| As the biggest Chinese city in size and with complex terrain, Chongqing is one of the earliest type of fog-haze air pollution, and the most air-polluted cities in China, which has not only brought harms to health of the citizens, but also influenced the city’s investment environment and competitiveness. Based on current situation, this paper firstly, studies the spatial distribution features and annual, monthly and diurnal variation of three main air pollutants (PM10, SO2, and NO2) in the urban areas of Chongqing from2002to2011. Secondly, method of K-means clustering analysis is used to classify the upper air and ground surface weather conditions which affect the diffusion of contaminant. The typical weather types that block the diffusion of pollutants summed up. The paper also analyzes the weather process characteristics of air pollution and the correlation between the local meteorological elements and the pollution concentration.The removal efficiency on atmospheric pollutants of different properties of precipitation is minutely discussed. Moreover, using the WRF-Chem model and observed data analysis, the paper emphatically studies the effects of local boundary layer meteorological features on the pollutants concentration under the conditions of sunny days, cloudy days and fogs in the urban areas of Chongqing. Mechanism of how meteorological conditions lead to serious air pollution is preliminarily uncovered. The main findings of the research are as follow:(1) The spatial distribution characteristics of air pollution in the main urban region of Chongqing:the highest PM10pollution areas are the six industrial and commercial core areas, and the SO2pollutant is mainly distributed in the industrial area, while the NO2pollutant mainly appears in the most prosperous business areas and busy roads.(2) The annual variation of the three main pollutants (PM10, SO2and NO2) concentration in the urban areas of Chongqing is declining on the whole, and the air quality is improved year by year. The highest pollution period is late autumn, winter and early spring (from November to March), especially in December and followed by November. The hourly average concentration of PM10presents the characteristics of "double peak and double valley" diurnal variation with a period of12hours. The peak appears at around12:00and22:00in winter half, while the lowest value appears at around7:00and18:00. But, the peak appears at around11:00and23:00in summer half, while the lowest value appears at around7:00and17:00.The concentration of SO2presents the characteristics of "single peak" diurnal variation. The peak appears at about12:00to13:00in winter half and10:00to11:00in summer half. The average of NO2concentration presents the characteristics of insignificant "double peak and double valley" diurnal variation. The main peak appears clear at approximately20:00and weak at about10:00to12:00.(3) The500hPa weather situations are classified by the method of K-means clustering analysis in Chongqing. The result shows that the high altitude weather situations are mainly the one trough and one ridge, zonal circulation, two troughs and one ridge, west high and east low and trough moving eastward. In the period of air pollution, the overall trend of the daily average surface temperature is increasing, the daily average pressure is decreasing, the daily average relative humidity changes mainly for pre period of pollution is decreasing first, then gradually increasing, and the wind speed remains from1m/s to1.5m/s.(4) Analysis showed that the main city of Chongqing meteorological boundary conditions have the most direct impact on the concentration of pollutants. From night to morning, because three typical representative weather conditions of foggy, sunlit and cloudy are in the thermal inversion layer and wind speed is small in high altitude and near the surface, the upward spread of PM10is weak. The pollutant concentration is significantly weakened with the declination of pollution emissions. Due to the high relative humidity and the adsorption of water vapor, the rate of pollutants concentration declines faster that in the fog than sunny and cloudy. After8:00, as the increase of pollution emissions by human activity, the pollutant’s concentration rises fast. However, during the8:00to13:00, the temperature rises more slowly in cloudy days than in sunny days and fogs, and the wind speed is too small in cloudy days directly leading to that the rate of pollutants concentration increasing is faster in cloudy days than that in sunny days and fogs. After13:00, boundary layer inversion is disappeared under the background of the three weather types. Therefore, the atmospheric diffusion is notably enhanced and pollutants concentrations begin to decline obviously. There are higher temperature and faster warming in sunny days, and the wind speed is large in high latitude and near the surface, so the concentrations of PM10decrease fast. The rate of warming is slower in foggy days than sunny days, so the rate of PM10concentration declining is smaller than in sunny days, which is similar in cloudy days.