Mexico City the model megacity for pollution study

As one of the world's biggest urban sprawls and home to some of the planet's worst pollution Mexico City makes the ideal subject for a science experiment on a grand scale.

A team from the USA’s National Centre for Atmospheric Research (NCAR) will be leading one of the most complex research projects ever to look at atmospheric chemistry to track the changes in air pollution as it drifts downwind from the vast city.

It will investigate changes in the chemical make-up of the pollution as it travels away from the city and try to assess its impact on regional and global air quality, eco-systems and the climate.

The results will be used to predict the combined impact of fast-growing tropical cities in the developing world.

“Mexico City’s pollution probably doesn’t have a global impact, but all urban areas together do, and the world is urbanizing,” said NCAR scientist Sasha Madronich, one of the project’s principal investigators.

“If we can understand the pollution impacts of Mexico City, we can apply this new knowledge to other urban areas across the globe.”

Mexico City has been chosen as it is representative of the fast-growing megacities of the developing world, being situated in the tropics, reliant on coal and wood for fuel and dirty diesel and petrol engines for transport.

Most existing models for studying the pollution impact of cities are geared up for the cleaner cities of industrialised nations and are of limited use when predicting the impact of the urban areas of emerging economic giants like Mexico, Brazil, India and China.

The project, Megacity Impacts on Regional and Global Environments, or MIRAGE will find out how far the pollution travels and look at how they affect not only air quality but also visibility and how they interact with pollution from other sources, such as agriculture or forest fires.

“We’re not looking so much at pollution inside the city because that’s already fairly well known,” said Mr Madronich.

“We’re looking at the outflow. 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.”

Because air pollution is complicated, both chemically and physically, and evolves over time and distance, scientists have traditionally faced difficulty in quantifying its components.

The MIRAGE team will use aircraft, ground stations and satellite observations to gather data on how Mexico City’s air pollution ages as it disperses in the first hours and days after emission.

The study is also unusual in that it will be looking at both particulate aerosol matter and polluting gases, rather than focusing on one or the other.

“In the past there have been air campaigns during which researchers have made lots of aerosol measurements, and other ones during which they’ve emphasized gas measurements,” said Mr Madronich.

“The uniqueness of MIRAGE is that it brings them together, allowing us to study interactions between gases and aerosols.

“The lifetime of organic aerosols may be longer than climate modellers have thought, and this could have a huge effect on climate.”

by Sam Bond

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