| The urban atmospheric particulate matter pollution was increasingly serious and had aroused wide concern of the government and the all sectors of society. Vegetation, especially the woody plants had an effect on controling and eliminating of atmospheric particulates by covering the surface to reduce particulate matter sources, retaining atmospheric particulate matter and affecting meteorological factors to indirectly affect the atmospheric particulate matter, and so on. Therefore, in recent years, reducing atmospheric particulate matter pollution with the aid of urban trees had the attention of the academia and relevant functional departments. There was lack of accurate study method on measuring particulate matter retention amount and particle size distribution of vegetation. There was also lack of systemic study on the typical stand configuration mode regulating the particulate matter (especially PM2.5, diameter, d≤2.5 μm) in Beijing. So this article carried out the related studies around the above two aspects.The research content mainly included:(1) To explorate a quantitative methodology to characterize the retention of PM2.5 and other atmospheric particulate matter by plant leaves. (1)Ginkgo biloba on the edge of urban road in Beijing was taken as an example to investigate the spatial variability of particulate matter retention amount on leaves within tree canopy in this study. PM2.5, PM10 and TSP retention amount by per unit area leaf were analyzed. (3) On the basis of research of existing scenic recreational forests types and basic features, the retention ability of stand to PM2.5, PM10, TSP and other atmospheric particulate matter and its relationship with meteorological factors were sdutied. And typical stand configuration modes regulating PM2.5 and other atmospheric particulate matter were proposed.The results showed that:(1) A methodology called elution-weighing-particle size-analysis (EWPA) was proposed to evaluate quantitatively retention amount and particle size distribution of atmospheric particulate matter by plant leaves. This method achieved a direct, accurate measurement about the quality and particle size distribution of atmospheric particulate matter retained by plant leaves, had superior operability. Its main features were:measurement processes were cleaning, centrifugation and drying, the particulate matter was collected and weighed by, then its particle size distribution was analysed by laser particle size analyzer. Finally, the mass of particulate matter retained by unit area of leaf and stand was translated from the leaf area and leaf area index; the particle size was in the range of 0.375 to 2000 μm. This method was applied to a Populus tomentosa stand which had not experienced rain for 27 days (the days when PM2.5 daily average concentration beyond level 2 standard days were up to 20 days) in Beijing Olympic Forest Park. The results showed that the average particle size of the atmospheric particulate matter retained by Populus tomentosa was 17.8 μm, and the percentages of the retained PM2S, inhalable particulate matter (PM10, d≤10 μm) and total suspended particle (TSP, d≤ 100 μm) were 13.7%,47.2%, and 99.9%, respectively; the masses of PM2.5, PM10, TSP and total particulate matter were 8.88×1016,30.6×10-6,64.7×10-6and 64.8×10-6 g-cm-2, respectively. The retention quantities of the Populus tomentosa stand to PM25, PM10, TSP and total particulate matter were 0.963, 3.32,7.01 and 7.02 kg-hm-2, respectively.(2) When the wind force was 0-2 grade and the air pollution was not serious, there was no obvious canopy spatial distribution pattern of the PM25, PM10, TSP retention amount by per unit area leaf of Ginkgo biloba on the edge of urban road. The distribution of PM25 and PM10 retention amount by per unit area leaf was relatively similar, PM25 and PM10 retention amount of lower leaves were greater, and TSP retention amount of higher leaves was greater. Besides height had significant effects on PM2.5 retention amount by per unit area leaf (P<0.05), other factors and interactions had no significant influence on PM2.5, PM10, TSP retention amount by per unit area leaf (P> 0.05). According to different research purposes, suggestion was put forward on the sampling method of the study on the atmospheric articulates retention amount in tree canopy, and certain scientific basis for the related research on the retention effect of urban vegetation to atmospheric particulate matter was provided.(3) When the wind force was 0-2 grade, there was no significant difference among daily average PM2.5 concentration [the average PM2.5 concentration during the observation period (9:00-15:00)] in the four stands at each pollution level (P>0.05). Stand PM2.5 concentration was positively correlated to air relative humidity (P<0.01), negatively correlated with air temperature (P<0.05), but uncorrelated with wind speed (P>0.05). Changed percentage of stand PM25 concentration compared to the space varied between -21.4% and 33.2%. The stand PM2.5 concentration decreased when the air humidity was low, and vice versa, and in this study the critical value of air humidity was 67%.(4) In this study, the size of particle matter retained by different forest tree species leaf ranged within 260 μm,and the average particle size was in the range of 12 to 39 μm. The atmospheric particulates retention ability of conifer species was greater than the broad leaved tree species. The three tree species which had the most PM2.5 and PM10 retention amount by per unit area leaf were:Pinus armandii, Ginkgo biloba, Sabina chinensis, respectively. The three tree species which had the most TSP retention amount by per unit area leaf were:Pinus armandii, Acer truncatum, Ginkgo biloba, respectively. In the comparation of PM2.5, PM10 and TSP retention amounts by per unit area stand, Pinus armandii-Ginkgo biloba mixed plantation and multiple-species-layered mixed plantation were the most.Based on the results, two typical stand configuration modes which aimed to regulate PM2.5 and other atmospheric particulates were put forward comprehensively, they were the "density crisscross pattern" and the "comprehensive control pattern". |
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