TRAPS
“WE LIVE SUBMERGED
AT THE BOTTOM OF
AN OCEAN OF AIR.”
Evangelista Torricelli, 1644
Hackney Wick is bordered by a road which seeps into its air. This burning hoop is an atmospheric device whose externalities are unaccounted for in its design. In order to modify this system it is necessary to design adequate traps for these free-floating effects. Designer traps for a designer atmosphere.
A collage of SEM images for airborne particulates. (A) General classification of airborne particulates; (B) particulates with seeds-coating composite morphology; (C) sulfate particulates with different morphologies. Source: Li, C., Ding, M., Yang, Y., Zhang, P., Li, Y., Wang, Y., Huang, L., Yang, P., Wang, M., Sha, X., Xu, Y., Guo, C. and Shan, Z. (2016). Portrait and Classification of Individual Haze Particulates. Journal of Environmental Protection, 7, 1355-1379. doi: 10.4236/jep.2016.710118.
Size comparison chart, showing soot and pollen (examples of PM2.5 and PM10, respectively) as compared to other particles.
PM2.5 and PM10 size in relation to the diameter of a human hair (0.6mm).
Plant foliage is the biggest natural particulate matter sink
on Earth. On a global scale, foliage spans over 400 million square kilometres, almost 80% of the planet’s total surface area.1 This massive surface, combined with
leaves’ suspension in the air, provides a highly suitable contact surface for the
deposition of atmospheric particulate matter – airborne dust containing heavy
metals and other pollutants.
Tree planting is one of the most effective methods toward combatting anthropogenic PM concentrations in urban environments. Biological remediation – the planting of vegetated green belts – can offer an efficient and sustainable method of reducing vehicular or industrial PM emissions in areas where the cessation or elimination of such emissions is often impractical or infeasible. Foliar PM capture is dependent on a variety of factors, from plant species and canopy type to leaf surface properties, density and micromorphology. Leaves can trap PM in several ways, from accumulation on hairs or trichromes to capture and retention through their stomatal pores.
1. Xiangying, W., Shiheng, L., Ying, Y., Zonghua, W., Hong, L., Dongming, P., Jianjun, C. (2017) Phylloremediation of Air Pollutants: Exploiting the Potential of Plant Leaves and Leaf-Associated Microbes. Frontiers in Plant Science, 8, 1318. doi: 10.3389/fpls.2017.01318.
Tree planting is one of the most effective methods toward combatting anthropogenic PM concentrations in urban environments. Biological remediation – the planting of vegetated green belts – can offer an efficient and sustainable method of reducing vehicular or industrial PM emissions in areas where the cessation or elimination of such emissions is often impractical or infeasible. Foliar PM capture is dependent on a variety of factors, from plant species and canopy type to leaf surface properties, density and micromorphology. Leaves can trap PM in several ways, from accumulation on hairs or trichromes to capture and retention through their stomatal pores.
1. Xiangying, W., Shiheng, L., Ying, Y., Zonghua, W., Hong, L., Dongming, P., Jianjun, C. (2017) Phylloremediation of Air Pollutants: Exploiting the Potential of Plant Leaves and Leaf-Associated Microbes. Frontiers in Plant Science, 8, 1318. doi: 10.3389/fpls.2017.01318.
Engraving of the Cowthorpe Oak,
Cowthorpe, North Yorkshire, England. The Cowthorpe Oak was said to be the largest oak tree in England, with a canopy that purportedly covered
2,000 m2. Source: British Library.
Scanning electron micrograph (SEM) of stomata on the surface of a cabbage plant leaf. Stomata (ovular openings above) are pores involved in the regulation of gas-water exchange between the leaf and the atmosphere. Source: SciencePhotoLibrary.
The London Plane tree (Platanus × acerifolia) is one
of the most widely planted species in the city. Plane trees are highly resilient, with little sign of senescence in even the oldest trees – huge swathes were planted across London during the industrial
revolution, and the vast majority still stand today. This longevity is due in part
to the tree’s most distinctive feature; its exfoliating bark.
Tree bark is characterised by lenticels, raised pores that facilitate gas exchange between the atmosphere and the plant’s internal tissues. In urban environments, these pores can become clogged by the deposition of high concentrations of particulate matter, interfering with air reaching the trunk and exacerbating the build-up of pollution within localised ‘street canyons,’ with adverse effects that can often mitigate the benefits of planting trees in the first place.
The exfoliating bark of the plane tree, however, regenerates at a rapid rate, shedding its clogged outer layers – ridding it of any harmful atmospheric pollutants that have accumulated – and growing fresh bark to replace it. As such, the London Plane is highly efficient at particulate matter entrapment. Known as ‘London’s lungs,’ plane trees in Greater London account for the removal of up to 2,000 tonnes of PM10 yearly.2
2. Treeconomics, i-Tree. (2015) Valuing London’s Urban Forest: Results of the London i-Tree Eco Project.
Tree bark is characterised by lenticels, raised pores that facilitate gas exchange between the atmosphere and the plant’s internal tissues. In urban environments, these pores can become clogged by the deposition of high concentrations of particulate matter, interfering with air reaching the trunk and exacerbating the build-up of pollution within localised ‘street canyons,’ with adverse effects that can often mitigate the benefits of planting trees in the first place.
