Fighting air pollution with urban greening

Planting the right trees in the right places isn’t just about beautifying the capital city of Delhi — it’s about building a living, breathing defence against one of the world’s worst urban air pollution crises

Trees, shrubs, and grasses do far more than add greenery to grey cityscapes. Vegetation intercepts airborne particles on leaves, branches, bark, and stems, physically reducing the amount of dust and pollutants residents inhale. Dense canopies stabilise exposed soil, cutting down on wind-blown dust resuspension - one of Delhi’s major pollution contributors. Plants also absorb gaseous pollutants such as oxides of sulphur and nitrogen directly through their stomata.

Additionally, trees add surface roughness that promotes atmospheric turbulence, which in turn prevents pollutant accumulation by encouraging greater dispersal and dilution. Green areas further stimulate the sedimentation of aerosols and dust particles. As a bonus, neighbourhoods with dense tree cover tend to run several degrees cooler than bare urban zones, delivering cleaner air and cooler streets simultaneously.

Research conducted across Delhi supports these benefits. Studies found that PM10 and PM05 concentrations were significantly higher in less vegetated areas like Anand Vihar and Jahangirpuri compared to greener localities such as Mandir Marg and Sri Aurobindo Marg. Well-designed vegetation barriers have also been shown to meaningfully reduce local PM5 levels near plantation zones, and dense shrub barriers around schools, hospitals, and pedestrian areas offer added protection by intercepting dust precisely at breathing height.

Choosing the right species

Not all plants are created equal when it comes to pollution tolerance. Some species are highly sensitive to pollutants and serve best as early-warning bioindicators, while others are hardy enough to withstand polluted conditions and actively help reduce the pollution load. The Air Pollution Tolerance Index (APTI) - calculated using a formula involving a plant’s ascorbic acid content, total chlorophyll, leaf extract pH, and relative water content - provides a scientific basis for species selection. Combined with biological and socio-economic factors, this yields an Anticipated Performance Index (API) that guides Green Belt development decisions.

A 2024 study by the Indian Agricultural Research Institute (IARI) identified Peepal (Ficus religiosa) as one of Delhi’s most pollution-tolerant tree species, followed closely by Karanj (Pongamia pinnata). Other recommended species for Delhi-NCR include Neem, Arjun, Jamun, Banyan, Shahtoot, Imli, Mango, and Amaltas - all valued for their high dust-capturing capacity, resilience under polluted conditions, and climate suitability. For large-scale ecological restoration along the Yamuna floodplain, biodiversity parks, and the Delhi Ridge, native species like Arjun, Jamun, Karanj, and Imli are particularly well-suited, offering additional benefits such as groundwater recharge, carbon sequestration, and biodiversity conservation.

Greening roads and flyovers

Urban road infrastructure offers significant untapped potential for strategic greening. Narrow medians of 0.5-1.2 m can be seeded with grasses like Dhoob and Vetiver to stabilise soil and reduce dust resuspension. Medium medians of 1.2-3 m width suit single to triple rows of shrubs such as Bougainvillea and Nerium oleander, while those between 3-4 m can support layered combinations of shrubs, grasses, and small trees like Frangipani. Larger medians exceeding 4 m allow for full-scale planting of trees, including Neem, Alstonia scholaris, Terminalia arjuna, Sheesham, and Karanj, combined with understorey shrubs and grasses for maximum pollutant capture.

Footpaths of 1.8 m width can be lined with shade-giving trees like Neem, Mango, and Terminalia arjuna, with native grasses planted in tree pits to bind soil. Even the underutilised spaces beneath flyovers can be put to work - climbers such as Vernonia grandiflora, Ficus scandens, Thunbergia grandiflora, and Ipomoea species can cover concrete surfaces, absorb pollutants, and introduce greenery into otherwise dead urban spaces.

Planning matters as much as planting

Experts are clear on one point: volume alone is not enough. Indiscriminate planting of large trees without proper planning can actually worsen local air quality. Dense canopies in confined spaces like narrow street canyons may trap pollutants and restrict air circulation, leading to higher localised concentrations. Effective greening demands careful attention to species selection, spacing, and placement to ensure vegetation enhances pollutant dispersal rather than obstructing it. In Delhi’s traffic-heavy corridors and densely built neighbourhoods, integrating trees with shrubs and grasses in a layered, planned manner is essential to maximise results.

Green belt design must also account for local climatic conditions, soil quality, water availability, and pollution load. Greening of adequate width and density using local species is strongly recommended - exotic species should be avoided to ensure easier maintenance and long-term survival. From a nursery perspective, sapling quality, transport, and early-stage care, including tree guards, watering, and manuring, are all critical to successful establishment. Notably, once established, native species typically require minimal maintenance.

A greener commitment

Urban greening is not a silver bullet, but it is an indispensable piece of Delhi’s clean air puzzle. When vegetation is planned scientifically - the right species, the right spacing, the right location - it functions as a living filter, a dust barrier, a temperature regulator, and an ecological anchor all at once.

The science is clear and the tools are available. What remains is the collective will to plant not just for optics, but with genuine purpose - turning Delhi’s streets, medians, and open spaces into a green infrastructure that works as hard as the city itself.

The science is clear and the tools are available. What remains is the collective will to plant not just for optics, but with genuine purpose

The writer is Dean (Academics) & Professor of Environmental Planning, School of Planning and Architecture, New Delhi; Views presented are personal.