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June 2005

Researchers to study air pollution near Mexico City

Teachers in Mexico City report that schoolchildren rarely use the color blue when they paint the sky. Instead, they use shades of brown and gray. With a population of about 22 million, Mexico City is notorious for its abundant air pollution.

Unfortunately, the city is hardly an anomaly. Air pollution, which has a whole host of negative effects on people and the environment, is one of the most visible environmental consequences of "megacities" like Mexico City. Demographers project that the number of these megacities, that is, cities with 10 million or more inhabitants, will reach 26 by 2015. The most explosive urban growth is occurring in developing nations. Lagos and Dhaka, for example, are growing 10 times faster than New York and Los Angeles.

Next February and March, about 40 NCAR researchers will head to Mexico to scrutinize the brown and gray air that hovers above the country's capital. During the project, dubbed MIRAGE (Megacity Impacts on Regional and Global Environments), they'll investigate the chemical transformation of air pollution as it flows from Mexico City to surrounding regions and the globe as a whole, with an eye toward applying what they learn to megacities around
the world.


Mexico City has some of the world's worst air quality. (Photo by Julio Etchart.)

Because air pollution is very complicated, both chemically and physically, and evolves over time and distance, scientists haven't been able to quantify it very well. By using aircraft and ground instruments to survey the air downwind of Mexico City, the NCAR team will observe how air pollution ages as it dissipates in the hours and days after emission.

"We're not looking so much at pollution inside the city because that's already known," explains ACD's Sasha Madronich, one of the project's leaders. "We're looking at the outflow so that for the first time we'll have an idea of how much pollution is outside the city and be able to understand its full life cycle."

MIRAGE is especially important because current models for air pollution were developed mainly for cities in industrialized nations, such as Los Angeles, and don't necessarily transfer well to megacities in the developing world. Smog in Los Angeles has a different chemical composition than the air pollution in Bombay or Shanghai, for example, where people may burn coal and wood and drive vehicles that emit more harmful chemicals.

"If we can understand this in the context of Mexico City, we can apply it to other urban areas around the world," Sasha says.

A unique package: aerosols and gases

Researchers will try to answer three broad questions during MIRAGE:

• How will future urban growth affect atmospheric composition on regional and global scales?

• What are the effects of changes in atmospheric composition on regional and global climate?

• To what extent can better urban design and planning reduce these impacts?

What differentiates MIRAGE from other field projects on air pollution, however, is the project's focus on both aerosols (airborne particles of dust, soot, and other pollutants) and gaseous pollutants (ozone, nitrogen oxides, hydrocarbons and their oxidation products, carbon monoxide, and sulfur dioxide). "In the past there have been air campaigns during which researchers have made lots of aerosol measurements, and other ones during which they've made lots of gas measurements," Sasha says. "The uniqueness of our package is that this is the first one that brings them together."

By studying how aerosols and gases interact with each other as they age, he and his colleagues will try to figure out if these interactions change the chemical and physical properties of the aerosols and what this means. For example, do changes in the chemical composition and physical structure of aerosols affect not only their optical properties but also their ability to nucleate clouparticles, which in turn can influence cloud lifetimes, reflectivities, and the formation of rain? As Sasha points out, this has important implications for climate. "There could literally be weather modification downwind of the city from the pollution," he says.

Mexico City is an ideal setting for MIRAGE because it is the world's third largest urban area, has some of the worst air quality anywhere, and is situated at low latitudes, as are most fast-growing megacities in developing nations. It is also located at high elevation (7,400 feet or about 2,255 meters), where the intense sunlight cooks up an acrid concoction of pollutants that is blown downwind into what is an otherwise only moderately polluted region. A network of ground-based air quality measurements has been in place in Mexico City for nearly two decades, and public and private labs currently conduct considerable air quality research. The city also offers good infrastructure and logistics for carrying out observations that require aircraft.

NCAR researchers, mainly from ACD and EOL, will collaborate with colleagues from the Department of Energy, NASA, NOAA, and various U.S. universities, as well as with more than a dozen Mexican partners from academia, the government, and private industry.



Sasha Madronich. (Photo by Carlye Calvin.)

