Large-Scale Outdoor Air Purification

• By Peter Crona & Flora Crona
• December 10, 2020
• technology

Governments and cities are successful in improving air quality by focusing on the sources of pollution. Fewer cars, more renewable energy, banning hazardous chemicals and greener factories can all have a great impact. The European Environment Agency has written a report [1] in which you can clearly see how emissions have been reduced in the EU-28 countries.

Figure 1. A graph showing emission over time of a number of pollutants. Screenshot is of Page 31 in [1].

Sadly, not everything is getting better everywhere. A recent report [2] states that America’s PM_2.5 concentration has increased by 5.5% between 2016 and 2018. It is also concluded that the increase is associated with 9,700 additional premature deaths during that period.

Figure 2. A graph showing PM_2.5 change in percentage from 2016. Screenshot is of Page 14 in [2].

China declared a war on air pollution in 2014 [3], and it appears to make steady progress, especially in a number of large cities [4].

There’s a lot of work going on to reduce emissions all over the world, but it takes time. Let’s now shift our focus to something that can have a more immediate effect, namely large-scale outdoor air purification.

Examples Around the World

Note that we have not chosen examples based on whether they are symbolic (to raise awareness for example) or actual attempts to improve air quality. Nor have we done a systematic search for examples based on any formal criteria. This blog post is just to be seen as a high-level exploration looking at some examples.

China

There are two particularly famous examples, one smaller which can be found in Beijing, and one very large that resides in Xi’an.

Beijing Smog Tower

See https://www.studioroosegaarde.net/project/smog-free-tower for pictures.

The smog tower in Beijing is 7 meters high, has a power consumption of 1170W and was designed by Studio Roosegaarde and put into usage 2016 [5]. It can clean up to 30,000 cubic meters of air per hour [6]. According to [7] the system does not use filters, but rather ionization. It positively charges particles and makes them adhere to a negatively charged surface.

According to [8], it has been questioned how effective the tower actually is, namely whether it is capable of creating a “bubble” of clean air, so that people can actually benefit from it when being in its vicinity. The article reports that at a 5 meter distance from the tower the PM_2.5 was 89, and at a 100m distance the PM_2.5 was 109. It is also stated that due to the poor results the tower has been renamed to the Smog Alert Tower by the China Forum of Environmental Journalists.

We suppose that it is relative. But given that outdoor air purification is a difficult challenge, we’re actually fairly impressed if indeed the tower reduced the pollution within 5 meters by 20mcg/m³ PM_2.5. Especially given that the smog tower has no filters. In our opinion, it does its job well in that case. Even if we also naturally wish that it could do even more.

Studio Roosegaarde, the company behind the smog tower in Beijing, has also created a bicycle with a built in air purifier for Ofo, a bicycle sharing company in Beijing. Based on the images, it looks like they simply put a filter at the front of the bicycle, tilted so that purified air is led towards the rider’s head [9].

Xi’an Solar-Assisted Large-Scale Cleaning System (SALSCS)

See https://www.antarcticajournal.com/air-purifier-in-xian-china/ for pictures, they have a pretty good explanatory picture.

The Xi’an tower is likely the world’s largest air purifier. It is in fact a scaled down version of what the designer would like to build next, which would be a 500 meter high tower [10]. There are conflicting numbers regarding the height. Some sources put it at 100 meters [10], whereas others put it at 60 meters [11]. Given that the paper [11] is co-authored by the stated head of research for the tower project, Cao Junji, we put our bets on that number. In either case, it is impressive. The paper states that the tower was finished in 2016.

We encourage you to read [11] if you are interested in more details. It is also a brief introduction to SALSCS if you are interested in the concept. Below follows some information from this paper.

Unlike the smaller smog tower in Beijing, this tower uses no fans (well, it actually has some, in case solar energy is not enough) nor ionization. It uses solar energy to heat air at the base of the tower, and then the tower acts as a chimney through which the heated air will flow. The air flow is used to push the air through filters in order to capture particles.

It is built with four air inlets to the tower, so that different filters can be used for the different sections (north, south, east and west). The western section uses a filter manufactured by Donaldson Company, Inc. The eastern section uses a domestic filter. The northern section uses a high air flow (HAF) filter manufactured by 3M. The south section uses no filter, or did not do so during the testing at least, for testing purposes.

