Urban trees are celebrated as essential shields against global warming, providing vital shade and carbon storage. However, different tree species emit Isoprene and Monoterpenes at variable rates. Combining high-emitting species with high-traffic areas (NOx hot zones) accelerates ground-level ozone (O3) production.
To defend against thermal stress, species like oaks, poplars, and eucalyptus emit natural chemicals called Biogenic Volatile Organic Compounds (BVOCs). When these biogenic gases mix with nitrogen oxides (NOx) from vehicular exhaust under intense summer sunlight, they trigger photochemical reactions that generate toxic ground-level O3.
As detailed in atmospheric modelling by the US Environmental Protection Agency (EPA) and forestry assessments by the USDA Forest Service (https://www.itreetools.org/), well-intentioned greening campaigns can inadvertently worsen summer smog unless cities strategically align urban forestry with atmospheric science.
The Invisible Scent of the City
On a scorching July afternoon, you escape the concrete glare and find refuge beneath a sprawling oak in your neighbourhood park. The air carries that crisp, resinous perfume of pine and sun-warmed leaves—it smells like nature at its purest, like everything right with the world. You breathe deep, feeling cleansed.
But that familiar scent is a chemical signal. It is the tree's silent distress call.
When temperatures spike and insects attack, trees release microscopic compounds into the air—defensive chemicals designed to protect them from heat stress and hungry pests. These include isoprene and terpenes, the very molecules responsible for that forest freshness we instinctively crave. They are natural, ancient, and essential to the tree's survival.
Yet, in the modern city, nothing exists in isolation. As these invisible molecules drift upward from the canopy, they do not simply dissipate into harmless oblivion. They drift into a sky already saturated with the ghosts of our morning commute. And that is where the trouble truly begins.
The Unintended Chemical Handshake
Here lies the paradox: nature's remedy becomes the city's pollutant. Under the relentless summer sun, BVOCs from trees encounter NOx—the gaseous byproducts of car engines, power plants, and industrial activity. This is not a gentle meeting. It is a photochemical handshake with volatile consequences. Ultraviolet radiation catalyses a reaction between these compounds, birthing ground-level ozone (O3).
The chemistry is deceptively simple:
BVOCs + NOx + sunlight → ground-level ozone + secondary organic aerosols
This is not the protective stratospheric ozone that shields us from radiation. This is tropospheric ozone—a corrosive respiratory irritant that inflames lung tissue, exacerbates asthma, and damages the very leaves that produced its precursors (See article by Vongelis et al. 2025 for details). The tree that offered shade now inadvertently contributes to the haze that clouds the skyline.
In the complex metabolism of the urban atmosphere, green infrastructure does not automatically equal clean air. The equation depends entirely on what else lingers in the air alongside those innocent leaf-emitted molecules.
The Tree Trial – Good Intentions, Wrong Species
For decades, city planners operated on a simple premise: more trees, better air. They planted fast-growing, shade-providing species—sweetgums with their star-shaped leaves, majestic oaks, and sturdy plane trees that lined boulevards from Paris to Los Angeles. These trees grew quickly, cooled streets, and gave cities an instant verdant identity.
But these species are also prodigious BVOC emitters. Oaks and sweetgums release high levels of isoprene, especially during heat waves (See Table below). High isoprene levels can react with other chemicals in the air to form O3. As climate change pushes urban temperatures, these trees emit more defensive chemicals, compounding the very smog they were meant to mitigate.
Eucalyptus, poplars, and Willows emit exceptionally higher amounts of isoprene. Despite huge urban forestry value, large oak populations in warm urban heat islands can significantly elevate local summer O3 levels when paired with heavy morning traffic NOx emissions.
Sweetgum is frequently used as an ornamental street tree due to its brilliant autumn foliage and tolerance to compacted urban soils. However, it emits high volumes of isoprene during hot summer afternoons, coinciding with peak ground-level ozone formation windows in metropolitan regions.
Consider the geometry of the problem: urban canyons formed by high-rise buildings trap both heat and emissions. Street-level exhaust stagnates beneath the canopy. Sunlight bakes this stagnant cocktail. The oak above, thriving in the heat, releases more BVOCs. The result is a localised ozone hotspot—an unintended consequence of good intentions.
This does not mean trees are the enemy. It means monocultures are. When cities plant the same high-emitting species block after block, they create predictable chemical reactors. Diversity, in this case, is not just ecological insurance—it is atmospheric necessity.
The Smart Greening Revolution
The future of urban forestry demands a shift in perspective: from plant anything green to plant what breathes well with the city.
Look to the low-BVOC heroes. Ginkgo biloba, with its fan-shaped leaves, emits negligible isoprene. Red maple offers brilliant autumn colour without the chemical baggage. Linden trees provide dense shade and fragrant blossoms while keeping their volatile emissions in check. These species allow cities to build resilient canopies that cool neighbourhoods without feeding photochemical smog.
But species selection alone is not enough. The broader vision requires integrated design:
- Spatial strategy: Planting low-emitters near high-traffic corridors, and high-emitters only in well-ventilated areas away from NOx sources.
- Soil hydration: Stressed trees emit more BVOCs. Adequate watering reduces stress and curbs emissions.
- Clean transport: Electrifying bus fleets and promoting cycling reduces NOx, breaking the chemical handshake at its source.
The smart green city does not choose between trees and clean air. It designs for both—layering ecology, chemistry, and engineering into a canopy that cools, cleans, and coexists with its human inhabitants.
