Urban greenery for air pollution control: a meta-analysis of current practice, progress, and challenges

Abstract

Most governmental initiatives in India, to leash down urban air pollution, have yielded little results till date, largely due to purely technocratic vision, which is shrouded by technological, economic, social, institutional, and political hardships. We present this reflective article on urban greenery, as a proposition to urban authorities (e.g., pollution regulators, environmental systems' managers, urban landscape planners, environmental policy makers), shift from purely technocratic way of thinking to thinking with nature, by strategic greening of urban spaces, for long-term air pollution prevention and control measures. To that end, we offer a meta-analysis of recent (post 2005) global literature using four-stage PRISMA (Preferred Reporting Items for Systematic Reviews and Meta-Analyses) approach. We open the narrative by briefing about main pollutant filtration mechanisms by trees, followed by cognitive aspects of species selection (e.g., deciduous vs. evergreen, air pollution tolerance index, environmental stressors). Till date, most Indian studies on urban greenery mostly but focused on physiological aspects of trees. Here, we draw attention of urban authorities to an equally compelling, but yet less explored, aspect: design criteria, with reference to two most common urban configurations, namely, street canyon and open road. With pictorial depictions, we enumerate various categories of street canyons and discuss aspect ratio (building height to street width) and various wind flow regimes (isolated roughness, wake interface, and skimming), that the urban authorities should be cognizant about to maximize pollutant removal efficiency. For open road, we discuss vegetation barriers, with special emphasis on canopy porosity/density functions. In the final sections, we reflect on a potential systems' thinking approach for on-ground implementation, comprising of revamping of urban forestry programs, research and development, community mobilization and stakeholder engagement, and strategic outreach. In addition, we emphasize on means to harness co-benefits of urban greenery, beyond mere pollutant removal, to garner support from urban residents' communities. Last but not the least, we also caution the urban authorities about the undesirable outcomes of urban greenery that will require more process-level research.

Keywords: Air pollution tolerance index (APTI); Aspect ratio; Biogenic volatile organic compound (bVOC); Canopy porosity/density; Community mobilization and stakeholder engagement; Dust re-suspension; Open road; Outreach and promotion; Social/community wellbeing; Street canyon; Urban air quality; Vegetation barrier; Wind flow regime.

Fig. 1

Comparative assessment of 24-h average concentrations of particulate matter (PM₁₀, expressed as µg m⁻³) in six Indian cities, reckoned among 20 most polluted cities worldwide, between 2019, and corresponding time periods in 2020. Note: solid red and blue broken lines, respectively, represent the Indian National Ambient Air Quality Standard (100 µg m⁻³) and World Health Organization threshold (10 µg m⁻³) for PM₁₀ (authors’ illustration; Data source: Central Pollution Control Board, India)

Fig. 2

Twenty-four-hour median concentrations of PM₁₀ and PM_2.5 (µg m⁻³) across 38 air quality monitoring stations in New Delhi between April 20 and May 30 in 2019, 2020, and 2021. Red bold line represents the Indian National Ambient Air Quality Standard (100 µg m⁻³) and World Health Organization threshold (10 µg m⁻³) for PM₁₀. Black circles in the top panel indicate air quality monitoring stations in New Delhi. The blue line represents the Indian National Ambient Air Quality Standard (60 µg m⁻³), and World Health Organization threshold (24 µg m⁻³) for PM_2.5 (authors’ own illustration; Data source: Central Pollution Control Board, India)

Fig. 3

Simplified schematic of the four-stage sequential literature extraction procedure using PRISMA (Preferred Reporting Items for Systematic Reviews and Meta-Analyses) (authors’ own illustration)

Fig. 4

Classificatory scheme for urban street canyons, with respect to building geometries and wind flow directions. H = building height; W = street width; L = street length (authors’ own illustration)

Fig. 5

a Simplified depiction of formation of primary wind vortex within urban street canyon with respect to perpendicular wind flow. b Wind flow regimes in various street canyon configurations commonly encountered in urban habitats

Fig. 6

Availability of a per capita urban green cover (m²/person) in different Indian cities, b per capita green cover in different world cities relative to four major Indian cities, and c city-wise percentages of green cover in India and world cities (authors’ own illustration; Data Source: OECD Stats; World Cities Culture Forum: http://www.worldcitiescultureforum.com/data/of-public-green-space-parks-and-gardens)

Fig. 7

A potential systems’ design to implement urban greenery, strategically supported and mobilized by an ecosystem of actors and agencies. It is an iterative process, envisioned within a robust institutional structure and political system (authors’ own illustration)

Fig. 8

Prioritizing action in pollution hot spots. Greening initiatives should be realized within a multi-layered systems’ design, cross-linking urban environmental policy outlook, opportunities of finance, and effectiveness of existing institutions (authors’ own illustration)