Observational Study Of PM_2.5 Mass Concentration And Aerosol Optical Characteristics In Shanghai Urban Area

In recent years, the environmental and climatic effects of atmospheric aerosolshave drawn much attention. Aerosols are not only with complex sources, but also withgreat uncertainty in space and time. Hence, long-term observations of aerosol physicaland chemical properties are necessary to understand the air quality status and toevaluate the aerosol climatic effect. In order to investigate the aerosol properties inYangtze River Delta region, two years （from January,2011to December,2012） ofcontinuous observations of aerosol concentration, optical characteristics and weatherconditions are performed in the campus of Fudan University, Shanghai. Based on theanalyses of the observational data, mean PM_2.5mass concentration and aerosol opticalproperties, their seasonal and diurnal variations, and causes of these variations arediscussed in this paper.The mean concentration of PM_2.5is43.8±28.7μg/m³, which is worse than theChinese air quality standard II. Compliance rates of24-hour average concentrationstandard I and II are47.9%and87.1%, respectively. PM_2.5mass concentration ishighest in winter and lowest in summer. Meteorological factors may produce greatinfluences on the seasonal variation of PM_2.5mass concentration: air temperature andPM_2.5concentration are negatively correlated; wind field could affect the source andconcentration of aerosols; precipitation could reduce PM_2.5. All the factors abovemake PM_2.5mass concentration in winter is higher than that in summer. Diurnalvariation of PM_2.5concentration shows a double-peak structure, and the two peaksappear around01LST and05LST, respectively.The mean value of aerosol scattering coefficient in Shanghai is210.9±181.8Mm^-1, with the maximum value appears in spring. The mean value of aerosolabsorption coefficient is18.7±13.0Mm^-1and the values are high in summer andautumn. Both scattering and absorption coefficient display a double-peak structure indiurnal variation. The two peak of absorption coefficient are more obvious,corresponding well with the traffic rush hours. Aerosol scattering and absorptioncoefficients have good correlations with PM_2.5mass concentration, with thecorrelation coefficients of0.94and0.86, respectively. Wind speed and direction couldaffect the aerosol scattering and absorption coefficients. When the wind comes from the east or the wind speed is large, coefficient values are relatively low. Conversely,when the wind comes from the west or the wind speed is small, coefficient values arehigh.Micro-pulse-lidar （MPL） data could be used to calculate the vertical distributionof the aerosol extinction coefficient, and then the mixing layer （ML） height andaerosol optical depth （AOD） could also be acquired. The mean height of ML is about0.79km, with its highest value appearing in summer. The diurnal variation shows asingle-peak structure, and high ML time in summer is much longer than that in winter.The average value of AOD is0.31. The ratio of AOD within mixed layer and AOD isabout0.69. That is to say, although aerosol extinction within the ML is of greatimportance, the aerosols above the ML should not be ignored. The ratio is high duringthe day and low at night. This is because parts of the aerosol particles remain in theremaining layer （RL） at night, increasing the proportion of aerosol extinction abovethe boundary layer.A typical air pollution process in Shanghai from14to15April2011isinvestigated by using observational data. The daily concentration of PM_2.5on14and15April were78.9μg/m³and115.9μg/m³, respectively. The air quality grade in thisprocess was worse than the Chinese air quality standard II of PM_2.5. The pollutionprocess was formed by pollutants accumulation due to stable synoptic situation andended by short-term precipitation and cold air from the northwest. Pollutants camefrom local and southwest Shanghai. The variation trend of aerosol scattering andabsorption coefficient are well consistent with that of PM_2.5mass concentration, withthe daily and hourly values greatly exceed the average. The vertical distribution ofaerosol extinction coefficient is mainly affected by concentration of pollutants,weather conditions and atmospheric vertical convection.