Smog formed by particulate matter and pollution is a serious problem in many growing urban areas. In the future, vertical forests and trees planted on building facades may be an important tool to combat pollution. Forest cities are under development in China, and active plant walls derived from space technology increase the air purification power of plants a hundredfold.

Air pollution such as oxides of nitrogen and sulfur, ground-level ozone and particulate matter is the single biggest environmental health risk in the world. Traffic causes a lot of the pollution, due to combustion emissions and road wear.

Smog development over China in December 2016. Image: NASA.

In October 2013, an especially dense and heavy fog of hazardous smog developed over northern China. It settled as a brown, dirty cover over Harbin, a city with eleven million residents. Visibility was reduced to merely tens of meters. Schools were closed, bus services cancelled, airplanes could not take off. Many people resorted to wearing mouth covers, even indoors. At their peak, the pollution rates were 50 times the WHO thresholds.

This was not a one-time event, but a recurring phenomenon that plagues many parts of East Asia, including the Chinese capital, Beijing, where the surrounding mountains often prevent wind from dispersing the smog. The situation worsens in the winter, when coal combustion intensifies to provide district heating, more cars are on out on the roads, and there is less precipitation to wash away floating particulate matter.

The smog consists of emission gases combining with aerosols, floating microparticles from soot and pollutants. Combustion is the main source: vehicle engines, power plants and household coal stoves. The soot particles contribute to global warming as well, and air pollution is estimated to cause 18000 deaths around the world every day – four times as many as the number of road casualties. In Africa, air pollution kills three times as many people as malnourishment.

Preventive measures, purification

In Europa and the USA, the situation has improved in many smog-ridden cities due to improved exhaust- and flue gas filtering and a transition to cleaner energy sources. In China, Northern India and other parts of Asia, where coal is still widely used, the issue is more pressing. A number of preventive measures have been taken, such as emission regulations and a push for electric vehicles and renewable energy sources. There are also attempts being made to clean up the pollution already in the air.

The Dutch company Invinity Group has designed a kind of smog vacuum cleaner, which can be placed on rooftops. It takes in air from a 300 m radius, and the eight meter long industrial filter is capable of processing 800 000 cubic meters of air every hour, removing the bulk of the particles.

Another method under development is to paint rooftops and building exteriors with titanium dioxide paint. The titanium dioxide, when exposed to UV light from the sun, absorbs NOx gases and degrades them to harmless nitrates that are washed away by the rain.

Green, natural filters

Planting trees and other plants is also a method that shows a lot of promise. Greenery contributes to lower air temperature, saving energy and reducing the buildup of ground-level ozone. Trees and plants also act like natural filters, absorbing pollution through their leaf pores, binding both carbon dioxide and smog. The roots and the microorganisms bound to them also play an important role in this process.

Green roofs is an established concept (read more in the article ”Green roofs for a healthier urban environment”); living vegetation planted on the roof insulates the building and traps hazardous particles. In the summer of 2016, Vasakronan planted the first vertical garden in Stockholm, Sweden, with 2 500 plants and 40 different species.

Vertical forests in cities

The vertical forest planned for Nanjing. Illustration: Stefano Boeri.

The Bosco Verticale skyscrapers in Milan, Italy have been called the world’s first vertical forest. The buildings, designed by the architect studio Stefano Boeri, are embedded with 21 000 plants. Others have followed suit, and it was recently announced that the same company will construct two green highrises in Nanjing, China. The two buildings – 108 and 200 meters tall, respectively – are to be finished in late 2018.

Nanjing’s vertical forest is the first piece of a larger puzzle. Sustainable mini cities, consisting of 100-200 buildings covered with plants, are already being designed. The first of these forest cities is planned to be erected within Liuzhou, a city with a million inhabitants, hopefully before 2020. Shijizhuang – one of the Chinese cities where air pollution is most severe – is next in line. These projects may offer a first glimpse of what the sustainable cities of the future may look like.

Where nature meets technology

There is an interesting twist to the greenery concept; when nature and technology are combined, they can reinforce each other and provide even more powerful solutions. The Finnish greentech company Naava is one example proving the point. The company has developed an active plant wall, that improves the air purifying power of the plants more than a hundredfold: a small plant wall containing 69 plants can have an effect on air quality equivalent to more than 8000 regular potted plants. The technology originally derives from NASA research with space stations in mind. One of the insights was that 98 percent of the purification happens in the plant root bacterial communities. The wall has a built-in fan system to increase air circulation, and the plants are potted in a customized substrate with activated carbon. The system is for indoor use, and is delivered in modules. Lighting, air circulation and watering is handled by an automated, sensor-controlled system.

Naava has high ambitions: the goal is to provide air purification for one billion people by 2025. The company is looking to expand to the Asian market and to develop outdoor solutions. Currently, a production facility is under construction on the East Coast of the USA. Others are exploring the same principles; the Norrköping based company Vertical Plants System, for instance, which is developing plant walls for public spaces.

Air pollution is one of the great challenges of urbanisation, but better preemptive measures and active cleaning is creating a brighter outlook. Smart use of greenery in urban settings is a promising and cost efficient way to handle particulate pollution, and when nature and technology combine, efficiency improves. Perhaps the megacities of the future will turn out to be smog-free sustainable developments, covered in high-absorbing greenery.

The article was published in February 2017.