Sharing Our World's Air
Pollution and its Globetrotting Ways
Atmospheric chemists have long recognized that air pollution can generate impacts far from its source. New satellite-borne instruments and software—both operating on a global scale—now map in unprecedented detail where pollutants emerge and where they travel. Several field studies are fleshing out this worldwide portrait through ground-based and airborne measurements with astounding speed and detail.
The acidic side of progress
Anyone living in the industrial cities of Victorian England experienced the sun-shrouding, lung-clogging effects of soot first hand. It was Robert Angus Smith who identified another threat from fossil fuel emissions: acid rain.
The Scottish chemist coined the phrase and introduced it in his 1872 book Air and Rain: The Beginnings of a Chemical Climatology. From data collected in Britain and Germany over two decades, Smith found sulfuric acid present in rainfall at levels as high as 21 parts per million. Smith also noted that “the stones and bricks of buildings, especially under projecting parts, crumble more readily in large towns where coal is burnt.” He attributed this to “the slow but constant action of acid rain.” It wasn’t until the 1970s that acid rain’s widespread effects on lakes and forests were fully recognized.
AT THE UNIVERSITIES
Bringing science to bear on pollution policy
“I’m definitely someone who’s using hard-core science tools to try to inform environmental policy decisions,” says Denise Mauzerall (Princeton University).
After completing her doctorate in atmospheric chemistry at Harvard University, Mauzerall spent three years as a postdoctoral researcher and visiting scientist at NCAR. Since 1999 she’s been an assistant professor in Princeton’s Woodrow Wilson School of Public and International Affairs. She and her group use science to attempt to shape U.S. and East Asia policy on air quality as it relates to health, energy, and climate change. Among her most recent projects is a study of how market-based cap-and-trade programs to limit emissions of nitrogen oxides (NOx, an ozone precursor) could be made more effective. Although these programs keep an overall lid on emissions, they offer little control over where and when emissions occur. This means, for example, that emission permits can be traded from locations of low to high population where the ozone produced from the emitted NOx can cause more damage. Likewise, a power plant might use more of its emission permits during the hottest days of summer, when demand for power is high but when conditions are also most favorable for ozone formation. “NOx cap-and-trade programs have been hailed as enormous successes because they’ve reduced emissions at relatively low cost,” says Mauzerall. “What I’m concerned about is whether we can adapt these systems to minimize impacts as well.” Mauzerall continues to use the MOZART model (see main article) to study the intercontinental transport of pollutants.
At the intersection of air chemistry and ecology
Elisabeth Holland brings a diverse array of training to a multifaceted job.
After an undergraduate degree in zoology and Spanish at Colorado State University, Holland completed graduate work in soil science and earned her CSU doctorate in ecology and environmental sciences. An NCAR scientist since 1989, Holland now leads the center’s Biogeosciences Initiative, whose goal is to incorporate the biological sciences into geophysics and atmospheric research. She and her colleagues study biogeochemical cycles and feedbacks with consequences that can be both important and unintended. For instance, fossil fuel emissions release nitrogen into the atmosphere, where rain carries it back to the soil. But excess nitrogen alters soil chemistry, changes the types of species found in an ecosystem, and can lead to oxygen-depleting, fish-killing algae blooms. In addressing these and other questions, says Holland, “I use the interplay of models and measurements to refine science questions and to ensure that the models adequately represent observations.” Holland served as a lead author for the 2001 assessment report from the Intergovernmental Panel on Climate Change and is doing so again for 2007. “Working on the reports has been a good experience in trying to summarize science in a way useful to policy makers,” Holland says. “Working with scientists from other countries and backgrounds also teaches you a lot about the tremendous privilege it is to do science.”
Although weather touches everyone on the globe, a thunderstorm in Ohio doesn’t easily reveal the fingerprints of a typhoon in Japan. The particles and gases that make up our atmosphere are a different story. When studied with care and precision, they serve as a chemical journal of biogenic and human activity—an index of how one continent’s industrial prowess or its vast swaths of forest can influence air quality many hundreds of miles away.
NCAR’s atmospheric chemists are in the vanguard of a new research emphasis on regional and global air quality. In 2004 they joined U.S. and European colleagues for a massive study of the river of air that flows across North America and beyond. The tools they employed included several pacesetting designs created at NCAR. Another NCAR group sampled the uptake of carbon in Colorado’s mountains amid the backdrop of a challenged forest ecosystem. Modelers and satellite specialists are breaking new ground in efforts to map and predict pollution around the globe. And in 2006, a long-sought study will quantify air chemistry in the region surrounding Mexico City, one of the planet’s expanding roster of megacities that shape the life—and the breath—of hundreds of millions of people.