The exfoliating bark of the plane tree, however, regenerates at a rapid rate, shedding its clogged outer layers – ridding it of any harmful atmospheric pollutants that have accumulated – and growing fresh bark to replace it. As such, the London Plane is highly efficient at particulate matter entrapment. Known as ‘London’s lungs,’ plane trees in Greater London account for the removal of up to 2,000 tonnes of PM10 yearly.2
2. Treeconomics, i-Tree. (2015) Valuing London’s Urban Forest: Results of the London i-Tree Eco Project.
Fallen plane tree bark. Source: Treetree.co.uk, Plane tree bark.
The variation in colour and texture of plane tree bark is a result of its natural exfoliation.
Large plates of mottled olive and grey-green outer bark peel away to
reveal a creamy buff-brown bark beneath, resulting in a disruptively patterned
camouflage-like appearance.
Source: Treetree.co.uk, Plane tree bark.
There are limits to the effectiveness of pollution
sequestration by urban greening alone. Above ground and below, a variety of
complex factors govern the application of bioremediation strategies in any
given urban area, from building heights and adjacencies to the negotiation of sub-surface
infrastructures, the availability of water and sunlight, and the prevailing
soil conditions. The ability of a species to retain particulate matter also depends
on weather conditions and patterns of dispersion within the immediate environment.
Many plants, though effective traps, merely retain particulate deposition on
the foliar surface, up to 50% of which may become airborne again through
resuspension in windy periods.3
Given these constraints, it is necessary to look elsewhere in order to find complementary and equally sustainable solutions to the growing problem of airborne particulates in urban areas.
3. Mo, L., Ma, Z., Xu, Y., Sun, F., Lun, X., Liu, X., Chen, J., & Yu, X. (2015). Assessing the Capacity of Plant Species to Accumulate Particulate Matter in Beijing, China. PloS one, 10 (10), e0140664. doi:10.1371/journal.pone.0140664.
Given these constraints, it is necessary to look elsewhere in order to find complementary and equally sustainable solutions to the growing problem of airborne particulates in urban areas.
3. Mo, L., Ma, Z., Xu, Y., Sun, F., Lun, X., Liu, X., Chen, J., & Yu, X. (2015). Assessing the Capacity of Plant Species to Accumulate Particulate Matter in Beijing, China. PloS one, 10 (10), e0140664. doi:10.1371/journal.pone.0140664.
Top:
A Giraffe trap. Source: Gell, A. (1996)
Vogel’s Net: Traps as Artworks, Artworks as Trap.
SUNSPACE
SUNSPACE
SUstaiNable materials SynthesiSed from by-Products and Alginates for Clean air and better Environment
Our investigation into particulate traps led us to a paper published in Frontiers in Chemistry, which described a new hybrid material offering an alternative to existing methods of PM reduction in urban environments. The paper outlined research undertaken by a team of researchers at the University of Brescia – in collaboration with partners at University of Brema, Trieste and Bologna, Joint Research Centre (JRC), the Italcementi–Heidelbergcement Group, and Delta Phoenix – into the development of SUNSPACE (SUstaiNable materials Synthesized from by-Products and Alginates for Clean air and better Environment), a porous material designed to reduce anthropogenic PM concentrations in the city. The research was supported by INSTM and Regione Lombardia.
Synthesised from industrial silicate by-products and algae-derived materials, SUNSPACE has a large pore volume that can entrap and confine aero-dispersed nano and micro PM (like the micromorphology of a leaf). SUNSPACE is a low-cost, low-impact and easily integrable material, and is designed to be applied directly as an external coating (as a plaster on a wall, or on roof tiles), and as such has a wide range of possible applications. SUNSPACE was recently awarded the Energy Globe Award for innovation in sustainability.
We made contact with Professor Elza Bontempi, Dr. Alessandra Zanoletti and PhD candidate Antonella Cornelio, with whom we met to discuss the work of their research group and to hear about recent developments in the materiality of SUNSPACE.
Zanoletti, A., Bilo, F., Borgese, L., Depero, L.E., Fahimi, A., Ponti, J., Valsesia, A., La Spina, R., Montini, T., and Bontempi, E. (2018). SUNSPACE, A Porous Material to Reduce Air Particulate Matter (PM). Frontiers in Chemistry, Volume 6, 2018, 534. doi: 10.3389/fchem.2018.00534.
Zanoletti, A., Bilo, F., Federici, S., Borgese, L., Depero, L.E., Ponti, J., Valsesia, A., La Spina, R., Segata, M., Montini, T. and Bontempi, E. (2020). The first material made for air pollution control able to sequestrate fine and ultrafine air particulate matter. Sustainable Cities and Society, Volume 53, 2020, 101961. doi: 10.1016/j.scs.2019.101961.
Meeting with Alessandra and Antonella via Zoom.
Slides from the meeting, outlining the basic principles of SUNSPACE, its sustainability, real-world applications and recent developments in its synthesis. Of particular interest to us were the recent investigations into the visual properties of the material led by Antonella as part of her PhD thesis. Source: Zanoletti, A., University of Brescia (UNIBS).
The SUNSPACE group were kind enough to send us samples both of SUNSPACE 1, synthesised with silica fume, and the more recent SUNSPACE 2, which substitutes the silica fume with bottom ash and calcium hydroxide to result in a lighter colour.
Top: SEM image of SUNSPACE after particulate capture. Source:
Zanoletti, A., Bilo, F., Federici, S., Borgese, L., Depero, L.E., Ponti, J., Valsesia, A., La Spina, R., Segata, M., Montini, T. and Bontempi, E. (2020). The first material made for air pollution control able to sequestrate fine and ultrafine air particulate matter. Sustainable Cities and Society, Volume 53, 2020, 101961. doi: 10.1016/j.scs.2019.101961.