A breath of fresh air in the city might be more complicated than it seems.
As cities worldwide plant millions of trees to create greener urban landscapes, a surprising scientific discovery is emerging: the very greenery planted to improve urban environments might be contributing to a different form of air pollution. This paradox lies at the heart of cutting-edge research into biogenic volatile organic compounds (BVOCs)—invisible gases emitted by plants that play a complex role in urban air quality. Understanding this hidden chemical language of plants is becoming increasingly crucial as cities strive to create healthier environments for their growing populations.
Walk through a pine forest and breathe in the fresh scent, or notice the earthy aroma after mowing the lawn—you're actually detecting biogenic volatile organic compounds. These chemicals are naturally released by plants and account for a staggering 90% of all volatile organic compounds in our global atmosphere 5 . While they're natural, their interaction with human-made pollutants creates a complex chemical dance in urban skies.
The most abundantly emitted BVOC, representing approximately 70% of total global emissions.
Known for their distinctive scents in conifers and citrus trees.
Heavier compounds that play a significant role in particle formation.
| BVOC Type | Chemical Formula | Relative Abundance | Primary Sources | Key Properties |
|---|---|---|---|---|
| Isoprene | C₅H₈ | ~70% of total BVOCs | Broadleaf trees (oak, poplar) | Highly reactive, influenced by temperature and sunlight |
| Monoterpenes | C₁₀H₁₆ | ~11% of total BVOCs | Conifers, citrus trees | Contribute to forest scent, form aerosols |
| Sesquiterpenes | C₁₅H₂₄ | ~2.5% of total BVOCs | Many flowering plants | Higher molecular weight, rapid ozone reaction |
| Other BVOCs | Various | ~16.5% of total BVOCs | Various plants | Includes alcohols, esters, carbonyls |
The central environmental challenge with BVOCs lies in their atmospheric reactivity. When these plant-emitted compounds interact with nitrogen oxides (NOₓ) from vehicle exhaust and industrial emissions in the presence of sunlight, they trigger complex photochemical reactions that produce ground-level ozone—a key component of smog and a respiratory health risk 1 5 .
Small increases in VOCs can lead to significant ozone production 1
Higher temperatures accelerate BVOC emission rates 1
Abundant nitrogen oxides enable ozone formation 3
"Although BVOCs are naturally emitted and not directly anthropogenic, their photochemical derivatives—tropospheric ozone and secondary organic aerosol—are recognized as hazardous pollutants under elevated anthropogenic influences" 5 .
To understand how significant this phenomenon can be, let's examine a groundbreaking 2025 study conducted in Guangzhou, China—a city that had actively expanded its urban green spaces as part of environmental improvement efforts 1 .
Researchers reconstructed leaf area index datasets specifically for urban areas, providing detailed information about vegetation density and distribution 1 .
Using the Model of Emissions of Gases and Aerosols from Nature (MEGANv3.1), the team estimated BVOC emissions from urban green spaces 1 .
The Weather Research and Forecasting model coupled with the Community Multiscale Air Quality model (WRF-CMAQ) simulated how these BVOC emissions affected ozone formation under different scenarios 1 .
The team compared scenarios with and without UGS-BVOC emissions to isolate their specific impact from other factors 1 .
Urban green spaces in Guangzhou emitted approximately 666 Gg of BVOCs annually, with isoprene and monoterpenes contributing significantly to these emissions 1 .
UGS-BVOC emissions accounted for approximately 33.45% of total VOC emissions in the city center—a substantial portion that had been previously underestimated in regional models 1 .
Including these emissions in models improved simulation accuracy, reducing mean biases in maximum daily 8-hour average (MDA8) ozone from -3.63 to -0.75 ppb in the city center 1 .
