| This paper aims to give full play to the protection function of trees and forests to PM2.5 and explore the PM2.5 mass concentration diurnal and seasonal variation in four types of roadside protection forests and PM2.5 changes in different horizontal distance. A typical protection forest lies between Xiangshan Road, Haidian District, Beijing and the Fifth Ring Road is chosen as the research object. The four types of protection forests are the evergreen coniferous Pinus thunbergii pure forest, deciduous broad-leaved Ginkgo biloba pure forest, evergreen coniferous Sabina chinensis pure forest and gingko and platycladus mixed forest, with an average width near 100 m. In those protection forests, a horizontal monitoring point was deployed in every 15 m from Jun, 2013 to Apr, 2014 to study the PM2.5 horizontal diffusion changes in different seasons and compare their advantages and disadvantages. Preliminary conclusions are as follows:(1) Comparison of PM2.5 mass concentration changes from high to low throughout the year were: winter> autumn> spring> summer. The PM2.5 concentration changed slightly when spring, summer and autumn comparing with each other, but the concentration of PM2.5 in winter was significantly higher than the spring, summer and autumn. The overall comparison of PM2.5 concentrations broadly consistent with the seasonal variation of the period of Beijing city, the variation of the overall air concentration of particulate matter in Beijing also winter> autumn> spring> summer, which shows that the air particulate matter in the road protection forests on the season impacts by the big environment in Beijing.(2). The trends of PM2.5 mass concentrations in different seasons are not the same. In spring, it showed "W" shape, there were two concentration peaks at 7:00 and 21:00, and the concentration reached two troughs at 1:00 and 13:00. PM2.5 concentration changes smoothly in summer, there were four "peaks" and 4 "trough". In autumn, the change of PM2.5 concentration were smooth in 9:00-19:00, after 19:00, concentration began to rise. It presented a "U" shape in winter and showed a significant "trough" period in 11:00-19:00. But in the same season, the trend of PM2.5 mass concentration in four different road protection forests were generally the same.(3). The horizontal changes in protection forests are not the same in different seasons. In spring, PM2.5 concentration in black pine forest was higher than both sides. The ginkgo and mixed forests PM2.5 concentrations were gradually reduced along the Fifth Ring Road to the Xiangshan Road, Sabina chinensis forest and black pine were similar in low middle high on both sides. In Summer, in addition to black pine, the PM2.5 concentrations in other three forest stands were all low, the black pine forest is highest. Similar changes appears in summer and autumn. The Black pine and Sabina chinensis changes the same trend in winter, from pollution sources to protection forests, it showed the change that drop first and then rise. The ginkgo and mixed forests gradually reduced along the Fifth Ring Road.(4). PM2.5 easily affected by surrounding environmental factors, in addition to a small impact from air pressure, the other measured factors were correlated with it. It had highly significant positive correlation with the TSP, PM10, PM1 and the real-time wind speed, maximum wind speed, average wind speed, temperature and light intensity was significantly negatively correlated with the relative humidity was significantly positively correlated, but have no correlation with air pressure.Through the study of the seasonal, diurnal and horizontal variation of PM2.5 mass concentration of the road protection forests, a series of laws and conclusions were discovered. It can provide some reference for future road protection forests construction. |
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