Size distribution and elemental composition of airborne particulate matter on four plant species in vertical greenery systems

Vertical greenery systems (VGSs) have been proven to greatly improve the urban thermal environment. However, their effect on the retention of airborne particulate matter (PM) in the road environment remains unclear. This study aimed to assess the quantity of size-segregated PMs on both the upper (adaxial) and lower (abaxial) leaf surfaces, the mass of size-segregated PMs on the leaf surface and within the leaf wax, as well as the elemental composition of PMs retained in leaf stomata of four plant species in VGSs in Guangzhou. The findings revealed that VGSs exhibited greater retention of fine (<2.5 mu m) PMs when compared to coarse (2.5-10 mu m) and large (>10 mu m) PMs. Among the four species studied, Tradescantia spathacea exhibited the highest PM retention in VGSs. The number density of accumulated PMs on the upper and lower leaf surfaces of the four plant species of VGSs at three heights varied significantly with species, heights, and their interaction (P < 0.05). The greatest stomatal block rate by PM occurred at a height of 0.6 m, the range was 91.1%-98.0%. The composition of PMs retained by the leaves primarily consisted of common element of C, O, Ca, K, Mg, and Si, which likely originated from natural sources. Heavy metals (HMs) such as Cr and Cd may have originated from brakes, while Zn and Cu might be attributed to tire wear and exhaust emissions. Therefore, VGSs effectively retained a substantial quantity of airborne PM and contributed to the mitigation of urban air pollution issues.