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Pollution and hot weather make unhealthy cocktail

Xavier Querol, deputy director of the Institute of Environmental Assessment and Water Research, is an expert on pollution detection. In relation to the current high levels of air pollution affecting Barcelona, his position is clear: "Sometimes it seems that the weather is the culprit, but pollution is mainly caused by cars"

DAVID SEGARRA | FEBRUARY 11TH, 2011

It is no secret that Barcelona suffers from occasional bouts of high levels of air pollution. But why does it happen? What are the factors behind these episodes? We talked to Xavier Querol, deputy director of the Institute of Environmental Assessment and Water Research of Spain’s National Research Council and expert on the detection of pollutants and their dispersion routes. Querol is clear: "Sometimes it seems that the weather is to blame for the pollution episodes in Barcelona. But pollution is mainly caused by cars."

Are there really so many cars in Barcelona?
In Barcelona, there are around 6,000 vehicles per square kilometer. This is a very high figure. In Amsterdam there are 1,000, and in Madrid 2,200, according to estimates. In those cities there are no streets with the traffic of Barcelona’s main thoroughfares such as Aragó, Consell de Cent, Mallorca, Aribau, or Muntaner.

Does it have something to do with the cities’ different layouts?
Paseo de la Castellana in Madrid is lined by short buildings and the pollutants disperse more easily. When compared to its equivalent in Barcelona, Avinguda Diagonal is surrounded by seven-story buildings which trap the pollutants in a more concentrated area.

Not all cars pollute the same.
No. One of our problems is that 45% of the vehicles on Barcelona’s streets are diesel. And it is a percentage that is on the rise: 70% of vehicles registered in 2010 were diesel. These are cars that were sold as being environmentally friendly because they emit less C02 per kilometer. But instead, they emit more nitrogen oxide and more particles.

Diesel vehicles emit less CO2, but more nitrogen oxide off more air particles

What role does Barcelona’s geography play?
It helps to concentrate the pollutants because the surrounding hills of Collserola make it difficult for the northerly winds to ventilate the atmosphere of the city. Furthermore, Barcelona does not get much rain and is very sunny, which is the worst combination from the point of view of air pollution.

Our climate doesn’t help, either.
The problem is always present, but when there is an anticyclone it is worse. The anticyclone often means there is a temperature inversion, the upper air is warmer and keeps cold air from rising, like a hat that prevents the contaminated air from escaping. When the anticyclone is over, the winds from the north clean the atmosphere. Anticyclones are common from October to May.

And in summer?
In summer, the main problem is recirculation. In the morning, the air moves in from the sea to the land. This air rises through the city, collecting pollutants, bounces of Collserola and rises back over the city at a higher altitude. The northwest wind, which blows in Barcelona when an anticyclone is over the Azores, sends this air back down toward the sea. Therefore, the air that re-enters the city the following day by the sea breeze is the same air as the day before! These episodes can last ten days.

What do we mean when we talk about pollutants?
Basically, there are three kinds that exceed the public health limits for the population: particulate matter, nitrogen oxide and ozone.

Let's start with ozone.
Ozone is a powerful oxidant that affects the respiratory system. In the case of Barcelona, it is produced from nitrogen oxide when it is transported north to the plains of Vic, a frequent movement particularly between the months of April to August. Along the way, the nitrogen oxide join together with other compounds, for instance, organic compounds released by forests, and this cocktail produces ozone 50 or 100 kilometers away from the city.

And particles?
Fifty percent of airborne particles are produced by automobiles. There are two types: large, coarse particles that can come, for example, from the abrasive wear of the brakes on vehicles. They are breathed in and even though they do not cause diseases, they exacerbate existing ones.

The smaller particles are a bigger problem, right?
Exactly. Diesel vehicles especially produce ultra-fine particles that can penetrate the pulmonary alveoli and promote cardiovascular disease since they can arrive to the bloodstream.

How small are they?
If a pollen grain is a football stadium, an ultra-fine particle would be like putting a tractor wheel in the middle. Remember that a pollen grain is tens microns big, and a thousand microns make a millimeter.

And, lastly, we have nitrogen oxide.
There is much debate about whether or not these contaminants have an impact, but we do know that if you increase NO2, you also see a jump in the amount of particulate matter. It is an example of the synergies that exist between different pollutants.

What solutions do you see?
The most important solutions are not technological ones but rather those which are related to our transportation habits. There are two types of people. We must convince those who live and work in Barcelona to always take public transport. The existing network is superb. And for those who live in the suburbs and have to commute each day, we must give them incentives to take public transport over the car.

And how can this be done?
Well, with a very good commuter rail service. Or with a bus service that has a bus lane on the highway, so that buses can keep to their schedule even when there are traffic jams.

And the person who lives in a neighborhood on the outskirts of Mollet and works in a polygon located near El Prat?
We can make a system of HOV (High Occupancy Vehicle) lanes equipped with a camera which permits cars with three or more occupants to take the equivalent of a bus lane.

Thanks to the Polytechnic University of Catalonia, such camera systems have been applied to HOV lanes in the Tunnels toll Vallvidrera.
And we can still do many more things. In 72 European cities, including Berlin, London, Vienna and Prague, the most heavily polluting car models are not allowed in the city center. Here we could do the same.

Clearly, there is not a lack of measures to take.
There are also measures that we call necessary but not essential, such as washing the streets, incorporating particulate filters on buses, controlling dust from constructions sites or lowering the speed limit. Each of these measures can only reduce pollution causes between 1- 2% each, and therefore individually they appear to be irrelevant. But if we implement all these changes together, we could reduce pollution by 15-30%, and that would be very important.

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