Chasing pollution in the C-130

Pending final approval by NSF, the research team will use the NCAR/NSF C-130 aircraft, as well as the DOE's G1 aircraft and a small NASA aircraft. They'll base the aircraft operations in Veracruz, a 45-minute flight east of Mexico City on the Gulf coast, to keep from getting tangled in Mexico City's heavy air traffic. Also, by taking off at sea level rather than in the higher, thinner air near the capital, the aircraft will be able to carry more weight and fuel and thereby remain in the air longer.

Mexico City's pollution plume generally spreads northeast due to prevailing winds from the southwest, although it can blow south during Nortes, winds from the north. "One challenge will be finding the plume, especially when it's a day or two old and over the Gulf of Mexico," says ACD's Frank Flocke, mission scientist for the C-130 during MIRAGE. "During a southwesterly flow or during a Norte event, the outflow should be reasonably well-defined and not just a big pollution cloud around the city."

On a typical mission, which will last about eight hours, researchers on the C-130 will sample the air upwind and directly above the city in addition to sampling the plume itself. They'll fly in stacked legs at different altitudes perpendicular to the plume to best sample the plume's whole extent.

“There could literally be weather modification downwind of the city from the pollution.”

—Sasha Madronich

The researchers plan to coordinate with an air traffic controller on the ground in order to avoid commercial aircraft coming into Mexico City, since they'll be operating a lidar (laser-based radar) on the plane. "We're shooting out lasers from the plane, so we need clear air space above and below," Frank says. "It may restrict us a little."

ACD's Alex Guenther and colleagues will set up ground-based instruments at the Technical University of Tecamac, situated about 25 miles (40 kilometers) northeast of the city, as well as at several locations within the city where Mexican and DOE researchers will also be conducting research.

"We'll be able to evaluate the Mexico City emission inventory with direct measurements of some emissions during MIRAGE using a system deployed on a tower within the city," Alex says.

The team will take measurements of gas and aerosol chemistry that can't be made from the aircraft. They'll deploy radar wind profilers to measure continuous wind speed and direction, determining the trajectories of air parcels that blow out of Mexico City. They'll also launch GPS radiosondes several times a day to make vertical profiles of wind speed and direction, temperature, and humidity from the ground through the lower stratosphere.

Sasha says he expects the field campaign to go smoothly, even though getting large amounts of equipment in and out of a foreign country can be a challenge. "It's nothing that NCAR isn't used to doing," he says. "We have very good relations with the Mexican researchers and government and we anticipate excellent collaborations for the good of understanding science."

Over the next few months, the team will begin planning educational and outreach activities, in addition to the normal involvement of graduate students, to complement the field campaign.

• by Nicole Gordon

For more about MIRAGE

The Urban Century?

Fifty years ago, 30% of the world's population lived in urban settings. Now, for the first time in human history, more people live in cities than do not. In 10 years, this number will reach about 60%. At the recent millennium, the world supported 411 cities that each contained more than one million inhabitants. While the majority of the world's urban population used to live in Europe and North America, today the most explosive growth is occurring in developing nations, particularly in Asia.


This map shows the world's 20 most populous urban agglomerations in 2004. With 21,503,000 inhabitants, Mexico City ranks third. (Data courtesy the World Gazetteer; illustration Mike Shibao.)

Migration within countries is the primary force remaking settlement patterns around the world. In the developing world especially, urban economic opportunities are drawing workers in large numbers from the countryside. During the last decade, for example, an estimated 150 million rural Chinese migrated to the nation's urban areas, filling megacities like Shanghai, Beijing, and Ghangzhou.

As globalization continues to impact economies around the world, these patterns of urbanization will continue. Demographers project that the number of megacities, that is, cities with 10 million or more inhabitants, will reach 26 by 2015. Cities that lack the infrastructure and services to support the influx of migrants will be strained to the breaking point, resulting in unsustainable growth and the pollution that so often accompanies it.

• by Nicole Gordon

Also in this issue...


Urban Century?

Tsunami relief

GLOBE in Thailand

Random Profile:
Alain Caya

U.S. emissions:
the full story

Delphi question:
FL0 construction issues

Just One Look


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