Study On Secondary Formation Of PM_2.5 In Hangzhou

PM2.5 is the particle matter that its aerodynamics diameter is smaller or equal to 2.5 urn. It is harmful not only to human health but also to the enviroment. Moreover, it is the important reason to the reducing of the city atmospheric visbility. Therefore, PM2.5 has become a hotspot in environment research in recent years. The third-Generation Air Quality Modeling System (Models-3/CMAQ) was used to simulate the spatial and temporal distribution of secondary PM2.5 in Hangzhou urban area. The aim is to understand formation of secondary PM2.5, feature of variation, trend in photochemical reaction in studied area (40x36 km) during the typical episode. The law of secondary PM2.5 formation from all sorts of precursors and source categories under typical meteorological conditions was explored. This would provide scientific basis to control the concentration of PM2.5 in urban area rationally and effectively.Mesoscale meteorological model version 5 (MM5) was used to simulate the three-dimensional hour-varying meteorological field with the high resolution of 1 km×1 km, SO₂,NOx emission inventories of point sources and stationary area sources and SO₂,NOx,VOCs,NH₃ emission inventories of mobile sources calculated by IVE (International Vehicle Emissions Model) were converted to IDA format, mobile line sources were converted to area sources according to grid border. According to the character of source categories, the time allocation, chemical split and spatial allocation are made furthermore by Sparse Matrix Operator Kernel Emissions (SMOKE) in order to meet the demand of Models-3 modeling with high spatial and temporal resolution.Based on the emission inventory and meteorological field with high resolution, the ordinary differential equations (ODEs) group of photochemical rates was established according to CB-IV mechanism while the ODEs were solved by means of Quasi-steady state approximation (QSSA) method. Community Multiscale Air Quality (CMAQ) was used to simulate the three-dimensional grid concentration and chemical components of secondary PM2.5. The episode started on June 30, 2007, 0:00 and ended on July 3, 2007, 23:00. The spatial and temporal distribution and the vertical distribution character of secondary PM2.5 were aslo analyzed. The results show that the secondary PM2.5 pollution level is quite high, and appears obvious feature of spatial and temporal variation. The distribution of secondary PM2.5 coincides well with that of mobile sources, both increasing from edge to center of the city. The concentration of PM2.5 also appears hour-varying disciplinarian: the PM2.5 concentration reach the highest value because of intense photochemical reaction despite the mixed layer is high at noon; the concentration is second high at morning because of the undeveloped mixed layer and the increase of source emissions due to busy traffic; the concentration at midnight is third high because of the low mixed layer and inversion effect; the concentration at evening drop to minimum due to full developped mixed layer and weak illumination. Among the average secondary PM2.5 concentration of surface layer, organic component account for 47.1%, followed by sulfate (20.4%), nitrate (16.5%) and ammonium (16.0%). The chemical component shows that controlling the emission of VOCs can effectively reduce the formation of secondary PM2.5.