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In the most recent Air Quality News Procurement Guide, Martin Guttridge-Hewitt explored the concept of ‘greening’ in which the health of urban spaces can be improved by introducing flora in innovative ways. In this article, Dr Sanja Potgieter-Vermaark of Manchester Metropolitan University a panellist at our recent Manchester conference – discusses her recent work with Living green walls

Urban green infrastructure (UGI) is a network of green spaces, water, and other natural systems, which has multiple functions and can provide direct and indirect enhancements to urban spaces. The effectiveness of UGIs to mitigate the risk of living in urban spaces with significantly reduced green spaces, has been researched widely. There is evidence of health benefits due to improved wellbeing, increased physical activity, and reduction in exposure to pollution (air and noise) and high temperatures. Further benefits include reduced flooding risks, social cohesion, protection of biodiversity, and cost savings.

Natural England launched the Green Infrastructure Framework in January 2023, to help towns and cities turn greener and aims to increase the green cover in urban residential areas to 40%. Greater Manchester has already moved towards increasing UGI (Manchester Green and Blue Infrastructure Strategy) of which implementation started in 2021 and is to be completed by 2025, during which it is estimated that a total of £250 million will have been invested in GI-related environmental improvements. The Greater Manchester council authority’s blue and green infrastructure framework highlights the successful implementation of urban green infrastructure and nature-based solutions in a complex multi stakeholder environment and is a notable example of how local planning authorities in the UK can create climate resilient areas for their communities.

One of the key branches of UGIs for densely populated urban areas is Living Green Walls (LGW). They can provide similar benefits to urban trees such as increases in biodiversity or localised air cooling but are more suited to urban areas where trees may not grow well due to issues such as narrow roads, lack of space, or soil compaction and underground utilities. They have been used and researched for a wide range of applications, such as water filtration & storm water management, acoustic damping, biodiversity & habitat creation, thermal insulation, and air quality management.

There are a variety of types, however LGW typically consists of a modular facade structure which acts as a container for a growth medium. The growth medium can be natural such as mineral/organic soils or completely synthetic structures which use textile matting as the rooting medium. The growth medium and high-density planting provide better performance characteristics (e.g Thermal insulation) than some of the lighter weight versions of UGIs like Green facades; which are typically comprised of trailing and climbing plants such as Hedera Helix (English Ivy).

Modular LGW can offer bespoke performance characteristics that can be achieved through the selection of plant species, substrates, and digital technologies. This information can be used within the assessment reports required for environmental reporting and mitigation schemes within the built environment sector such as the Building Research Establishment

Environmental Assessment Method (BREEAM).

Environmental remediation features for the built environment sector are especially relevant for local planning authorities as all public buildings now require an ‘Excellent’ BREEAM accreditation score and any new builds with a construction value greater than £500,000 must achieve a BREEAM ‘Outstanding’ rating.

Specific air quality benefits of living walls:

One of the key areas being studied is how LGW can be utilised to improve air quality in Manchester

• How do plants improve air quality?

The main way the pollutants are captured by LGW is via deposition of the fine particles and gases onto the leaves’ surfaces as air passes through the foliage. There are a multitude of physical and environmental factors which influence how the LGW performs. Plant traits and design characteristics such as large surface area, high leaf area density, and foliage longevity are key direct influences, whereas environmental factors such as humidity, wind speed and street canyon ratio affect how the pollutants interact with the foliage and localised air pollution concentrations.

Measuring living wall performance

Measuring the air quality performance of LGW is a complex task due to the array of variables involved, this means a combination of lab studies and in-situ research is required to provide contextually relevant information about LGW.

• In-situ digital monitoring

One of the ways we measured the effectiveness of LGW is by integrating high performance air sensors amongst the foliage to get proxy measurements of its performance when compared to an array of ARUN (Automatic Urban and Rural Network) air quality sensors measurements in the city of Manchester.

