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by Carol Rasmussen

Mexico City

Mexico City has some of the world's worst pollution. (Photos by Carlye Calvin.)

It takes a big effort to solve a big problem. This March, some 300 scientists from almost a hundred institutions worldwide converged in Mexico to begin finding an answer to one of the world's nastier atmospheric problems: the dense air pollution from megacities.

"The world is urbanizing," says NCAR's Sasha Madronich, who headed one of four complementary field campaigns in and around Mexico City during March. The U.N. Population Division estimates that sometime in the middle of this decade, for the first time in history, more than half of the people on Earth will live in cities. Along with population growth, rapid urbanization has led to the growth of megacities (those with populations over 10 million). Whereas 50 years ago New York was the only city that size, now there are more than 20, mostly located in poorer countries in Asia and Africa.

With utilities such as power and water often stressed past the breaking point, these new urban giants produce air pollution that is different in composition from that of older, better studied midlatitude cities. Also, according to Hanwant Singh (NASA), leader of another of the four Mexican field projects, "these megacities generate atmospheric concentrations of pollutants that routinely exceed international standards. Ultimately, this is very much tied to human health."

When NCAR scientists began planning to study megacity pollution seven years ago, they chose Mexico City because it's extremely polluted yet relatively close to home. Madronich explains, "If we can understand the pollution impacts of Mexico City, we can apply this new knowledge to other urban areas across the globe." The project, named Megacity Impacts on Regional and Global Environments (MIRAGE), attracted investigators from 49 universities (35 in this country). It eventually was joined by three other studies of Mexican air quality on different scales and with different goals (see sidebar - The rest of MILAGRO). The four projects are collectively known as the Megacity Initiative: Local and Global Research Observations, or MILAGRO (Spanish for "miracle"). The coordinated field phase took place throughout the month of March.

A fresh voice from the field

Julien Wang, an undergraduate at The Johns Hopkins University, is a protégé in UCAR's Significant Opportunities in Atmospheric Research and Science (SOARS) program, an initiative to increase the presence of underrepresented groups in the geosciences. She and protégé Marco Orozco (California State University, Los Angeles) had an opportunity to visit the MIRAGE project in the field in Veracruz. Wang reported, "Raj Pandya [director of SOARS] suggested that I take a trip down to Veracruz and take a look at how the data collection and organization is done. I thought that this would give me a better understanding of the overall project as well as the data itself. . . . In Veracruz, Danny McKenna (NCAR) showed me around the Operations Center. I soon met some of the other scientists who worked on putting together the daily weather briefing, which consisted of pollutant concentration forecast graphs and the CO plume over Mexico City for the next few days." Wang visited the C-130 and learned about the sweltering cabin conditions during the flights. "I was very surprised that all the instruments and computers were able to withstand the heat," she commented. Later, she got a chance to fly on the U.S. Forest Service's Twin Otter. "There were five scientists on board, each with complicated instruments hooked up to two or three computers at once," she reported. "The Twin Otter was looking for forest fires in Chiapas. . . . As the plane spiraled down toward each forest fire, the air got very brown. There were five fires and in all of the cases, all I could see was smoke."

Wang concluded, "I have come to appreciate data collection much more now since I have seen how much effort goes into flying the planes, setting up the instruments, and monitoring the plane's flight from the ground. I believe that I would like to be part of this kind of scientific research that involves both field work and technical work. . . . I'm just not sure I would like to fly for eight hours on a turbulent plane with the cabin at 100 degrees Fahrenheit!"

The four simultaneous field projects required a huge commitment of resources as well as people: six aircraft, three ground sites, balloons, sondes, lidars, weather and chemical models, and more. "The logistics were challenging," says Madronich, but he has nothing but praise for the cooperation on the Mexican side of the border. "The aircraft controllers in particular were extremely helpful in coordinating six airplanes buzzing through the air they control."

Madronich and his MIRAGE colleagues focused on four questions: how far the pollution plumes extend, how the constituent chemicals are transformed downwind, how these chemicals affect visibility and regional or even global climate, and how they interact with pollutants from other sources, such as agricultural fires. The main tools they used were the instrumented NSF/NCAR C-130, based in Veracruz; ground-based instruments including GPS radiosondes and tethered balloons, based at the Technical University of Tecamac, some 40 kilometers northeast (downwind) of Mexico City; and satellite observations.

The researchers collected air samples and data on both aerosol particles (including inorganic and organic particles, dust, and soot) and gases (including oxidants, reactive nitrogen, hydroxyl and peroxy radicals, volatile organic compounds, sulfur dioxide, and carbon monoxide). Madronich explains that most field projects focus on either one or the other. "MIRAGE brings them together, allowing us to study interactions between gases and aerosols."

Conditions on the C-130 were especially challenging; during the eight-hour flights, the cabin temperatures reached around 38°C (100°F). Nevertheless, the researchers flew all 88 planned hours.

"I think the campaign was scientifically very successful," Madronich sums up. "The weather was fairly well along the lines of what we had expected. We saw a great mixture of different pollution conditions; we went through urban pollution, but also lots of smoke from regional and agricultural fires, and dust events. On one occasion we followed the pollution plume all the way to the U.S. Texas coast, and on another day all the way to the Yucatan peninsula."

Participants hope to have a preliminary data set available by 1 September. Although no conclusions can be drawn yet, Madronich notes, "The air was more polluted than we thought, and the pollution may have changed the gas-phase chemistry in unexpected ways."

C-130 interior

The C-130 interior being prepared for one of a dozen research flights.


There were 49 university/institutional collaborators with NCAR in MIRAGE. Here, doctoral student Robert Osborn (Texas A&M University) adjusts the Differential Supersaturation Separator, an instrument used to study particles around which clouds can form (cloud condensation nuclei).


On the Web

MIRAGE home page

MILAGRO home page

Julien Wang's complete report


The rest of MILAGRO

In addition to MIRAGE, the MILAGRO campaign included three other experiments.

MCMA-2006, the Mexico City Metropolitan Area–2006 Experiment, focused on air quality in Mexico City and on the scientific data needed to make policy decisions on air pollution control. This experiment was the smallest in area, limited to the Mexico City basin, and the only one that did not employ aircraft. It was led by Luisa Molina (Molina Center for Energy and the Environment) with funding from NSF, the U.S. Department of Energy (DOE), and several Mexican and European research agencies. The project looked at emissions of aerosols, volatile organic compounds, and other gases, and it collected weather and solar radiation data.

MAX-Mex, the Megacity Aerosol Experiment, focused on the properties, transport, and reactions of aerosols in and around Mexico City. It was intermediate in scale between MCMA-2006 and MIRAGE. Under the leadership of Jeffrey Gaffney (Argonne National Laboratory), the program deployed a DOE Gulfstream-1 and a NASA King Air and used instrumentation at all three MILAGRO ground sites to measure aerosol distribution and the extent of pollution plumes. Funding came from DOE.

INTEX-B, the Intercontinental Chemical Transport Experiment–B, is a NASA-funded experiment focusing on long-range transport of pollution, global atmospheric photochemistry, and the effects of aerosols and clouds on radiation and climate. Hanwant Singh is the project leader. INTEX-B researchers made observations with NASA's DC-8 and J-31 aircraft and collected satellite data to support both their project and MIRAGE. The INTEX-B field area was the largest of all the MILAGRO projects, extending from north of Houston to south of Acapulco and from central Mexico well over the Gulf of Mexico. The Mexican field phase took place in March, as did the other MILAGRO experiments, and then the DC-8 and C-130 moved to the Pacific Northwest for observations of pollution reaching North America from Asia. The experiment continued through May 15.


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