By Perkins, Sid
Science News , Vol. 172, No. 10
Jeffrey S. Gaffney, a sunburn-prone atmospheric scientist, set off one morning in March 2006 for a day of field work in Mexico City--without his hat and sunscreen. At Mexico City's altitude, 2,240 meters above sea level, sunlight beating down through the thin air delivers as much as 30 percent more ultraviolet radiation than reaches coastal regions. "I thought I'd be fried at the end of the day, for sure," he recalls.
But Gaffney, a researcher at the University of Arkansas in Little Rock, came through the day unscathed. The sunburn that he'd feared never developed because much of the sun's ultraviolet radiation was blocked by the city's notoriously dirty air. Some of the radiation was scattered back to space by the high-altitude haze smothering the city; some was absorbed as it stimulated chemical reactions within that soup of pollutants.
He and more than 450 other researchers from Mexico, Europe, and the United States had come to Mexico City for a monthlong study of air pollution in, above, and around the metropolis. The MILAGRO campaign--the Megacity Initiative: Local and Global Research Observations--was designed to scrutinize local and regional air quality, the effects of pollutants on human health, and the influence of Mexico City's pollution on regional and global climate. Mexico City, whose 20 million inhabitants endure constantly nasty air, was an ideal study site.
Any urban area with more than 10 million residents is considered a megacity. Many studies suggest that these gigantic urban areas have become the world's dominant sources of atmospheric pollutants. Some of their emissions are long-lived and apt to drift far from home. Pollution from Asian megacities routinely wafts to North America (SN: 12/12/98, p. 374; SN: 1/1,/00, p. 4). Other research suggests that as much as one-third of the low-altitude ozone in some parts of Europe originates in North America.
MILAGRO data will enable scientists to better understand how air pollution evolves, says Gaffney. Increasingly, field studies and lab experiments indicate that the color, chemical reactivity, and other characteristics of airborne particles and droplets can change dramatically over time. Such findings may eventually affect how such pollutants are regulated.
Because Mexico City is the world's second-largest megacity, many of the problems observed there may foreshadow troubles to come in smaller, rapidly growing urban areas throughout the world. On the other hand, computer simulations suggest that many smaller megacities export a proportionately higher share of their pollution to distant regions even than Mexico City does.
A GROWING PROBLEM In 1800, only 3 percent of the world's population lived in cities. Today, more than half of our planet's 6.6 billion people reside in urban regions. By 2030, if forecasts by the United Nations are correct, three-fifths of the world's projected population of 8.3 billion will live in cities.
The growth of population in cities leads to increased energy consumption within relatively small areas, a trend that typically decreases air quality and increases the prevalence of adverse health effects, says Luisa T. Molina, an atmospheric scientist at the Massachusetts Institute of Technology. Those consequences are particularly pronounced in Mexico City, she noted in May at a meeting of the American Geophysical Union in Acapulco, Mexico. In recent years, for example, ground-level concentrations of ozone in Mexico City have exceeded the country's air-quality standards 284 days per year, on average. Geography doesn't help: Mexico City lies in a broad basin ringed by tall mountains that can block the movement of air masses that might clear out pollution.
Furthermore, the city's rapid spread in recent decades has aggravated its pollution problems. Mexico City now covers about 1,500 square kilometers--about 10 times as much land as it occupied just 50 years ago, says Molina. …