Continuous air quality monitoring systems can be advantageous not only to measure the effectiveness of new building adaptations, but they can also be installed when a building is undergoing construction; this can evidence that developers and contractors are compliant with considerate construction schemes. This is especially relevant as the construction sector is responsible for over 30% of PM10 emissions.

A study to assess the effectiveness of LGW to improve local air quality is being trialed with a joint venture between Bruntwood group and Trafford Council for their regeneration of Altrincham town centre. This urban redevelopment has been designed to actively enhance work life balance and improve the wellbeing of its communities by improving local air quality, biophilia and biodiversity. The LGW used for the trial is the patented HYVERT system designed by urban greening company; I Want Plants.

This transparent data approach can help build trust between stakeholders, which is especially relevant when partnerships span different priorities and sectors.

• Chemical analysis

Measuring the mass concentration of airborne particulate matter is important for overall indication of health risks, however it does not provide the full picture as to how harmful the material is. The chemical composition of particulate matter can have considerable influence on health risks.

Biomonitoring is an effective technique that can be used to measure an area's pollutant composition. It can involve performing chemical analysis on vegetation within an area to see what pollutants have been absorbed by the biomass.

This approach provides a good representation of spatial and temporal data. E.g A deciduous tree loses foliage each year so any pollutants found would have to be within that period.

As HYVERT LGW often spans large swaths of a building's surface and is regularly maintained on a monthly basis, the wall can be utilised as a means of biomonitoring across urban street canyons with detailed temporal scales. The buildup of fine particulate matter on the leaves surface enables the HYVERT LGW to be chemically profiled for the presence of potentially harmful elements such as Cu, Co, Ni, Fe, Pb, and Cr. These elements are preferential to analyse as they are often found in quantifiable concentrations in particulate matter and are from common pollution sources such as combustion and traffic.

We profiled the HYVERT LGW biomass using XRF (X-Ray Fluorescence), to see if this rapid analytical method could compliment results already obtained by traditional analytical methods such as inductively coupled plasma with mass spectrometry (ICP-MS) could be applicable for urban biomonitoring. We found that although the XRF method was not able to achieve the same limits of detection as ICP-MS it was still highly effective, fast, non-destructive, and a greener screening method for demonstrating the benefits of the HYVERT LGW.

As LGW increases in abundance due to UGI investment this alternative low-cost biomonitoring approach can fill the gaps in particulate matter composition data and could be used to supplement continuous digital monitoring systems used in the urban areas.

Policy synergies:

As the new Biodiversity Net Gain policy kicks off in November 2023, new developments will need to demonstrate an increase in net gain by a minimum of 10%. For small sites below a 5000 m2 footprint, the net gain must be created directly on the plot, which is particularly challenging for built-up urban areas.

Vertical green infrastructure suits this niche, and thus presents an opportunity to combine regional initiatives such as Manchester's Clean Air strategy with the larger national government policies such Biodiversity Net Gain, by selecting hardy pollution capturing plants along with localised species which provide food sources and habitats for wildlife.

Other built environment policies also require a multifunctional retrofit solution. The new Energy Performance Certificate (EPC) requirements mean that from 1st April 2023, new and existing leases for commercial properties are moving from a Minimum Energy Efficiency Standards (MEES) rating of “F” to “E”, with all properties being required to be at an “B” standard by 2030.

There are a range of solutions for retrofitting older buildings, but external wall insulation typically provides the highest EPC increase, limits any disruption to the interior usage and does not reduce internal floor space.

By investing in energy efficiency technologies like HYVERT LGW at an early stage the initial capital expenditure of the LGW can be offset by the gains made through overall energy usage reduction which ranges between 16-31%, along with the increases of property and land value which average 2-2.5%.

Ongoing research is being conducted as a part of an Innovate UK funded Knowledge Transfer Partnership with Manchester Metropolitan University and industry partner I Want Plants to demonstrate how their high performance HYVERT LGW system can be used, tested, and enhanced for the myriad of UGI applications.