Green roofs reduce urban microplastic pollution by billions

By Dr. Priyom Bose, Ph.D.Reviewed by Lauren HardakerJun 13 2025

In a breakthrough for cleaner cities, scientists reveal that green roofs aren’t just beautiful. They are powerful pollution barriers that capture billions of airborne microplastics every year.

A recent study published in Communications Earth & Environment investigated the effectiveness of modular green roofs in intercepting and retaining microplastics. Research revealed that green roofs intercept microplastics from urban air, and this strategy could be leveraged in designing future stormwater and air-quality management strategies.

Study: Green roofs act as the first barrier to intercept microplastics from urban atmosphere. Image credit: LP2 Studio/Shutterstock.com

Microplastic Pollution in the Urban Landscape and Preventive Strategies

Scientists have detected microplastics in aquatic, terrestrial, and atmospheric ecosystems closely associated with human activities. Microplastics are introduced into coastal cities through complex processes involving the ocean, inland areas, and urban environments. Among various routes, stormwater transport, atmospheric deposition, and riverine input are common routes of microplastic pollution.

Over the years, researchers have focused on developing strategies to reduce urban atmospheric microplastic deposition. Most of these horizontal strategies, including bioretention ponds, constructed wetlands, and vegetated swales, have demonstrated remarkable efficiency in intercepting microplastics. Urban rooftops are primary receptors of both atmospheric wet and dry deposition and could be exploited as an opportunity to intercept atmospheric pollutants.

What Are Green Roofs?

Green roofs are roofs covered with plants and vegetation. These are also popularly known as vegetated roofs. Green roofs are typically made of a waterproofing membrane, a growing medium like soil, and vegetation or plants. These roofs provide significant environmental benefits, such as air cleaning, stormwater retention, and runoff purification.  Over the years, the popularity of green roofs has accelerated substantially. Understanding whether green roofs can intercept and reduce microplastic pollution is essential.

About the Study

The current study installed a pilot green roof module to assess its ability to intercept microplastics under various rainfall conditions. A total of four groups were designed, comprising a blank group (BL), a control group (CK), and two experimental groups (RR and SL).

The BL group supported a traditional roof configuration without a green roof module using a polypropylene (PP) box. The CK group contained a green roof module without plants. The RR was planted with Rhodiola rosea in the experimental green roofs, whereas the SS green roof contained Sedum lineare. Both these plants are common green roof species in Shanghai, China. Except for BL, all other experimental settings contained a rainfall generator, a collector, and a green roof module.

The planting soil comprised bio-organic fertilizer, vermiculite, peat perlite, and fermented alcohol. Based on intensity, the current study simulated four types of rainfall events: light, moderate, heavy, and torrential. The rainfall events were repeated three times and lasted 1-2 hours per feeding. Two types of microplastics, rubber in fragment form and polyurethane (PU) in fiber form, were used for the experimental findings.

Study Findings

The current study measured effluent microplastic concentrations from green roofs after independent rainfall events. Microplastic concentration ranged between dozens and hundreds, and its number decreased within the first 20 minutes.

The concentration reached two to five times higher than that in the later stages of rainfall. Importantly, this earl spike was not due to the classic ‘first flush’ effect seen in surface runoff but rather due to the rapid percolation of microplastics through the substrate and limited retention capacity and the green roof soil. The inflow microplastic concentration remained constant throughout the rainfall event, supporting this distinction.

This trend highlights that due to their substrate properties, green roofs allow microplastics to drip downward rapidly with rainfall rather than being gradually washed off like accumulated pollutants on impervious surfaces.

Typically, green roofs experience long-term dry and wet cycles, which may alter their internal structures and affect the trapping of pollutants. For instance, cracks and channels formed during dry spells may facilitate microplastics' passage more readily in the first 20 minutes of subsequent rainfalls. Each pilot green roof model exhibited an average microplastic interception rate of over 97.5% for simulated rainfall events. This indicates the efficacy of green roofs in capturing atmospheric microplastics and filtering them out during wet depositions.

The study quantified where microplastics were retained: the planting soil layer trapped between 66% and 92% of intercepted microplastics, while the overground part of vegetation contributed modestly (up to about 24% in some cases, depending on plant species). The contribution of roots and other layers was minimal. The retention of fibers by the vegetation layer was lower than that of fragments, partly because fibers are more easily resuspended by air turbulence.

Considering Shanghai has approximately 3.56 million m² of green roofs, the annual flux of atmospheric microplastics intercepted by green roofs could exceed 1.70 × 10¹² n yr⁻¹, assuming a minimum interception efficiency of 97.5%. Green roof performance may vary due to differences in roof inclination, plant selection, substrate composition, and local climate conditions.

Other factors affecting green roofs' microplastic interception efficiency are temporal variations (e.g., prolonged droughts) that may influence the filtration and hydrological processes. Prolonged droughts can also decrease retention efficiency and may even facilitate the secondary release of previously captured microplastics.

The study also cautions that these estimates are based on controlled conditions and may not represent all real-world scenarios; further large-scale field investigations are needed.

Among different rainfall events, the highest microplastic concentrations were estimated during light rainfall, ranging between 40 n L−1 and 367 n L−1, while the lowest concentrations were obtained at heavier precipitations. Microplastics interception efficiency was found to increase at higher precipitation intensity. Since the surface of green roofs is permeable, the infiltration process could dominate the effluent microplastic concentration.

In contrast to RR and SL, effluent microplastic concentrations were slightly lower in CK. This was attributed to the effect of plant roots on the substrate's porosity. Sedum lineare formed a denser root system, creating more channels for water flow, allowing more microplastics to pass through, slightly reducing retention efficiency compared to the unplanted control.

Furthermore, fragments exhibited slightly higher removal efficiency than fibers in both CK and RR treatments, with efficiencies of 98.6% and 98.5%, respectively, compared to 96.7% and 97.0% for fibers. The elongated, flexible shape of fibers makes them more likely to bypass interception and more susceptible to resuspension into the atmosphere, meaning a fraction of captured fibers may be lost during windy or turbulent events. The study found that up to 7.9% of retained microplastics, mainly fibers, could be re-released by air turbulence, potentially limiting the effectiveness of green roofs as a permanent sink for these pollutants.

Long-term green roof use could generate microplastics due to the aging and degradation of its plastic components. However, the plant root system and soil microbes could aid in plastic degradation. The study found that PP components, commonly used in green roof construction, underwent measurable aging and fragmentation, releasing new microplastics over time, particularly in the presence of plants and wet-dry cycling.

The authors recommend using non-plastic materials for specific roof layers, such as fabrics and drainage materials, to minimize the risk of green roofs becoming a source of microplastic pollution. Periodic maintenance or substrate replacement may also be necessary to sustain long-term performance and reduce risks of secondary pollution.

Additionally, the study highlights possible innovative remediation strategies, such as introducing soil invertebrates such as earthworms or mealworms, which may aid in degrading or mineralizing accumulated microplastics, reducing long-term accumulation, and maintaining filtration performance.

Conclusions

The current experimental findings support the installation of green roofs on regional and city scales, which could substantially reduce atmospheric microplastic pollution cost-effectively. Green roofs were more effective at filtering microplastics and preventing their downstream transport under higher rainfall intensities.

However, the study emphasizes that fibers are more likely to escape capture or be re-released, and the materials used in green roofs should be carefully selected to avoid adding to the problem. Ongoing maintenance and monitoring are also important, especially under changing climatic conditions.

In the future, large-scale research is required to understand the variability of green roof performance in real-world settings. The study findings suggest exploring biological solutions, such as using soil invertebrates, to help manage microplastic accumulation over time.

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