direct evidence for this scenario came to light when Heikki Tervahattu, an atmospheric scientist at the University of Helsinki collected particles over Helsinki early in 1998. When he and his colleagues looked at those aerosols with a scanning electron microscope, they noticed that many of the droplets pulsated, enlarging in one part while shrinking in another. This odd behavior, which suggested that a film may have coated these aerosols, spurred the researchers to perform chemical analyses of the droplets. The team reported their observations online April 11 in the Journal of Geophysical Research (Atmospheres).
The results indicate that the droplets began as salt spray over the North Atlantic Ocean and accumulated industrial pollutants as they passed over France, Germany, and southern Scandinavia. Tervahattu and his colleagues also detected long-chain, carbon-based molecules associated with the aerosols.
During subsequent analyses, the scientists blasted the droplets with an ion beam. Those tests, reported online Aug. 31 in the same journal, showed a crystal of sea salt at the heart of each aerosol droplet and a fatty acid coating no more than a molecule or two thick. The scientists determined that the film is primarily palmitic acid, which is produced when a microorganism’s cell membrane disintegrates.
Though thin, the film significantly affects a droplet’s physical, chemical, and optical properties, says Tervahattu. For example, the water-repellent end of each fatty acid molecule points toward the outside of the aerosol particle. As a result, once completely coated by a layer of fatty acids, the droplet would have been much less likely to grow by attracting more water vapor, Tervahattu suggests. Because droplets must grow above a certain size to fall as rain, organic films on a cloud’s particles might squelch precipitation. At this early stage in the research, the film’s global impact on rainfall remains uncertain.
Cool it, or not
New research suggests that the aerosols produced by human activity may not cool Earth’s climate as much as some scientists have predicted.
Ulrike Lohmann and Glen Lesins of Dalhousie University in Halifax, Nova Scotia, recently used satellite observations to validate a climate model that estimates the indirect effects of human-generated aerosols on various properties of the most familiar aerosol of all–clouds. Sulfate- and carbon-rich emissions from automobiles and industrial activity have substantially increased the number of aerosol particles now present in the atmosphere, as compared with preindustrial times, say the researchers.
Because the individual droplets are smaller, clouds tainted with industrial aerosols tend to scatter radiation back into space more effectively than pristine clouds do. There’s less precipitation because the water droplets don’t grow to a size at which they’d fall out of the cloud. That, in turn, means that the clouds last longer than they would if the artificial aerosols weren’t present. These indirect effects, when combined, add up to a net global cooling.
When Lohmann and Lesins removed the human-produced aerosols from the mathematical model they were testing, the simulation’s results for cloud reflectivity and cloud-droplet size didn’t match those estimated from satellite observations. By including those aerosols in the model, results better matched reality, the researchers note. They report their findings in the Nov. 1 Science.
During their analyses, Lohmann and Lesins discovered that pollutant aerosols influenced the clouds in the simulations more strongly than they did in the satellite observations. So, the researchers recalibrated their model. The revised simulations suggest that human-generated aerosols block 40 percent less of the incoming solar radiation than the previous model had indicated. The new data indicate that aerosol-derived cooling effects may not be as large as many might hope.
That’s cold comfort for a world full of industries and automobiles that spew a seemingly ever-increasing amount of planet-warming greenhouse gases.
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