Understanding the washoff processes of PM2.5 from leaf surfaces during rainfall events

Abstract

The PM on plant surface being washed into soil during the rain is one of the key processes for plants to reduce atmospheric PM. This study attempted to explore the PM2.5 washoff processes from different tree leaves during rain events (<16 mm/h) using an artificial rainfall simulation experiment. We aimed to improve our knowledge of processes associated with PM2.5 reduction and to provide a basis for accurately evaluating the ability of plants to reduce PM2.5. This is the first study showing that the PM2.5 washoff processes from leaves follow quartic functions and 4 pattern curves under different conditions were categorized. They respectively explained the PM2.5 washoff processes in broad-leaved trees with large leaves and simple crowns (bimodal curved), in conifer species with small leaves and complex crowns under high rainfall intensity (unimodal curved), in trees with extremely complex crowns under high rainfall intensity (continually-rising curved), and in conditions under which extremely high water interception efficiency but rather low water storage capacity occur (U-shaped curved). These curves indicate that the amount of PM2.5 on leaves was not necessarily reduced in rainfall events. The general ranking of the average values of PM2.5 number on leaves surface during rain events was Cedrus Deodara (40.3 × 103/cm2), Japanese Maple (33.0 × 103/cm2) > Dragon Juniper (14.7 × 103/cm2), Dawn Redwood (12.6 × 103/cm2) > Common Boxwood (6.4 × 103/cm2), Lotus Magnolia (4.1 × 103/cm2). The cycles of PM2.5 accumulation and removal on broad-leaved trees might be shorter than that of conifers, meaning that they may have a better PM2.5 washoff efficiency during rain, which is opposite to the PM2.5 deposition efficiency. Higher rainfall intensity can reduce the cycle length and enhance the PM2.5 washoff efficiency.