A high-resolution study of PM2.5 accumulation inside leaves in leaf stomata compared with non-stomatal areas using three-dimensional X-ray microscopy.

ABSTRACT

Plant leaves retain atmospheric particulate matter (PM) on their surfaces, helping PM removal and risk reduction of respiratory tract infection. Several processes (deposition, resuspension, rainfall removal) can influence the PM accumulation on leaves and different leaf microstructures (e.g., trichomes, epicuticular waxes) can also be involved in retaining PM. However, the accumulation and distribution of PM on leaves, particularly at the stomata, are unclear, and the lack of characterization methods limits our understanding of this process. Thus, in this study, we aimed to explore the pathway through which PM2.5 (aerodynamic diameter ≤ 2.5 µm) enters plant leaves, and the penetration depth of PM2.5 along the entry route. Here, an indoor experiment using diamond powder as a tracer to simulate PM2.5 deposition on leaves was carried out. Then, the treated and non-treated leaves were scanned by using three-dimensional (3D) X-ray microscopy. Next, the grayscale value of the scanned images was used to compare PM2.5 accumulation in stomatal and non-stomatal areas of the treated and non-treated leaves, respectively. Finally, a total PM2.5 volume from the abaxial epidermis was calculated. The results showed that, first, a large amount of PM2.5 accumulates within leaf stomata, whereas PM2.5 does not accumulate at non-stomatal areas. Then, the penetration depth of PM2.5 in stomata of most tree species was 5-14 µm from the abaxial epidermis. For the first time, 3D X-ray microscope scanning was used to confirm that a pathway by which PM2.5 enters the leaves is through the stomata, which is fundamental for further research on how PM2.5 translocates and interacts with tissues and cells in leaves.