Role of roadside vegetation as a passive method for urban air particulate matter absorption and its capturing efficiency under different conditions

Abstract

It has been widely discussed that air particulate matter (PM) has become a serious environmental issue which put great threats to human health and life-span. The respiratory and neurodegenerative disease are considered highly related to PM pollution. Most European urban residents are considered under the threat from PM2.5, and ultra-fine PM has even been reported to harm human DNA. With the acceleration of urbanization, traffic-related PM is becoming a large portion of anthropogenic PM in the urban area, finding an efficient way to mitigate the urban PM pollution is thus quite imperative.

Green vegetation has been accepted as efficient phytoremediation for urban PM reduction by former researches, but most studies focused on the PM capturing mechanism by vegetation which has large planting area, like city forest, urban green land, and city wetland. The efficiency of roadside vegetation which faces directly to the source of urban PM pollution was, however, rarely reported. This study tries to select the most efficient roadside plants by comparing the PM capturing efficiency of 12 common urban plant species with different leaf traits and leaf surface characteristics in Hanover, Germany; to summarize the similarity of highly efficient plant species; to explore the internal and external factors which lead to the efficiency disparities; and attempts to propose optimal using patterns with efficient roadside plants under different urban conditions.

Among all tested plants, Taxus baccata, Pinus nigra, Berberis thunbergii and Hedera helix were found to be the most efficient species; evergreen plants which had needle-shaped leaves were generally more efficient than deciduous species with broad leaves; species with small leaf area tended to possess higher efficiency than species with large leaves. Plant species with rough leaf surface which is resulted from a large amount of existing deep grooves, trichomes and stomata were found quite effective for PM capture. In addition, leaf wax and leaf hydrophilicity were also motivators for high capturing efficiency. In the view of finding optimal using patterns, needle-leaved plants were found efficient for both PM10 and PM2.5 capture under the high traffic pressure, while broad-leaved species was optimal for PM2.5 capture under the light traffic pressure; leaf surface with a height range of 1-2 m was the most effective area for large PM absorption and leaf wax was effective for fine PM reduction at all height ranges. In brief, complex planting pattern which combines both efficient needle-leaved and broad-leaved roadside vegetation, or a vertical green wall which is covered by efficient roadside plant species are recommended by us as optimal using patterns.

This study especially highlights the role of roadside vegetation as eco-friendly phytoremediation for urban PM pollution absorption and is supposed as a theoretical basis for future roadside green infrastructure management and city planning.