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Particulate matter (PM) in the atmosphere is a threat to human health. Wetland plants were confirmed to accumulate particles on the leaf surface; at the same time, rainfall could wash-off particles and accelerate the whole removal process, however, the dynamic processes occurring during rainfall events on wetland plants remain unclear. In order to provide sustainable strategies for authorities to take measures, we need to figure out how to reduce PM on leave surface by artificial rainfall efficiently. Four wetland species (Scirpus validus, Typha orientalis, Phragmites australis, and Iris wilsonii) were selected to examine for leave surface accumulation and simulate the experiment. We estimated the wash-off ability of rainfalls with three different intensities (15, 30, and 60 mm h−1) and determined the proportions of different PM size-fractions washed by the rains. The results showed that particles accumulated on the surface could be washed off efficiently (78% ∼ 89%) by the simulated rainfalls. The removal rates were high in the first 30 min and large particles comprised a large proportion of the removed particles. The rainfall with the intensity of 30 mm h−1 removed the most particles among three different rainfall intensities. When the rainfall intensity increased, fine particles (PM 2.5) could be washed off more easily. Moreover, with a thinner wax layer, fine particles on wetland plants' leaf surfaces might be more easily removed by the rains. While wash off the plants, spraying rains with the intensity of 30 mm h−1 for about 30 min every time (high intensity with shorter time) may be particle removal efficiency. Image 1 • Indicated the PM removal process dynamics during rainfall on wetland plants. • Leaf surface PM could be washed off efficiently (78% ∼ 89%) by the simulated rainfalls. • The removal rates were high in the first 30 min during rainfall. • The rainfall with the intensity of 30 mm h−1 removed the most particles. [ABSTRACT FROM AUTHOR]

[Display omitted] Apparently, plant leaf surface particulate matter (PM) can be effectively removed by rainfall, leaving space on the leaf surface to adsorb and retain PM again. At present, most of the relevant studies adopt the method of artificial rainfall simulation to quantify rainfall characteristics in terms of rainfall intensity, and mostly focus on one category of plants. Studies of rainfall intensity on different categories of plants are lacking. Therefore, in this study, common urban greenery plants were classified into four categories, namely, trees, shrubs, herbs, and aquatic plants. Artificial rainfall was used and three different gradients of rainfall intensity, 30 mm/h, 45 mm/h and 60 mm/h, were set. The mass of per unit leaf area PM removal was calculated using the elution weighing method and the filter membrane method was used to classify the PM into three particle sizes: coarse (10–100-μm), fine (3–10-μm) and ultrafine (0.4–3-μm). It was devoted to investigating the differences in the effect of rainfall intensity on the removal of PM from the leaf surfaces of different categories of common urban greenery. The results showed that the removal of coarse particles by rainfall was the highest under different rainfall intensities. At the same time, the removal effects of rainfall on leaf surface PM of herbs were the highest under different rainfall intensity, while that of trees was the lowest. Among the same category of plants, Salix matsudana had better particle retention capacity among trees, Buxus sinica among shrubs, Poa pratensis among herbs, and Acorus calamus among aquatic plants. In addition, all tested plants had the highest mass of PM removed from leaf surface units at high rainfall intensity (60 mm/h). However, at a higher rainfall intensity (45 mm/h), the mass of PM on plant unit leaf surface removed was lower than the minimum rainfall intensity (30 mm/h). The results of the above study can provide a basis and theoretical support for the selection of species for urban green space restoration, as well as a reference for the establishment of urban cleaning systems from the perspective of the relationship between rainfall intensity and PM on plant leaf surfaces. [ABSTRACT FROM AUTHOR]

