Vegetation has an incredible ability to absorb pollutants. Trees like planets, beech and maple have the potential to absorb about 20 kg CO² per day. Respectively, 7 tonnes/year. This corresponds to the annual CO2 emissions from one Dane (2014).
The closer to the pollutant source the plant is the better ((Vos et al., 2013; Pugh et al., 2012). Give the street a dense green base of herbaceous plants and bushes at the same height as the exhaust pipes, and the vegetation will work to cleanse the air. We suggest Benved (Euonymus) and Firewood (Pyracantha), which are particularly good at the absorption of NOx and Ozone, and common Vedbend (Hedera Helix) that are good for absorption of CO2 and for oxygen production and fire tower.
Since the bushes 'only' can store 40% of the landfill while 60% are washed by rain, underlying vegetation is also required. We suggest bioswales: long smells of grass, sand, and soil that suck bad air while collecting and filtering rainwater, so there is less pressure on the treatment plants. The reduction of metal concentrations in bioswales ranges from 20-60%. Soil particles will attract and bind much of the remaining concentrations of the metals. Bioswales also function as effective buffers between cars and cyclists, providing an additional angle of commuter safety.
Another advice is to secure that the polluted air is removed from breathing height. Despite natural countermeasures, there will still be polluted air in the street – the trick is to steer it away from pedestrian level. A big part of H.C. Andersen Boulevard has rows of houses on both sides, and they act like bumpers in a bowling alley, preventing exhaust-laden air from leaving the street level.
To lift pollutants above the roofs, we must create favorable channels for natural breezes, ensuring a good wind corridor away from the street level. Just like bushes, trees are efficient air purifiers, because they absorb carbon dioxide, release oxygen, moisture the air and filter it from pollutants. But trees can also hurt the wind flow if they are poorly positioned because dense treetops can trap unhealthy air in the street.
The solution is to plant porous, relatively transparent trees so that the air can penetrate the branches enabling filtration without obstructing ventilation channels (Tiwary et al., 2005). We suggest common Platanus (Platanus x hispanica) and Maple (Acer Pseudoplatan), particularly good at the absorption of CO2, and NOx and ozone beech (Fagus sylvatica) and Avnbók (Carpinus betulus), which absorbs NOx and ozone.
Give an advantage to efficient vehicles. H. C. Andersen’s Boulevard is a major thoroughfare for Copenhagen traffic, but we can make the road less crowded by making it advantageous to choose a space-efficient vehicle. By eliminating two lanes out of the six, each estimated to be around three meters in width, we will free 600 m2 of extra space on 100 m road. This could become fast lanes for buses, carpools, electric cars or perhaps wider bicycle paths. If we cannot get the cars off the street entirely, we can at least try to reduce the density of daily traffic by making it more attractive to choose an alternative. And if we understand the users of this street, like where they are coming from and to where they are going, we can create tailored custom solutions for this street in particular -- But these alternatives need space.
The air pollution on H. C. Andersen’s Boulevard will not be solved overnight, but with the help of clever urban design, we can solve parts of the associated problems without necessarily redirecting traffic.