Reduction of traffic-related particulate matter by roadside plants: effect of traffic pressure and sampling height.

Atmospheric particulate matters (PM) caused by urban traffic system put residents' health at serious risk. As a method of phytoremediation to mitigate this risk, roadside plants show significant potential to remove PM from the air. However, the relationship between traffic pressure, height, and leaves' capturing capacity is rarely reported. In this study, two common effective roadside plants (Hedera helix, Taxus baccata) with typical leaf shapes (broad and needle-leaved) were selected to explore the foliage capacity under high, middle and low traffic burden. A green wall covered by H. helix was tested to find out the relationship between heights and the amount of accumulated PM. Although the PM capturing capacity varied between different traffic pressures, needle-leaved species generally accumulated more PM than broad-leaved species. For PM10 capturing, needle-leaved species showed higher capacity under all traffic pressures except under the low traffic pressure. For PM2.5 capturing, needle-leaved species accumulated more PM only under the high traffic pressure. Needle-leaved plants were more sensitive to the change of traffic pressures, its PM capturing capacity changed notably between different traffic pressures because its leaf wax is more susceptible to be corroded. Leaf surface contact angle was slightly affected by the change of traffic pressure for broad-leaved species, but for needle-leaved species, it changed greatly. Leaf surface was the main zone for large PM capturing because the large PM was more likely to be stuck in grooves on the leaf surface, while leaf wax was mainly for fine PM absorption because PM with small sizes could adhere to the surface of the wax crystal. By comparing the amount of captured PM by leaf surface of urban roadside plants under different traffic pressures and height ranges, this study optimizes the benefits of roadside plants as traffic-related PM filter under different traffic conditions.