Atmospheric particulate matter (PM) deposition which involves both dry and wet processes is an important means of controlling air pollution. To investigate the characteristics of dry and wet deposition in wetlands, PM concentrations and meteorological conditions were monitored during summer at heights of 1.5 m, 6 m and 10 m above ground level at Cuihu Wetland (Beijing, China) in order to assess the efficiency of PM2.5 (particles with an aerodynamic size of <2.5 μm) and PM10 (particles with an aerodynamic size of <10 μm) removal. The results showed: Daily concentrations of PM, dry deposition velocities and fluxes changed with the same variation trend. The daily average deposition velocity for PM10 (3.19 ± 1.18 cm·s–1) was almost 10 times that of PM2.5 (0.32 ± 0.33 cm·s–1). For PM2.5, the following dry deposition fluxes were recorded: 10 m (0.170 ± 0.463 μg·m–2·s–1) > 6 m (0.007 ± 0.003 μg·m–2·s–1) > 1.5 m (0.005 ± 0.002 μg·m–2·s–1). And the following deposition fluxes for PM10 were recorded: 10 m (2.163 ± 2.941 μg·m–2·s–1) > 1.5 m (1.565 ± 0.872 μg·m–2·s–1) > 6 m (0.987 ± 0.595 μg·m–2·s–1). In the case of wet deposition, the relative deposition fluxes for PM2.5 and PM10 were 1.5 m > 10 m > 6 m, i.e. there was very little difference between the fluxes for PM2.5 (0.688 ± 0.069 μg·m–2·s–1) and for PM10 (0.904 ± 0.103 μg·m–2·s–1). It was also noted that rainfall intensity and PM diameter influenced wet deposition efficiency. Dry deposition (63%) was more tilted towards removing PM10 than was the case for wet deposition (37%). In terms of PM2.5 removal, wet deposition (92%) was found to be more efficient. [ABSTRACT FROM AUTHOR]

Today, particulate-matter (PM) pollution has become one of the most severe air-pollution problems. As the most commonly used method in daily life, phytoremediation can use plant organs (such as leaves) as biological filters for pollutants to repair the atmosphere. At the same time, rainfall can remove PM from plant-leaf surfaces and enable them to adsorb PM again. By simulating natural rainfall, the rainfall characteristics are quantified as rainfall intensity and rainfall duration, and we use the washout-weighing method to obtain the amount of PM removed from the leaf surface. Then, use a scanner to scan the leaves after rain to get their images, and use Image J software to process the images to obtain leaf area. Finally, the amount of PM removed by rain per unit leaf area can be calculated. It will be used to explore the impact of different rainfall intensity and duration on the removal of PM from the leaf surface of wetland plants. The results showed that under three rainfall intensities used in this experiment, the removal of PM from plant-leaf surfaces all increased with an increase in rainfall duration. When the particle size is 10-100-μm, and the rainfall intensity is 30 mm/h, the removal amount of plant particles tested in this experiment is the largest. With increased rainfall duration, the removal of PM from plant-leaf surfaces increased sharply at first, then slowly, and finally tended to be stable. The removal efficiency of PM on the blade surface is most apparent at the early stage of rainfall, and then gradually weakens. Among the four wetland plants tested in this experiment, in the range of 10-100-μm, the number of PM on the leaf surface of Scirpus validus is the largest, and the optimum rainfall intensity is 30 mm/h; in the range of 2.5–10-μm, the number of PM on the leaf surface of Typha orientalis is the largest, and the optimal rainfall intensity is 30 mm/h; in the range of 0.45–2.5-μm, the number of PM on the leaf surface of Iris wilsonii is the largest, and the optimal rainfall intensity is 15 mm/h. Wetland species with high particle accumulation capacity can provide references for vegetation restoration of degraded wetland plants and plant cultivation in constructed wetlands. At the same time, the best rainfall intensity and duration for removing particulate matter on the surface of plant leaves were obtained through experiments, which provided a reference for the design of automatic plant irrigation systems and dust removers in different scenarios. • Rainfall intensity and duration affect the particulate removal process significantly. • The maximum particle removal amount occurs at 30 mm/h precipitation. • The most easily removal particle size ranges from 10 to 100 μm. • Three stages that change with the duration of rainfall: rapid growth, slow growth and stable period. [ABSTRACT FROM AUTHOR]