Quantification of the traffic-generated particulate matter capture by plant species in a living wall and evaluation of the important leaf characteristics

Traffic-generated particulate matter (PM) is a significant fraction of urban PM pollution and little is known about the use of living walls as a short-term strategy to reduce this pollution. The present study evaluated the potential of twenty living wall plants to reduce traffic-based PM using a living wall system located along a busy road in Stoke-on-Trent, UK. An Environmental Scanning Electron Microscope (ESEM) and ImageJ software were employed to quantify PM accumulation on leaves (PM1, PM2.5 and PM10) and their elemental composition was determined using Energy Dispersive X-ray (EDX). Inter-species variation in leaf-PM accumulation was evaluated using a Generalized Linear Mixed-effect Model (GLMM) using time as a factor; any differential PM accumulation due to specific leaf characteristics (stomatal density, hair/trichomes, ridges and grooves) was identified. The study showed a promising potential for living wall plants to remove atmospheric PM; an estimated average number of 122.08 ± 6.9 × 107 PM1, 8.24 ± 0.72 × 107 PM2.5 and 4.45 ± 0.33 × 107 PM10 were captured on 100 cm2 of the living wall used in this study. Different species captured significantly different quantities of all particle sizes; the highest amount of all particle sizes was found on the leaf-needles of Juniperus chinensis L., followed by smaller-leaved species. In the absence of an apparent pattern in correlation between PM accumulation and leaf surface characteristics, the study highlighted the importance of individual leaf size in PM capture irrespective of their variable micro-morphology. The elemental composition of the captured particles showed a strong correlation with traffic-based PM and a wide range of important heavy metals. We conclude that the use of living walls that consist largely of smaller-leaved species and conifers can potentially have a significant impact in ameliorating air quality by removing traffic-generated PM pollution to improve the wellbeing of urban dwellers