WEBVTT

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They are considered to be the green lung of the city.

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Street trees are cool islands in the inner cities that are heating up

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due to climate change.

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By evaporating over their leaves, they lower the ambient temperature.

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Through the leafy roof, they cast shadows in the summer heat.

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But trees do not always and under all circumstances promote the city

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climate.

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They can also be the reason that the pollutants from the traffic can

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not be properly transported from the streets.

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This is the surprising result of simulations in the wind tunnel at the

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Karlsruher Institute of Technology.

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Another surprise, hedges in heavily traveled streets are better for

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air quality than narrow trees.

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In cities, road traffic is usually the main source of air pollutants.

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There are already a lot of investigations.

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However, they have always examined the city or the street without

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vegetation, without trees.

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But if you go for a walk through our city and open your eyes, then you

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see that there are some street trains that have a lot of trees, tree

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groups or even alleys, rows of trees.

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And then the question arose, what effect does such a row of trees have

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on the wind conditions in the street?

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And of course, this is also connected with the transport of these

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traffic exhaust gases, which are released in the street.

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They have to be transported out somehow.

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Otherwise, these concentrations would continue to increase and the air

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quality would be unbearably bad.

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Dr. Christoph Gromke experiments at the Laboratory for Building and

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Environmental Aerodynamics of the Karlsruher Institute of Technology.

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There are several wind tunnels available to simulate the load of the

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wind on buildings, but the spread of pollutants in built areas is also

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one of the research tasks.

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For the question of what street trees do with the air quality, a 1 to

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100 scale model was set up in the wind tunnel.

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We have a typical urban street gorge in the inner city area of ​​a

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city.

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Let's say it's about 200 meters long.

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On the right and left we now have closed rows of houses from buildings

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that are about four or five floors high.

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This is the model we are depicting.

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And we are now examining the pollutants released by the traffic on the

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street in the street area, how they spread in this street gorge or how

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they are now being transported.

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The background is that the air pollutant concentrations should not be

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too high.

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A crucial factor is the formation of the trees in the model.

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We understand trees as porous bodies that can be permeated.

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Imagine a treetop, there is the trunk, then there are many branches,

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branches and then there are leaves.

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From the outside, this often has the shape of a ball or an ellipsoid.

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And inside there are still many air spaces, so it's a porous body.

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The crucial thing for us now is what resistance this tree or this

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porous body has to an air flow.

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Every body and also porous bodies practically slow down the air in a

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certain way.

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A part of the air goes over the outside, goes around it.

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A part of the air just flows through.

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In sum, this leads to the slowdown or reverse of the flow.

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And that's exactly what we're trying to reproduce in our wind tunnel.

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Then the model is exposed to air flows from different directions in

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the wind tunnel.

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The pollutants of the traffic are introduced as measurable trace gases

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into the miniaturized street space.

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We have a trace gas that we introduce into our model, where the

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traffic releases the exhaust gas.

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This trace gas is released on the ground.

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This is then distributed with the wind that we now put on it.

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It thins out, it is transported.

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Everywhere in our model we have measuring points.

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Sometimes that's a few tens, sometimes a few hundreds.

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Measuring points where we then measure the concentration of our trace

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gas.

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The result was a surprise.

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We have about 200 meters of road and then buildings on the right and

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left, i.e.

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closed houses.

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The natural wind that blows through the street gorge thins out and

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transports it.

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Trees are porous bodies that now exert resistance.

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This also influences and slows down the flow.

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We were able to prove that these air flows in street gorges with

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alleys, with rows of trees, partially block the flow, slow down the

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flow.

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That means we have less ventilation.

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We have less dilution of the pollutants.

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And that's why there are higher concentrations in the street space.

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The effect is getting stronger.

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The denser the alley trees are, the more their crowns connect.

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The emergence of the natural wind that now blows over the roofs of the

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street gorge and the street space itself takes place at this

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interface, at this transition area, in the roof area.

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The more material there is now, e.g.

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tree crowns, the more difficult it becomes for the wind to penetrate

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the street gorge and to go out again.

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That means with the same emission strength we have higher

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concentrations in the street space.

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The research team was even more surprised when they repeated the

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attempt in the wind tunnel with hedges in the middle of the street.

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Imagine you have a street-parallel hedge that now goes over the entire

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street gorge.

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It is now placed in the middle between the roadways.

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And we simply investigated how such a hedge affects the concentrations

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of traffic emissions in the street space.

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And we found that hedges, especially in the ground-near area, i.e.

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where the concentrations are released, lead to significantly lower

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concentrations.

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Especially in these areas in the street space, where we would have

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found the highest concentrations without the hedges.

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Scientists have not yet been able to investigate why this is the case.

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But there are hypotheses.

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This hedge on the ground directs the wind and thus changes the vortex

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system near the ground.

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This is more local on the ground, not in the entire street gorge.

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But it changes where the exhaust gases are released and where the

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passers -by stop.

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This means that fewer pollutants are transported to the passers-by at

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least on one side.

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Accordingly, the recommendation falls to those responsible for city

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planning.

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Especially in heavily trafficked streets, where many emissions are

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released, you can say that hedges have a favorable effect.

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And you have to consider that trees block the natural ventilation.

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On the other hand, which climatic benefits do trees have in view of

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the temperature in the street space?

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The whole picture, how trees not only affect the air quality, but also

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the temperatures in the urban heat centers, will be the subject of

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another experiment in the wind tunnel of the Laboratory for

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Environmental Aerodynamics.

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It will start soon.

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In this we will examine the combined effects of vegetation in the

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street space on air quality and climatic parameters.

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So on the heat balance in the street space, but also on the building

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energy demand.

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Trees, because they cast shadows, can lead to the fact that they do

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not turn on the air conditioning in the summer.

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Conversely, it can lead to trees, due to the shading, that the heating

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period is a little longer.

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Of course, this must be calculated and calculated over the cycle of a

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whole year.

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Stefan Fuchs, Karlsruhe Institute for Technology