{"id":102752,"date":"2023-06-15T09:40:42","date_gmt":"2023-06-15T08:40:42","guid":{"rendered":"https:\/\/greenblue.com\/?p=102752"},"modified":"2023-07-07T11:04:19","modified_gmt":"2023-07-07T10:04:19","slug":"on-the-road-to-cleaner-air-with-green-infrastructure","status":"publish","type":"post","link":"https:\/\/greenblue.com\/gb\/on-the-road-to-cleaner-air-with-green-infrastructure\/","title":{"rendered":"On the Road to Cleaner Air With Green Infrastructure"},"content":{"rendered":"

Clean Air Day<\/strong><\/a>, the UK\u2019s largest campaign on air pollution, is taking place on Thursday 15th June 2023<\/strong>. This article marks the occasion by focusing on links between air quality and green infrastructure.<\/p>\n

The largest source of unclean air in the UK is the transport sector, with the highest levels of pollutants found alongside busy roads. To tackle this issue, the government\u2019s 2019 Clean Air Strategy<\/a><\/strong> identifies<\/p>\n

\u201cintelligent tree planting and better access to green infrastructure\u201d <\/strong><\/p><\/blockquote>\n

as part of a mix of actions required to be pursued, with tree planting acknowledged alongside other measures such as promoting active travel, public transport, and zero and ultra-low vehicles.<\/p>\n

This suggests that green infrastructure has a role to play in improving air quality. Indeed, the avoided mortality and morbidity due to air pollution removal by UK urban green and blue space were quantified by the Office for National Statistics<\/strong><\/a>, estimating the economic value arising from these avoided health costs in 2015 alone equalled \u00a3136m.<\/strong><\/p>\n

But what does \u201cintelligent tree planting\u201d<\/em> look like to improve air quality?<\/p>\n

Creating cleaner air oases using green infrastructure<\/h3>\n

Polluted air alongside busy roads is not only a very hostile environment for people, impacting negatively on their health. It is also damaging to green infrastructure including trees and hedges. This means the decisions made early at the design stage will determine whether we end up with, quoting Ferranti<\/strong> et al. (2019), \u201crelatively cleaner \u2018oases\u2019\u201d or \u201crelatively dirtier \u2018hotspots\u2019\u201d.<\/em><\/p>\n

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The Old Bailey, London (Source: The Tree Council: treecouncil.org.uk)<\/figcaption><\/figure>\n

Trees are not inert assets such as lamp posts or other grey infrastructure. They are living breathing things – \u201cfluid entities\u201d<\/em>, quoting Barwise and Kumar<\/strong> (2020), responsive to the environment they are in. Well-informed and carefully considered tree species selection is critical for ensuring that any trees planted to act as a vegetation barrier will thrive. Furthermore, over their lifespan, the trees will deliver desired benefits such as shade and temperature regulation whilst being resilient to withstanding pests and climate change.<\/p>\n

The best practice<\/a> <\/strong>guidance on the topic was brought together by Kumar et al. (2019). It offers design suggestions of what and where to plant depending on the type of road environment and offers a list of 19 tree species<\/strong> considered suitable for air pollution abatement either as trees or hedges.<\/p>\n

Barwise and Kumar (2020) expand on this work, producing a more comprehensive initial list of viable tree species for the UK to act as vegetation barriers<\/a>, by drawing on the earlier tree species selection tool<\/a><\/strong> by Hirons and Sj\u00f6man.<\/p>\n

 <\/p>\n

The list brings together data for 61 tree species and their key traits such as canopy density, leaf surface area, and drought tolerance. It also takes into account disbenefits in the form of pollen and naturally volatile organic compounds produced by trees.<\/p>\n

To help further guide a selection process of green infrastructure for improved air quality, Barwise and Kumar (2020, p. 15) suggest that practitioners ask these four questions:<\/p>\n

    \n
  1. Will the species thrive under the given environmental conditions?<\/li>\n
  2. Do the known ecophysiological and morphological characteristics of the species complement the conditions of the urban context (street canyon or open road)?<\/li>\n
  3. Will the species contribute minimal emissions?<\/li>\n
  4. Does the species bear traits that are deemed to be beneficial for pollution abatement?<\/li>\n<\/ol>\n

    An online tool to design your own hedge<\/a><\/strong>\u00a0that takes into account different types of road environments was developed by Barwise et al. (2021). The authors tested the tool prototype with members of the public, with plans for future versions aimed at green infrastructure practitioners as its users.<\/p>\n\n\n\n\n
    Atmospheric dispersion<\/strong>: comprising \u201cthe transportation and dilution of pollutants from the pollutant source, and the various roles that vegetation may play<\/em>\u201d<\/td>\n<\/tr>\n
    Dry deposition<\/strong>: \u201c describes the process by which pollutants are deposited on solid surfaces, thereby reducing ambient atmospheric concentrations<\/em>\u201d<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

    Table 1 Definitions of the two mechanisms for green infrastructure to tackle air pollution. Source: Barwise and Kumar (2020, p.6):<\/p>\n

    Minimising pedestrian exposure to air pollutants using green infrastructure: does it work?<\/h3>\n

    Overall, the air quality and green infrastructure dynamics are complex, with different elements not easy to untangle and control (e.g., Barwise and Kumar, 2020, Ferranti et al. 2019, Kumar et. al, 2019). In urban areas, the impact of green infrastructure including trees on cleaning the air is unfortunately minimal. This is unless it is strategically placed to act as a barrier that modifies wind flow, re-directing pollutants away from pedestrians and other users (Ferranti, MacKenzie, and Levine, 2019).<\/p>\n

    For example, with regards to tackling urban pollution caused by nitrogen dioxide (NO2), one of the pollutants for which the UK experiences enduring breaches of legal limits<\/a><\/strong>, the Air Quality Expert Group concluded in its 2018 report that using green infrastructure was, in general, futile due to its failure to act as a good sink for this particular pollutant.<\/p>\n

    Policy-wise, the 2018 report by the Air Quality Expert Group suggests devising an<\/p>\n

    \u201cintegrated policy which separates people spatially from major pollution sources (especially traffic) as far as possible and in which vegetation is used between the sources<\/span> and the urban population maximises its beneficial effects.\u201d<\/strong><\/p><\/blockquote>\n

    Consequently, there is not a one size fits all approach, continue Barwise and Kumar (2020): instead, the design and choice of green infrastructure for vegetation barrier purposes are context-driven, with some local environments more suited to act as effective pollutant sinks than others. Different types of urban environments will require different solutions, such as an open-road environment versus one comprising a street canyon \u2013 reviewed in detail by Abhijith et al.<\/em> (2017).<\/p>\n

    Barwise and Kumar (2020) conclude that green infrastructure is<\/span> an effective solution for tackling air pollution that works at a local level, but not necessarily at a larger scale, e.g., in a town or city. On p. 12 of their review, the authors offer these recommendations:<\/p>\n