During pollution episodes, UGS-BVOC emissions added up to 8.9 ppb to MDA8 ozone levels—a significant increase considering the U.S. EPA standard is 70 ppb 1 .
| Impact Metric | Impact Range | Context & Significance |
|---|---|---|
| Monthly Mean Ozone Increase | 1.7-3.7 ppb (+3.8%-8.5%) | Combined effect of BVOC emissions and land use change |
| Urban Monthly Mean Ozone | 1.0-1.4 ppb (+2.3%-3.2%) | From BVOC emissions alone in urban areas |
| MDA8 Ozone During Pollution Episodes | Up to 8.9 ppb increase | Significant contribution during worst air quality days |
| Model Improvement | Bias improved from -3.63 to -0.75 ppb | Including BVOCs dramatically improved prediction accuracy |
Understanding BVOC emissions and their impacts requires sophisticated tools and techniques. Researchers employ a diverse array of methods to detect, measure, and predict these elusive compounds:
| Research Tool | Primary Function | Key Features & Applications |
|---|---|---|
| PTR-MS (Proton Transfer Reaction-Mass Spectrometry) | Real-time BVOC detection | High sensitivity, minimal sample preparation, field-deployable 4 5 |
| GC-MS (Gas Chromatography-Mass Spectrometry) | Precise compound identification | Separates and identifies specific BVOCs; gold standard for verification 5 |
| MEGAN (Model of Emissions of Gases and Aerosols from Nature) | Predicting BVOC emissions | Incorporates temperature, light, vegetation type; used in climate models 1 5 |
| Enclosure Systems | Leaf-scale emission measurements | Portable chambers capture emissions from individual plants 6 |
| WRF-CMAQ (Weather Research and Forecasting & Community Multiscale Air Quality) | Modeling air quality impacts | Simulates complex atmospheric chemistry and pollutant transport 1 |
| Eddy Covariance Systems | Ecosystem-scale flux measurements | Measures vertical exchange of BVOCs between vegetation and atmosphere |
These tools have revealed that BVOC emissions aren't constant—they vary by plant species, time of day, temperature, light levels, and even stress factors like drought or insect damage 4 6 . For example, research in Montreal and Helsinki found that street trees often had different emission patterns compared to park trees of the same species, highlighting how urban growing conditions influence BVOC release 6 .
The solution isn't to eliminate urban greenery—the benefits of urban forests for cooling, mental health, and carbon sequestration are too valuable. Instead, researchers suggest smarter approaches to urban greening.
Choosing tree species with lower BVOC emission potentials represents one of the most practical management strategies. Research consistently shows significant variation in emissions between species 6 . Isoprene emissions, particularly significant for ozone formation, vary dramatically—oaks and poplars are high emitters, while maples, birches, and many conifers are typically lower emitters 5 6 .
Rather than planting monocultures, cities can create diverse canopies that balance high- and low-emitting species while considering the proximity to pollution sources. This includes avoiding planting high-BVOC species in areas already struggling with high ozone levels, considering the placement of trees relative to streets and industrial areas, and balancing the carbon sequestration benefits of fast-growing high emitters with the air quality benefits of low emitters 1 5 .
Since BVOCs need nitrogen oxides to form ozone, controlling NOₓ emissions from vehicles and industry remains crucial. As one review notes, "stringent controls on anthropogenic precursors (e.g., anthropogenic volatile organic compounds (AVOCs)) could serve as a complementary measure to mitigate secondary pollution" 5 . This dual approach addresses both sides of the ozone formation equation.
Emerging technologies like biofiltration show promise for controlling VOCs from concentrated sources. This biological process uses microorganisms to break down VOCs and has proven effective for industrial applications, offering a more environmentally friendly alternative to incineration 2 .
The discovery that urban greenery contributes to ozone formation isn't a reason to abandon tree-planting initiatives, but rather an opportunity to approach urban forestry with greater sophistication. The cities of tomorrow can be both greener and cleaner if we listen to the silent chemical language of plants and understand their interactions with our urban atmosphere.
As research continues to untangle these complex relationships, scientists are developing more accurate models and detailed emission databases to guide urban planners 6 . The future of urban air quality management lies in integrated strategies that acknowledge the dual nature of our botanical companions—recognizing that while plants provide essential services to cities, they must be selected and placed with careful consideration of their atmospheric chemistry.
By embracing this nuanced understanding, we can move beyond simplistic "greening" toward truly sustainable urban ecosystems where both people and plants can breathe easier.