Charting the course of U.S. pollutants
Going with the flow took on new meaning in the summer of 2004 for NCAR’s Alan Fried and James Walega. The two were among several scientists and technicians aboard a NASA DC-8 aircraft as it worked its way eastward across the United States over several weeks. While over the Rocky Mountains, the airborne scientists sampled jet-stream flow containing pollutants from Asia. Moving east, the group measured emissions generated by power plants, cities, and forests across the Midwest and South. The cross-country trek ended in New England, where the DC-8 joined a fleet of other aircraft, along with ground stations and a NOAA ship, to assess the quality of air exiting the region for the Atlantic and points east.
The New England study was part of the Intercontinental Chemical Transport Experiment–North America (INTEX-NA), a major collaborative effort to study gases and aerosols (airborne particles) as they leave the continent. A second INTEX field phase in 2006 will diagnose the quality of air entering North America from Asia.
The large-scale vision of INTEX is noteworthy in itself. NASA’s Hanwant Singh calls it “the first time a coordinated worldwide campaign has been launched to establish a benchmark reading from which global atmospheric policies can be developed.”
NCAR brought to INTEX-NA several of its uniquely tailored instruments for assessing air chemistry from a fast-moving aircraft. On the DC-8, Richard Shetter and colleagues flight-tested an improved instrument for detecting actinic flux. This index of light reaching molecules from all directions is an important gauge of solar-driven processes in the atmosphere. Aboard a NOAA P-3 aircraft, NCAR’s Frank Flocke operated a new instrument that can measure small amounts of PAN, or peroxyacetyl nitrate—a key agent in low-level ozone—in two seconds or less. Its predecessor took about two minutes.
Fried’s group focused on formaldehyde, an intermediary that reveals how quickly ozone and radicals (highly reactive molecules) are being produced and how hydrocarbons are reacting with oxygen and sunlight. The laser-based spectrometer he brought on board the DC-8 measures formaldehyde levels as low as 30 parts per trillion. Fried, along with colleagues Dirk Richter and Petter Weibring, is collaborating with Frank Tittel (Rice University) on a new system that relies on a mix of laser-beam output in a crystal (lithium niobate, commonly used in cell phones). The blend produces laser light in a longer, mid-infrared wavelength ideal for detecting trace gases. The new device should be lighter, smaller, more stable, and more robust than its predecessor—all pluses for an airborne system.
Tittel, a laser expert since the 1960s, calls Fried’s new instrument the “gold standard” in detecting trace amounts of formaldehyde. “It’s such an important gas,” Tittel adds, “not only in atmospheric chemistry but also in industrial safety.”
An eye in the sky for emissions
The eyes of meteorologists opened wide in the 1960s when the first images of Earth from space revealed a world of cloud formations and behavior previously unseen. Atmospheric chemists are in a similarly heady state as data roll in from the first space-borne device to provide long-term mapping of a key pollutant, carbon monoxide (CO), across the globe.
Launched in 1999 aboard NASA’s Terra satellite, the Measurements of Pollution in the Troposphere instrument (MOPITT) uses a technique called correlation spectroscopy to detect CO from space. Several billion tons of CO enter the atmosphere each year, roughly split between plant sources and human activity (anthropogenic sources). MOPITT infers the amounts and locations of CO while circling the globe. “CO is involved in much of the chemistry of the lower atmosphere, and it’s now one of the few pollutants that we can measure from space, thanks to MOPITT,” says NCAR’s David Edwards.
In early 2003, the MOPITT team, led by NCAR and the University of Toronto, released images of pollution wafting across the Pacific from China and Southeast Asia, with CO up to four times the background levels. With only a few days’ processing time required—and with CO as a tracer that persists for several weeks—the MOPITT imagery is prompt enough to serve as a guide to regional pollution events as they unfold. During the INTEX-NA field campaign, MOPITT data provided a large-scale context for the localized aircraft and ground-based measurements—“very useful information when deciding where to fly,” says Edwards.
