The Spatial-temporal Distribution And Deposition Characteristic Of PM_2.5 Acidic Components In Beijing

As the primary pollutant of haze, PM2.5 has got wide attention, in which the acidic components are the main parts. How will the underlying surface of urban forests and urban green spaces influence PM2.5 deposition, and how will the PM2.5 concentration and its acidic components be in the small area above urban green space? The acidic components in PM2.5 in disparate area with different plants are rarely studied in Beijing. We carried out the investigation of PM2.5 and acidic components in PM2.5 in four sites:the gate of Beijing Forestry University, the lawn in campus, the Olympic Forest Park and the Jiufeng National Forest Park. The results are as follow:The annual concentration of PM2.5 and monocarboxylic acid showed the trend as gate> lawn> Olympic> Jiufeng. The seasonal characteristic of them is as winter> spring> autumn> summer. The concentrations of PM2.5 and monocarboxylic acid showed different diurnal variation characteristics during different seasons. The sources, vegetation, inversion layer and photochemical reaction may be the important factors which affect the spatial and temporal distribution of PM2.5 and monocarboxylic acid. Meteorological condition and biogenic emission have influence on monocarboxylic acid concentration, but anthropogenic sources were still the most important factors. The concentration of NO3- varied as gate> lawn> Olympic> Jiufeng. SO12- concentration had the same characteristic except in winter. Different with NO3- and SO42-, NH4+ concentration showed the variation as lawn> gate> Jiufeng> Olympic. The concentration of acidic water soluble inorganic ions was lowest during summer. In winter, each concentration of NH4+, NO3- and SO42- was higher during nighttime than daytime. The concentrations of NO3- and SO42- were higher during daytime in spring, summer and autumn. The contribution of mobile source played the most important part in all four sampling sites.Generally speaking, tree canopy can promote the deposition of PM2.5 and water soluble inorganic ions. The plant in Jiufeng showed stronger capacity to promote deposition than that in Olympic. The deposition velocity in summer was highest, and the deposition was higher during daytime than nighttime. While the tree canopy play a negative role in deposition process of monocarboxylic acid.Meteorological condition has influence on vertical deposition and the pollutant diffusion process. The photochemical reaction may be affected by temperature and relative humidity. In certain range, higher humidity can increase the weight of particulate, and make it easy to deposite. And higher wind speed can not only promote the horizontal diffusion but also promote vertical deposition.