Numerical Simulation On Air Pollution Over Complex Terrain

The mesoscale weather model WRF and CMAQ model were used to simulate the characteristics of meteorological field and air polluted distributions in Xigu district of Lanzhou city in winter from the 27th of January to the 2nd of February in 2005, which are based on the analysis of atmospheric boundary-later observed and air quality monitoring. The capacities of WRF and CMAQ models over complex terrain were testified by comparing simulated results and observed data, the effects of meteorological fields from three PBL (Planetary Boundary Layer) schemes (YSU, MYJ and ACM2) on the simulated results of CMAQ model were discussed; The differences of three PBL schemes (YSU, MYJ and ACM2) on the retrieve of meteorological characteristics in lower layer were analyzed, the effects of meteorological fields from three PBL schemes on the simulated results of CMAQ model were also discussed. The results can improve the accuracy of numerical simulation and air pollution forecasting in Lanzhou city. The spatial and temporal distributions of air pollutants in Lanzhou city were also investigated by analyzing the contribution of the various physical-chemical processes on the concentration of SO2 and NO2; some sensitive numerical experiments were used to study the effects of higher emission sources on air quality in Lanzhou city. This would provide some scientific foundations for controlling air pollution and improve air quality in Lanzhou city.(1) Comparing the results of the observations and simulations shows that the metrological variables have similar characters in PBL with three PBL schemes in WRF, but the temperature from MYJ scheme was lower than that from the other two schemes during the night, with opposite results during the daytime, which is caused by stronger turbulence due to the short-wave radiation during the daytime.(2) Local closed MYJ parameterization presents more reasonable daily changes on surface temperature than the other non-local closed parameterizations (YSU and ACM2) due to the facts that traces and energy were dominated by local turbulence, which results in the weak turbulent transportation in winter stable condition especially in valley city.(3)Investigating observed and simulated profiles of the mean temperature shows that three PBL schemes have good characters during the night, while the temperature profiles simulated by ACM2 (combined with local and non-local scheme) PBL scheme in WRF in is better lower layers than that from the other schemes during the daytime. It also presents that MYJ gives more reasonable results in upper layer. (4) Spatial and temporal characters of pollutants were well simulated by WRF model combined with CAMQ model with different PBL parameterization, which also testified the capacity of studying pollutant problems by CMAQ model.(5) The simulated concentrations of air pollution using the ACM2 PBL scheme was in WRF model combining with CMAQ model were consistent with the observed data The correlation coefficients of simulated and observed SO2 and NO2 daily averaged concentration are 0.61 and 0.57 respectively. It is because that'the turbulent exchanges are well simulated by ACM2 PBL scheme. The same exchange coefficients adopted by air quality model and meteorology model is another reason.(6) The simulated PM10 concentration was not satisfied due to the uncertainty of course particles. Simulated PM2.5 concentration may be related to TSP concentration, which indicates that the chemical transport process in CMAQ model can illustrate the formation, change and clarity of fine particles, which must be tested for the coarse particles.(7) The horizontal and vertical distributions of SO2, NO2 and PM2.5 in Lanzhou city show that they are affected by the spatial distribution of emission sources and meteorological fields. There are two maximum concentrations of pollutions in Lanzhou, which located around Tieluju in Chengguan district and Lanhua hotel in Xigu district. Pollutant concentration decreases linearly with height. The vertical gradients of the concentration are smaller during daytime than that in night because of the stronger turbulent mixing in the daytime.(8) The results of sensitive experiments of the emissions of higher emission sources show that:SO2 and NO2 emissions of higher emission sources accounts for 59% and 72% in total emissions; and SO2 and NO2 concentration caused by higher emission sources accounts for 46% and 49% in daily averaged concentration. Controlling the emissions of higher emission sources is the effective method to control air pollution in Lanzhou city.(9) Analysis on physical and chemical processes of different pollutions shows that pollution emission is the main factor over effecting SO2 surface concentration, and the vertical diffusion is the secondly one, while chemical processes have significant effects on NO2 concentration.