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What happens to air pollution as it heads downwind from its source?

By 2015, nearly half of the world’s population will live in urban centers in developing nations. Air pollution exported from these megacities will adversely affect human health, agriculture, natural ecosystems, regional visibility, and even the global environment.

mexico city

Mexico City is among the world's most polluted cities. (Photo by Julio Etchart)

While researchers acknowledge the importance of urban air pollution at the regional and global level, they have not been able to quantify it well. Pollution is very complicated, both chemically and physically, and it evolves over time and distance.

To get a better understanding of what happens to air pollution after it leaves a city, NCAR scientists and their Mexican counterparts are launching a major field campaign as part of the Megacity Impacts on Regional and Global Environments project (MIRAGE).

Their objective is to investigate the chemical transformation of pollutants that flow from a large metropolitan area to surrounding regions and the globe as a whole.

“MIRAGE research results may be the first step in understanding the complexity of the physical and chemical processes that occur when pollutants leave one of the major tropical cities in the world,” says Mireya Moya, a scientist at the National Autonomous University of Mexico and one of the project researchers. “This will have socioeconomic benefits for Mexico City as well as for other societies.”

Testing the air

Sasha Madronich

NCAR scientist Sasha Madronich (Photo by Carlye Calvin, ©UCAR.)

In February and March 2006, MIRAGE scientists will survey the air downwind of Mexico City, using the NSF/NCAR C-130aircraft as well as ground-based instruments and satellite observations. NCAR is collaborating with scientists from U.S. universities and more than a dozen Mexican partners, including the National Autonomous University of Mexico, the Iberian-American University of Mexico, other state universities, national and municipal government agencies, and the country’s national oil company.

The scientists specifically want to understand 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 NCAR scientist 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. If we can understand this in the context of Mexico City, we can apply it to other urban areas around the world.”

By observing ozone production, aerosol evolution, nitrogen oxide budgets, and oxidized hydrocarbons, the scientists hope to answer three broad questions:

• 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?

quoteThey’ll also ask some questions specific to the region. “There could literally be weather modification downwind of the city from the pollution, so the project will try to find that out,” Madronich says. In addition, MIRAGE will look at the extent to which local biomass burning in central Mexico contributes to poor air quality.

Mexico City is an ideal setting for MIRAGE because it is one of the world’s largest cities and is situated in the tropics, as are most of the world’s fast-growing megacities. Its emissions are apparent for several hundred miles downwind in a surrounding region that is otherwise only moderately polluted. A network of ground-based air quality measurements has been in place for nearly two decades, and public and private labs currently conduct considerable air quality research. Mexico City also offers good infrastructure and logistics for carrying out observations that require aircraft.

“This type of collaboration of scientists from different countries should benefit both sides,” Moya says. “Mexican institutions cannot perform this research without the support of foreign institutions, and scientists in both countries will learn about the outflow of pollutants from an urban area.”



Researchers are developing new approaches to computer modeling to focus on the regional impacts of a changing climate. (Courtesy Weather Research and Forecasting, NCAR)

How will climate change affect the United States and Canada by the middle of this century? An international team of scientists will seek answers by simulating North American climate from 2040 to 2070 in unusually fine detail.

Most projections of future conditions rely on global climate models run on supercomputers that, despite their sophistication, lack the detail to simulate behavior within a state or region. Led by NCAR’s Linda Mearns, the North American Regional Climate Change Assessment Program (NARCCAP) will use an ensemble of global climate models and high-resolution regional climate models to produce simulations with about triple the resolution of most projections of future climate.

The combination of tools will allow scientists to incorporate relatively small topographical features, such as mountain ranges, lakes, and complex coastlines, that can have significant effects on local and regional climate. An important research benefit of the effort, which is designed to complement similar projects in Europe and South America, will be the ability to compare the results of fine-scale with coarser-scale modeling to more clearly determine the added value of high-resolution projections of future climate.

NCAR scientists will work with colleagues at U.S. universities and laboratories, the University of Quebec and the Ouranos Consortium in Montreal, and Britain’s Hadley Centre for Climate Prediction and Research. The project is expected to spawn additional international collaborations as researchers continue to fine-tune projections of the impacts of climate change on North America.

More Collaborations in the Americas:

Looking at Tropical Pollution

Thinking Continentally, Forecasting Locally

Training Forecasters in Latin America

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