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The City and the Sky


The City and the Sky

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Rush hour on Paris’ Champs Elysées means a constant stream of traffic.

And, according to Matthias Beekmann of France’s biggest scientific research group, the exhaust fumes are doing more damage to the city’s air than any other man-made factor.

“Traffic is the biggest source of pollution in Paris,” he says.

One in five French people live in the Paris region, putting on a list of 20 so-called ‘megacities.’

The French capital is now the focus of a broad European Union research project to trace the impact on the atmosphere and climate of the pollution above our streets.

Beekmann explains its goals: “We want to find out what happens to pollution not just on an urban scale, but on a much wider scale. What we’re interested in is the impact of emissions, or pollution, of one city on a continental scale or even further, on a global scale.”

To understand city pollution, first you have to head out of town. On the road we joined Frank Drewnick, a physicist at the Max Planck Institute for Chemistry who told us what his role in the project involves:

“We’re driving with our mobile lab, and as we’re driving we’re measuring the composition of the air, the particles and gases it contains,” he said.

In a field on the north eastern edge of Paris, Drewnick is measuring the background air pollution flowing into the city from other parts of Europe.

The team need an isolated spot, and even park their generator wagon 100 metres away to avoid picking up particles from the engine.

Now they can take a clear reading of particles and gases that are blown here from far away.

“The particle emissions are coming from the Benelux countries, that is the nearest source, but a big part is coming from eastern Europe where a lot of coal-fired power plants are emitting a lot of sulphur dioxide into the air,” explains Drewnick.

Back in the city, the project is in full swing.

Matthias Beekmann and project coordinator Alexander Baklanov are in the middle of a winter session of data gathering.

They will compare the readings with pollution data gathered over a one month period last summer.

Instruments on the roof of a lab in central Paris are operating 24 hours a day, tracking what are known as ‘organic aerosols,’ as Beekmann explains:

“Aerosol is dust, very fine particles which float in the air and which man breathes in. That is why it’s bad for one’s health. Organic means this aerosol is made up of chemical elements like carbon, oxygen and hydrogen.”

An Irish team has joined the massive one-month field work campaign.

Their spectrometer identifies pollution particles from cars, heating systems and wood fires. When a strong westerly wind blows they even pick up particles of sea salt from the Atlantic.

Their research is on a very local scale, and the changes in air quality reflect the rhythms of daily life. Robert Healy, a post-doctoral researcher at University College Cork is on the team. He told us:

“The cleanest period is always 3-4 o’clock in the morning, because you don’t have much activity, people aren’t doing anything, so you see a large spike at 7 o’clock in the morning for the traffic rush and then a lull for the mid afternoon and then a big spike again for 5 or 6.”

The particles and gases emitted by cars or factories stay in the air, some for days, others for hundreds of years.

And they can have an impact well beyond their immediate surroundings, says Beekmann:

“A particle emitted by a car for example can be taken up vertically into the atmosphere, into the surrounding air and rise up to hundreds of metres, even a kilometer depending on the season. There is more in summer than in winter. Then it can be carried along by the wind for hundreds or even thousands of kilometres.”

Climate expert Bill Collins from the UK’s Met Office in south-west England is analysing how megacities affect air quality and climate on a global scale.

The pollution he is studying doesn’t respect borders. As Collins points out, it can be transported huge distances:

“So we have a plot here of the total pollutant levels of this particular compound called ozone, and you can see how the weather systems are blowing it around the world.

“We’ve combined three megacities here together, New York, Washington and Boston, and you can see there’s a nice big red plume – well I say nice, it’s obviously not nice for the people experiencing it – that gets blown across the Atlantic by the strong westerly winds that we experience over the north Atlantic quite frequently, and this is slamming into the UK and western Europe.”

While ozone has negative implications for human and plant health, other pollution from megacities can influence the climate.

“There are two aspects pollution can have on climate, one is the long lived greenhouse effect, everyone is familiar with carbon dioxide that lasts for centuries in the atmosphere, so your pollution coming out of your car exhaust will last and it will affect the climate, on a long time scale.”

“But something I’m more interested in is the shorter lived reactive components. One of these is sulphur dioxide, which reacts in the atmosphere quite quickly to make something we call sulphate aerosols, that are basically white bright particles that reflect sunlight back, and so they cool the globe, they cool the atmosphere,” says Collins.

He wants to find out how much that cooling effect offsets CO2 warming in the short term.

While urban pollution has been studied for decades, the attempt to quantify and then model how these gases and particles interact with the climate is an ambitious new goal, as Alexander Baklanov explains:

“From the scientific point of view the scale of research, and the scale of all these processes which we analyse is also quite broad, and in this is probably one of the unique aspects of this project, that for the first time we try to study the processes starting from the scale of the street canyon and going further to the scale of the megacity, the regional scale and changes on the global scale.”

At their current population levels, the world’s 34 largest cities alone will make the planet around a quarter of a degree warmer by the end of this century.

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