There are rolling doors that can be used to control which sections that are open. I.e. to control through which filters air can pass.

India

India has installed a smaller tower in Delhi and there’s a plan to build a new one. There’s also another interesting example where a company used radio waves to purify air during a marathon. India has also tried using water cannons.

Smog Tower at Lajpat Nagar Market

See picture over at https://healthpolicy-watch.news/indian-clean-air-advocates-bitterly-critical-of-supreme-court-decision-pushing-questionable-smog-tower-technology/, which is also an interesting, and critical, article

A fairly small tower meant to purify the air for people visting the Lajpat Nagar market. It is hard to find much information about it, but [12] and [13] states that the tower is 6 meter height and can purify up to 600,000 cubic meters of air, or a circumference area of up to 750 meter. Furthermore, it is claimed to remove nearly 80% of PM_2.5 and PM₁₀. We failed to find any details about what kind of filter it is using.

Smog Tower at Connaught Place

It is hard to find details about this one, and it is not built yet. An article from October 9th 2020 claims the goal is to build it within 10 months [14]. We also could not find reports about the technology they plan to use. It appears that it will suck in polluted air from the top and blow it out at the bottom. It is mentioned that it might use HEPA filters of class H14, as well as activated carbon and a pre-filter [14].

According to [15], the height is not decided on, but 20 meter is mentioned. The article also makes a somewhat surprising claim that it is expected to reduce the particulate matter by 50% 200 meters against the wind and 1km in the direction of the wind.

We wouldn’t trust the details too much, the sources are not “official” and the statements are vague and eyebrow-raising.

Technologies

The examples above have mainly used various types of filters or ionization. However, there are some other interesting technologies out there.

Pulsed Radio Frequency Energy

This example in India sticks out a bit. According to [16], radio waves were used during a half-marathon to reduce air pollution. However, the article also discusses that there’s a lot of controversy surrounding both the usage of the technology during the race and its actual effect. Also safety concerns are raised.

Using this technology at such a large-scale doesn’t seem to be proven efficient. With that said, it would be quite wonderful if the technology does work and is safe. The manufacturer has a website where they describe the technology [17].

TiO₂ Coatings

TiO₂ is commonly used for photocatalytic air purification. It has also been used on buildings [18] as well as roads to break down pollutants such as NO_x [19]. The idea is indeed promising.

Imagine if we were able to paint roads or buildings to effectively purify the air. There’s a lot of research looking at how to improve TiO2 based photocatalysis, for instance [20] and [21] are two examples of looking at how to expand the wavelengths it works with, so that it not only works with UV light, but also visible light. This has a lot of potential for aiding in the design of air purifiers for VOC, NO_x and other gases that for instance HEPA filters are ineffective against. And let’s hope it can also raise its efficiency when used outdoors.

Air Purifying Facades

See pictures over at https://purcity.com/products/gaps/

An interesting concept is turning buildings into air purifiers. GapS [22] is a product designed to be used as a double facade. According to the manufacturer, you put it outside on your existing facade. It has sensors that will release collected rain water to self-clean. The double facade has a gap and an air flow is created through a chimney effect.

We did not find any information about GapS except that from the manufacturer. The manufacturer also claims that GapS can trap CO₂, VOC and PM_2.5/PM₁₀, which we found a bit surprising. We’re not convinced due to the lack of proof of their claims, and we could not find any information about existing installations. But, the idea is interesting and not entirely unique. For example, different facades are discussed in [23].

Water Cannons

See a picture over at http://www.china.org.cn/environment/2014-05/13/content_32367666.htm

In an attempt to combat air pollution water cannons have been used in India [24] as well as Xi’an [25] (yepp, the place with the big smog tower). However, [25] criticizes it by claiming that it is not effective against more than dust from construction sites. In particular, it is claimed to not be effective against smaller particles, such as those making up PM_2.5.

Conclusion

I hope you enjoyed this little exploration looking at large-scale air purification examples and technologies. It is by no means exhaustive, but just meant to give you a glimpse of some ideas out there.

Perhaps you, just like we, think that it makes more sense to invest in reducing air pollution at its sources. It does indeed seem that most attempts to purify the air outdoors, in particular using smog towers, are criticized. But remember to keep an open mind, and if you wish to criticize someone, do it with the intention to help them. We win nothing through hate and if nobody tries anything the majority think is crazy, we won’t get far.

References