| Taken typical road protection forest–the urban arterial road?Anli road?and the beltway forest?the North Fifth Ring?in Beijing as an object,concentration of atmospheric particulate matters?PM?in horizontal and vertical,micro-climate factors,traffic flow and other indicators were measured in real-time by methods of small-scale quantitative measurement.Four contents were analyzed in the paper,including the temporal and spatial distribution of PM concentration within the road protection forest,the influence effects of forest structure on the diffusion of PM concentration,the purification effect of different functional areas and the influencing factors on PM concentrations.The main conclusions were as follows:1.Atmospheric particulate matter concentrations within road protection forest had significant temporal variation.?1?Concentrations of TSP and PM10 in spring and winter were significantly higher than that in summer and autumn,while the highest concentration of PM2.5and PM1 was in winter.?2?During the observation period,air particulate matter showed similar diurnal variation in four seasons,concentration of four diameter was higher in night than in daytime,with peaks at 7:00 and 19:00,valley in 13:00 in daytime,but it was different for peaks and valley in night.?3?Diurnal variation of PM were inconsistent in different weather conditions,but diurnal variation of four diameter in same weather condition were similar.2.Spatial distribution of PM in urban arterial road belt:?1?Horizontal distribution:the maximum membership grade of TSP and PM10 all turned up in the central partition belt or co-rotating partition belt in three transects.The membership grade was highest in co-rotating partition belt,followed by the central partition belt,and fluctuated to the outside in transect Y1.Membership grade in transect Y2 gradually decreased from the central partition belt to the forest,reached the lowest in 15m within forest and maintained steady state.?2?PM2.5 and PM1showed a declining trend from the central partition belt to the forest in transect Y1.It was decreased from the central partition belt to the road edge but increased after entering forest for PM2.5 and PM1 in transect Y2.?2?Vertical distribution:the daily average membership grade was in the order of 1.5m>4m>6m in the central partition belt while the maximum membership grade in other monitoring points appeared in 4m or 6m and with minimum in 1.5m in a day.the highest PM concentration was in 4m or 6m and the minimum in 1.5m for central partition belt,co-rotating partition belt and non-motor belt in most time.From the forest edge to 35m within forest,it showed highest concentration in 1.5m,and lowest in 6m during daytime but at night it was highest in 6m and lowest in 1.5m.3.Spatial distribution of PM in beltway forest:?1?Xiangshan road and North Fifth Ring were two linear pollution sources,concentration of PM nearby the edge was highest,with the increase of the distance away from the roadbed,concentration of PM was lower and lower until remained relatively stable within 3045m.?2?Vertical distribution:It was mainly polluted in1.5m for majority belts in traffic peak-hours,as time goes on,the pollution level gradually increased,pollution concentrated in the top of canopy most time in night.The maximum membership grade was mainly turned up in 1.5m in Xiangshan Road while the minimum was in the middle or at the top of the canopy.The maximum membership grade turned up in the top of the canopy in North Fifth Ring while minimum in 1.5m.The maximum was in the middle or at the top of the canopy but minimum in 1.5m for the other monitoring points.4.Concentrations of PM in different beltway forest with a certain spatial differences,it was highest in road and road parallel?traffic road region?followed by living area parallel with road,while it was lower in waterway parallel with road and green space parallel with road.The diffusion of PM concentration was inconsistant in different belt,PM concentration in green area showed a decreasing state away from the North Fifth Ring road and maintained stable concentraiton in the range of 4060m away from the roadbed,while PM concentration in the other belts were fluctuated.Purifying effects on atmospheric particulate matter different sections in beltway presented Green area>Water area>Living Area>Traffic road region.5.Forest structure affected the diffusion of atmospheric particulate matter:?1?There were three ways for the diffusion of PM,directly upward vertical diffusion,through the canopy of high porosity or from the bottom of the space under the branches into the forest directly,and flowing crown diffused into the forest.The transmission distance were short for TSP and PM10but far for PM2.5 and PM1.?2?Dust detention functions in tight structure was stronger than in sparse structure,the regulation effects of plants on PM were strong within a certain scope of pollution,while existed a certain tolerance threshold,when the pollution intensity was too large,tight structure belts was not conducive to the diffusion of particles and played a blocking role in PM.?3?The canopy of street trees prevented the exchange of airflow,the diffusion motion of PM was limited to some extent between the ground and the crown canopy which resulted in the accumulation of PM and increased the pollution.With canopy density greater,the vertical diffusion of PM was stronger inhibited.6.Percentage of three finer particles showed significantly inconformity among different seasons,different times,different weather and different belt in road protection forest.TSP was mainly polluted by particle smaller than 10?m in normal but polluted by 10 to 100?m in sunny.PM10 was mainly polluted by particles between 2.5 and PM2.5 within the range of 12.5?m.It presented linear relationship among four diameters,with highest correlation between PM2.5 and PM1,the relationship in summer was the most strongest in four seasons.7.PM concentrations were affected by many factors.?1?Meteorological factors:PM concentration was positively correlated tolight intensity and air temperature in summer.PM concentration showed a highly significant or significant negative correlation with light intensity,wind speed,air temperature but showed significantly positive correlated with relative humidity in spring,winter,autumn and annual average.?2?It showed a negative linear correlation between atmospheric mixing layer height and PM,the correlations tended to increase with particle decreasing.?3?Traffic flow and four atmospheric pollutants were significant positive correlated with PM10 and PM2.5.?4?Purifying rate of road protection forest showed a significant positive correlation with belt width and canopy density but negatively correlation with porosity.In view of purifying effects,the optimum range of plant communities in belt width was in the range of 40m to 50m,the canopy density was from 0.70 to 0.85 and best porosity from 0.15 to 0.30.Finally,the regulating technology of road protection forest for purifying PM was put forward in order to provide technical support for optimizing the channel configuration structure and the construction of urban green space. |
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