Particulate matter capturing capacity of roadside evergreen vegetation during the winter season

In the urban atmosphere, particulate matter (PM), especially PM discharged by vehicle traffic, is a serious threat to residents’ health. As the PM capturing ability of most deciduous plants is significantly weakened during winter, it is important to further understand the PM removal capacity of evergreen species. Four representative roadside evergreen plants (two tree species and two shrub species) were tested along one main street in Hanover, Germany. The results showed that, in winter, notable differences existed among the evergreen species in terms of their PM capturing capacity. In general, Taxus baccata was the most efficient species, while Prunus laurocerasus was the least efficient. The capacity of each species varied in each month, but it was generally observed that T. baccata was the most stable and efficient species for both PM10 and PM2.5 capture during the whole winter. Its PM capturing capacity reached its peak value in December and then gradually declined. From November to January, Pinus nigra showed a high PM capturing capacity at first, but its capacity later declined sharply. Though Hedera helix had two peak values, in December and in February, its capacity was still much lower than that of either of the two needle-leaved species. P. laurocerasus was the most inefficient species during all winter months. Through SEM observation, it was found that the abundance of ridges and grooves increased the roughness of the leaf surface of T. baccata, and thus, there was sufficient room on the leaf to capture PM. Among all tested species, the leaf surface of P. laurocerasus was the smoothest, and the fewest particles were observed on its leaves. A negative correlation was found between the leaf surface contact angle and its PM capturing capacity. P. laurocerasus showed the largest contact angle and the lowest PM capturing capacity, while the most efficient species, T. baccata, had the smallest contact angle. This indicated that hydrophilic leaves tended to have a higher capacity for PM capture. This study highlights the importance of evergreen roadside plants for PM pollution management during winter and provides insights for further roadside green infrastructure planning to improve urban air quality.