Beyond leaf traits: context dependent particulate matter accumulation across outdoor, indoor and active green walls

Nature-based Solutions (NbS) have been shown to effectively remove air pollutants, such as particulate matter (PM), from the atmosphere, with this capacity being largely dependent on the plant species-specific morpho- logical traits. However, other factors including chemical interactions and atmospheric and microclimatic pa- rameters can also affect PM removal efficiency. To test this hypothesis, we compared PM accumulation across five plant species with distinct morphological traits (Chlorophytum comosum, Hedera helix, Monstera deliciosa, Nephrolepis exaltata, Tradescantia zebrina) grown on outdoor (OGW) and indoor (IGW) green walls. Additionally, four of these species were tested in an active green wall (AGW) installed within a closed chamber with forced ventilation, where M. deliciosa could not be included because of space constraints. Leaf surfaces were analysed via Scanning Electron Microscopy (SEM) to characterise the micromorphological features associated with PM accumulation. The deposited PM load was quantified using a combination of gravimetric and conductimetric approaches, while the PM chemical composition was assessed by Microwave Plasma Atomic Emission Spec- troscopy (MP-AES). Significant statistical differences in PM capture were observed among the species. Furthermore, the species exhibited distinct PM accumulation patterns across the different green wall systems, showing species-specific affinities for certain potentially toxic elements. This demonstrates that PM retention is not driven by leaf morphology alone. Among species, Chlorophytum comosum performed best in OGW and IGW. Finally, the presence of the plants in the AGW reduced PM concentration by approximately 50%, while high humidity enhanced wet deposition and increased fine-particle capture, confirming the critical role of microcli- matic factors in air purification.