Abstract
Street canyons are generally highly polluted urban environments due to high traffic emissions and impeded dispersion. Green infrastructure (GI) is one potential passive control system for air pollution in street canyons, yet optimum GI design is currently unclear. This review consolidates findings from previous research on GI in street canyons and assesses the suitability of different GI forms in terms of local air quality improvement. Studies on the effects of various GI options (trees, hedges, green walls, green screens and green roofs) are critically evaluated, findings are synthesised, and possible recommendations are summarised. In addition, various measurement methods used for quantifying the effectiveness of street greening for air pollution reduction are analysed. Finally, we explore the findings of studies that have compared plant species for pollution mitigation. We conclude that the influences of different GI options on air quality in street canyons depend on street canyon geometry, meteorological conditions and vegetation characteristics. Green walls, green screens and green roofs are potentially viable GI options in existing street canyons, where there is typically a lack of available planting space. Particle deposition to leaves is usually quantified by leaf washing experiments or by microscopy imaging techniques, the latter of which indicates size distribution and is more accurate. The pollutant reduction capacity of a plant species largely depends on its macromorphology in relation to the physical environment. Certain micromorphological leaf traits also positively correlate with deposition, including grooves, ridges, trichomes, stomatal density and epicuticular wax amount. The complexity of street canyon environments and the limited number of previous studies on novel forms of GI in street canyons mean that offering specific recommendations is currently unfeasible. This review highlights a need for further research, particularly on green walls and green screens, to substantiate their efficacy and investigate technical considerations.
Highlights
Air pollution is a major environmental hazard to human health, predominantly responsible for damage to respiratory and cardiovascular systems, leading to reduced lifespan (WHO, 2006; Remy et al, 2011; Lelieveld et al, 2015) as well as lung cancer and other diseases (WHO, 2009; Miri et al, 2018; Luo et al, 2020; Willis and Petrokofsky, 2017; Bahrami Asl et al, 2018)
Reviewed studies regarding the mitigation of air pollution by urban greenery due to deposition and aerodynamic effects, with special attention given to the design guidelines resulting from these studies and their applicability in the process of urban planning
Topics include: passive methods influencing airflow patterns in the built environment (Gallagher et al, 2015); particulate matter (PM) removal by vegetation (Janhall, 2015); ecosystem services offered by Green infrastructure (GI) in streets (Gago et al, 2013; Mullaney et al, 2015; Sal mond et al, 2016; Willis and Petrokofsky, 2017) and the air pollution mitigation and other environmental benefits offered by green roofs (Berardi et al, 2014; Rowe, 2011; Vijayaraghavan, 2016)
Summary
Air pollution is a major environmental hazard to human health, predominantly responsible for damage to respiratory and cardiovascular systems, leading to reduced lifespan (WHO, 2006; Remy et al, 2011; Lelieveld et al, 2015) as well as lung cancer and other diseases (WHO, 2009; Miri et al, 2018; Luo et al, 2020; Willis and Petrokofsky, 2017; Bahrami Asl et al, 2018). Topics include: passive methods influencing airflow patterns in the built environment (Gallagher et al, 2015); PM removal by vegetation (Janhall, 2015); ecosystem services offered by GI in streets (Gago et al, 2013; Mullaney et al, 2015; Sal mond et al, 2016; Willis and Petrokofsky, 2017) and the air pollution mitigation and other environmental benefits offered by green roofs (Berardi et al, 2014; Rowe, 2011; Vijayaraghavan, 2016) Past reviews such as Abhijith et al (2017) have highlighted the need to consider diverse urban conditions, from complex street canyons to open road environments, when selecting or configuring GI. Underlying mechanisms of pollutant concentration reduction by GI at local scale
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