To maximise the cooling and carbon benefits of urban canopy while suppressing ground-level ozone formation, city planners implement targeted atmospheric and urban forestry strategies.
Strategic Species Selection (The Low-BVOC Mix)
Shift planting guidelines to enforce a 70/30 ratio favouring low-BVOC species (e.g., Maples, Linden, Ginkgo, Ash, and Dogwood) over high-isoprene emitters like Oaks and Sweetgums along high-traffic corridors.
Spatial Separation from NOx Hotspots
- Avoid planting high-isoprene-emitting species directly adjacent to congested highways, bus terminals, or industrial zones where vehicle tailpipe NOx concentrations peak.
- Reserve high-emitting shade trees for large suburban parks or windward greenbelts far from dense traffic gridlock.
Combined Urban Transport Policies
Ground-level ozone requires both BVOCs and NOx. Since trees cannot be easily removed, accelerating electric vehicle (EV) adoption and public transit electrification drastically cuts the NOx side of the chemical equation, neutralising the ozone-forming potential of existing oak and plane tree canopies.
Microclimate & Canopy Temperature Management
BVOC emissions increase exponentially with leaf temperature. Maintaining deep soil moisture via permeable pavements, rain gardens, and bioswales keeps tree canopies cool through transpiration, reducing stress-induced chemical emissions during heatwaves.
Tree Planting in Cities of Bangladesh for Low BVOC Emissions
Selecting native and climate-adapted tree species with low BVOC emission profiles is crucial for Bangladesh's dense urban centres (such as Dhaka, Chattogram, and Gazipur), where vehicular NOx levels are exceptionally high.
Tropical field studies in South Asia demonstrate that several ubiquitous, shade-providing native species emit negligible-to-low levels of isoprene and monoterpenes, making them ideal candidates for urban greening without worsening summer ground-level ozone.
Neem (Azadirachta indica)
BVOC Profile: Extremely low to non-detectable isoprene emissions.
Urban Suitability: Highly resilient to urban heat island effects, drought, and air pollution. Provides a broad evergreen canopy while filtering particulate matter (PM2.5).
Sonalu/Golden Shower (Cassia fistula)
BVOC Profile: Negligible/below detection limit for isoprene
Urban Suitability: Excellent medium-sized ornamental street tree with vibrant yellow blooms. Tolerates compacted urban soils and traffic pollution well.
Chhatim/Devil’s Tree (Alstonia scholaris)
BVOC Profile: Very low to non-detectable foliar isoprene flux
Urban Suitability: Grows rapidly, developing a multi-tiered horizontal canopy that provides shade along major avenues and highway medians.
Bakul (Mimusops elengi)
BVOC Profile: Low isoprene and terpene emission rates
Urban Suitability: A dense, evergreen native shade tree widely used in urban parks and residential green spaces; they are highly effective for noise reduction and local cooling.
Jarul/Queen’s Crape Myrtle (Lagerstroemia speciosa)
BVOC Profile: Low BVOC emission potential relative to other flowering tropical broadleaves
Urban Suitability: Outstanding roadside aesthetic tree that withstands waterlogging during monsoons and high urban heat during dry pre-monsoon months.
Kadam (Neolamarckia cadamba)
BVOC Profile: Moderate-to-low overall BVOC potential compared to high-emitting tropical species, such as Eucalyptus or Ficus.
Urban Suitability: Fast-growing native tree with massive leaf area, offering exceptional microclimate cooling when planted in urban parks or along riverbanks.
Native Species to Avoid in High-Traffic Zones in Bangladesh
For high-traffic, high-NOx urban corridors in Bangladesh, limit or avoid heavy plantings of:
- Eucalyptus (Eucalyptus spp.): Extremely high isoprene and monoterpene emitter
- Banyan & Sacred Fig (Ficus benghalensis & Ficus religiosa): High isoprene emitters under intense solar heat. Reserve these magnificent trees for rural areas or expansive parks rather than congested roadside medians.
- Mango (Mangifera indica): Moderate-to-high isoprene emitter during hot pre-monsoon flowering/fruiting periods. Better suited for backyard agroforestry than narrow traffic corridors.
- Jackfruit (Artocarpus heterophyllus): High isoprene emitter. Excellent for backyard agroforestry, urban parks, and residential gardens, but avoid planting it along major, high-traffic arterial roads.
- Java Plum/Jam (Syzygium cumini): High isoprene & monoterpene emitter. Valuable for biodiversity, fruit yield, and shade in urban greenbelts or larger public parks, but its volatile emissions make it sub-optimal for congested highway medians where tailpipe NOx levels peak.
Urban Planning Rule of Thumb
Plant high isoprene and/or monoterpene-emitting fruit trees, such as Jackfruit, Java Plum, and Mango, in the green zones (parks, residential yards, and community gardens) where they offer fruit and shade away from heavy traffic. For roadside medians and high-traffic avenues, prioritise non-isoprene or extremely low-BVOC species, such as Neem (Azadirachta indica), Sonalu (Cassia fistula), and Bakul (Mimusops elengi).
Conclusion
The urban canopy is not a simple solution; it is a living, breathing chemical participant in the city's atmosphere. Trees cool our streets and lift our spirits, yet under the summer sun, their natural defences can mingle with tailpipe emissions to create ozone—a hidden cost of greening.
However, hidden cost is not a reason to abandon urban forestry. It is a call to do it smarter. By choosing low-BVOC species, designing for airflow, and cutting fossil fuel emissions at the root, we can grow cities where canopies and clean air rise together. The paradox dissolves when we treat trees not as decorations, but as partners in atmospheric chemistry.