The retention characteristics for water-soluble and water-insoluble particulate matter of five tree species along an air pollution gradient in Beijing, China.

The air purification potential of plants has been widely studied and recognized. However, their specific capacities in retaining water-soluble (WSPM) and water-insoluble (WIPM) atmospheric particulate matter (PM) are still unclear. In order to recommend tree species with high air phytoremediation ability, the retention characteristics for WSPM and WIPM of five tree species under different haze pollution levels and PM retention durations in Beijing were evaluated after introducing ultrasonic cleaning procedure to the conventional leaf cleaning methods. The daily PM amount retained these species in the six central districts in Beijing (SCBD) was roughly estimated based on the field tree survey data in 171 plots randomly distributed within the Fifth Ring Road. The updated leaf cleaning method improved the evaluation accuracy for WSPM and WIPM by 54% and 31%, respectively. The particles retained by the broadleaf and coniferous species were mainly composed of WSPM (71%) and WIPM (64%), respectively. The diameter distribution of PM varied markedly with species, PM retention duration, and pollution level. However, it always showed a unimodal pattern for WSPM and no uniform patterns for WIPM. The average relative capacities of different species in retaining WSPM of TSP (PM ≤ 100 μm) were more stable with time, and the corresponding rank was Sophora japonica > Salix babylonica > Ginkgo biloba > Pinus tabuliformis > Sabina chinensis. Whereas, as to the WIPM of TSP, their order changed to S. japonica > P. tabuliformis > S. babylonica > G. biloba > S. chinensis. During the study period, the TPM (WIPM+WSPM) of TSP retained by these species per day in the SCBD reached 132.6 t (76.1 t WSPM + 56.5 t WIPM), accounting for a considerable proportion of the daily dust-fall amount. These findings can contribute to selecting greening tree species and managing the urban forest to improve urban air quality.