Autumn and winter air phytofiltration - Are plants able to biofilter air during peak pollutant emissions?

Air pollution is highest in winter. The high concentration of particulate matter (PM) and trace elements (TE) after the growing season is influenced by increased pollutant emissions, unfavorable meteorological conditions, and the low efficiency of air phytofiltration. Plants that can remove pollutants from the air during the growing season are leafless in autumn/winter, and therefore unable to capture PM/TE effectively. This study investigated the ability of nine species of leafy evergreen plants to accumulate PM (surface and in-wax PM; PM_2.5 and PM₁₀) and TE in autumn and winter. Plant material was harvested in November and December from the park in Wuhan, China. The amount of accumulated pollutants depended on the species. The shrubs (Loropetalum chinense, Pittosporum tobira, Rhododendron simsii) and grass (Ophiopogon japonicus), were more effective at phytofiltration of PM and TE per leaf area unit than the trees. However, to better understand the potential of plants to accumulate PM in relation to a unit of land area, the leaf area index (LAI) has to be considered. Ligustrum lucidum and P. tobira characterized by low LAI, despite having PM deposition comparable to other trees and shrubs, exhibited a markedly reduced efficacy of pollutants accumulation in relation to square metre of land they occupy. In contrast to the TE concentration in winter, PM deposition on plants did not always increase after the autumn, probably due to the park's low density of vegetation, PM resuspension by wind, and a decrease in the plants' physiological activity. Seasonal variations in pollutants accumulation among species were recorded during the autumn/winter. This study reinforces the need for biodiversity and higher-density urban greening to optimize post-growth air phytofiltration. A holistic, year-round air pollution mitigation strategy should be provided by incorporating more diverse evergreen plant species with complementary phytofiltering properties.