Coagulation effect of aero submicron particles on plant leaves: Measuring methods and potential mechanisms

Aero submicron particles (d < 1 μm) have attracted widely attention due to their difficulty in removal from the air and serious threat to human health. Leaves are considered as important organs to purify particulate matter and alleviate air pollution. However, the current research mainly focuses on the removal capacity of particulate matter by urban plants at different scales, there are relatively few studies on the change of particle diameter at the air-leaf interface during this process. This study is one of the first to propose the existence of coagulation effect of aero submicron particles on the leaves, and a sweep-resuspension method and X-ray microscope were used to measure such size changes of two typical subtropical broad-leaf plants. The results showed that the size of submicron particles increased significantly during the migration from atmosphere to leaf surface: the average particle size increased from 0.48 μm at emission to 3.40 μm on the leaf surface, while the proportion of submicron particles decreased from 95% to less than 20%. The sweep-resuspension method was easy to implement, the data was easy to obtain, and the cost was low, therefore it could be widely used in the determination of the coagulation effect. The coagulation effect was also inferred as an important mechanism used by plants to reduce particulate matter. In the process of particulate removal: coagulation effect and dry deposition are actually two steps that occur simultaneously and interact. This finding refined the understanding of particulate removal processing, and laid a foundation for further research on factors affecting coagulation, which can be helpful for optimizing tree species selection and plant arrangement.