MOPITT is also a boon to longer-term research. Now that MOPITT has the better part of a decade under its belt, Edwards and colleagues can examine how pollution levels in the atmosphere change from year to year. For instance, as smoke and pollutants from intense Russian wildfires moved east in late 2002 and early 2003, MOPITT traced a spike in CO levels extending as far as North America.
NCAR scientists have also analyzed CO pollution over Europe by using MOPITT data and the Model for Ozone and Related Chemical Tracers. MOZART, a three-dimensional system devised by NCAR, the Max Planck Institute, and NOAA, helps separate out the contributions from different pollution sources. While the majority of Europe’s anthropogenic CO originates from the continent itself, the study found just under a third arrives from North America and Asia, another sign of pollution’s extensive reach.
The impact of a burning forest on air quality is broader than the smoke and toxins it spews out. It may take decades before a forest scarred by drought and wildland fire can remove carbon dioxide from the atmosphere as efficiently as a healthy forest does. In a world where carbon budgets may soon affect fiscal budgets, the future of fire-prone forests looms especially large.
“Carbon is the currency of forestry,” says NCAR scientist David Schimel. He joined NCAR’s Britton Stephens and Colorado State University’s Dennis Ojima and Tomislava Vukicevic in mid-2003 to head the Atmospheric Carbon in the Mountains Experiment (ACME), which studied the chemical input and output of alpine forests along Colorado’s Front Range. Schimel points out that the steady rise in global levels of carbon dioxide (CO2), recognized since the 1950s, masks a variety of regional processes. Between the global picture and localized data, “you’d like to have a bridging measurement. The idea of ACME is to bridge those scales.”
While aircraft passed over the pine and spruce forests just northwest of Denver, tower-mounted sensors sampled the area’s meteorology and chemistry around the clock from May to July. The early data on forest respiration—photosynthesis by day, emissions by night—were “much more dramatic than we’d dared hope for,” says Schimel. “Some of the signals were roughly ten times bigger than anticipated.”
One question to be addressed through ACME is whether the forests of the Rocky Mountains are actually removing 30 to 40% of the nation’s carbon emissions, as Schimel and colleagues had estimated in previous work. Between a multiyear drought, extensive fire damage, and widespread tree loss through a beetle infestation, the West’s forests may not be helping to offset U.S. emissions as much as once thought. “Hopefully, ACME will give us credible estimates at the statewide level,” Schimel says.
Megacities: an urban question mark
The counterpoint between natural and human influences on air chemistry reaches a crescendo around megacities, the urban centers with more than 10 million residents each. In 1950, New York was the planet’s only megacity; by 2015, the United Nations predicts there will be 27 megacities, the vast majority in developing countries. As they generate vast amounts of pollution, megacities also serve as laboratories for the fate of gases and aerosols that fan out into surrounding rural areas.
NCAR is leading the charge to examine the air in and around Mexico City through a major field campaign planned for 2006. The NCAR initiative, Megacity Impacts on Regional and Global Environments (MIRAGE), encompasses the field project and related modeling studies. “We’re hoping to look at this problem from many points of view,” says lead scientist Sasha Madronich.
The air chemistry of megacities varies widely, from the soot and smoke prevalent in such developing cities as Calcutta to the hydrocarbons and nitrogen oxides emerging from tailpipes in more prosperous, vehicle-dominated locales such as Los Angeles. The emissions themselves evolve as they leave town and interact with oceans, forests, or other natural features, says Madronich. “When aerosols get hundreds of kilometers downwind from a megacity, they’re not the same aerosols you had initially. Their chemical and radiative features have changed.”
For the Mexico City project, NCAR will draw on the strength of its research ties with universities and government labs, along with a bevy of instruments developed in house. “For the first time,” says Madronich, “we have the capacity to rapidly measure key reactive compounds, and we have an increasingly favorable climate for international collaboration.”
Gregory Carmichael (University of Iowa) hopes to use MIRAGE data to compare and contrast the airborne footprint of Mexico City to that of megacities in Asia. He’ll be keeping an eye toward the solutions and alternatives that can blossom in a large urban area, albeit a polluted one. “We typically view megacities in a negative environmental context, and often persons living in megacities are exposed to high levels of pollutants,” says Carmichael. “However, megacities are often also at the forefront of environmental action.”
In the end, MIRAGE’s acronym could prove ironic. As Carmichael points out, the project may well produce one of the most concrete pictures to date of air chemistry around a great urban area. “MIRAGE will provide data to quantify the megacity footprint at urban, regional, and—by implication—global scales.”