(5) Through comparative analysis of inversion and diurnal temperature showed that the reason of weak inversion and thin inversion is small temperature difference between day and night in winter months in main city of Chongqing, unlike other northern cities (such as Lanzhou, etc.). Just the characteristic of weak inversion and thin inversion is caused slightly polluted weather more than moderately polluted weather in Chongqing city. This is obvious differences between in Chongqing city and in northern city. Through analysis of the changes of turbulent kinetic energy and correlation analysis of turbulent kinetic energy and pollutant concentration in different weather that the turbulent kinetic energy is stronger, vertical diffusion of pollutants is stronger, and the concentration of pollutant is lower.Diffusion capacity of pollutants is transported by the mean wind speed in horizontal advection significantly stronger than it is transported by the turbulent in vertical direction.(6) WRF-Chem model has good simulated effects on atmospheric boundary layer in urban areas of Chongqing. Compared the two boundary layer programs, surface temperature and boundary layer wind field simulated by MYJ is better than that of YSU. The WRF-Chem model can simulate the diurnal variation of PM10, SO2and NO2concentrations in Chongqing main city, but the concentrations were significantly smaller. The concentration of pollutants simulated by MYJ is also better than that of YSU. The results of numerical simulation show that the height of boundary layer has effects significantly on pollutants concentration. As atmospheric boundary layer is stable and PBLH is relatively low at night, the vertical diffusivity of atmosphere is weak. As a result of less emission after nightfall, PM10will decrease gradually, and the pre-dawn reached its lowest point.As the increasing of solar radiation by day, PBLH increases rapidly and the vertical diffusivity space of pollutants enlarges evidently. Correlation coefficient between PBLH and PM10at8:00-17:00is examined. The result shows that PBLH is negatively related with PM101or3hours later. So, for PBLH’s effect on PM10, there is a lag effect of1or3hours. The PBLH trends vary in different weather conditions, which results in obvious difference in the effects on pollutants concentration. In foggy and cloudy days, PM10decreases evidently2-3hours after increasing of PBLH. However, PM10decreases evidently1-2hours after increasing of PBLH in sunny days. The reason of PM10concentration change delay1-3hours of PBLH change is that PM10(or other contaminants) is gradually changed under the action of the atmospheric dispersion. It is a slow gradual process. SO the meteorological condition change of atmospheric boundary layer is faster, dispersion response time of pollutant is shorter.Analysis of effects of PBLH variation on PM10can better explain the characteristics of diurnal variation of PM10. Therefore, forecast of PBLH can briefly figure out the hourly variation trends of PM10, which provides technological support for pollutants forecast. The study also finds that variations of boundary layer wind field and pollutants concentration are good indicators. The urban areas of Chongqing are influenced by downdraft at night, which has weak effect on the vertical diffusivity of pollutants. Artificial emission increases in the morning. Downdraft weakens the vertical diffusivity of pollutants, which results in rapidly increasing of pollutants concentration. In the contrast, when there is updraft, the vertical diffusivity of pollutants is strong and the increasing speed of pollutants slows down. It’s especially obvious when there is a high upper air wind speed and strong updraft. The strong pumping action strengthens the vertical diffusivity of atmosphere significantly to control the increasing of pollutants. In the afternoon, there is mainly strong updraft in the boundary layer as a result of strong solar radiation. The vertical diffusivity of atmospheric boundary layer is the strongest, which results in an obvious decreasing trend of the pollutants concentration, and reached the day in the valley point of the pollution concentration. When it is from pollution to non-polluted in atmospheric boundary layer, the wind speed will be significantly enhanced and is updraft, the boundary layer height will be significantly increased and the turbulent kinetic energy will be significantly enhanced. The air pollutant concentrations would be obviously reduced when horizontal diffusion and vertical diffusion capabilities are enhanced in the atmosphere.(7)Precipitation has obvious scavenging effect on atmospheric pollutants. In addition, precipitation at different levels presents the exponential change of the scavenging effect on PM10, SO2, NO2and API reduction rate. The removal effect of continuous precipitation is best in the first two days. And the second day is better than the first day. The removal effect of hourly precipitation on PM10and SO2is better than that on NO2. And the night is better than the daytime. The removal effect of precipitation is better in the rising phase than the declining phase.Based on the research of temporal variation of air pollution and meteorological conditions, and used of the mechanism of precipitation can effectively remove contaminants,and combined with the special primary pollutant PM10diurnal variation characteristics in the main city of Chongqing, rocket and artificial precipitation and water spray from high-rise from20:00to7:00can be used to improve air quality. And from8:00to12:00, aircraft rainfall or high-rise sprinkler can be better used to block the PM10concentration increasing. |
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