Typical types and formation mechanisms of haze in an Eastern Asia megacity, Shanghai

摘要

An intensive aerosol and gases campaign was performed at Shanghai in theYangtze River Delta region over Eastern China from late March to early June2009. This study provided a complementary picture of typical haze types andthe formation mechanisms in megacities over China by using a synergy ofground-based monitoring, satellite and lidar observations. During the wholestudy period, several extreme low visibility periods were observed withdistinct characteristics, and three typical haze types were identified,i.e. secondary inorganic pollution, dust, and biomass burning. Sulfate,nitrate and ammonium accounted for a major part of PM_2.5 mass duringthe secondary inorganic pollution, and the good correlation betweenSO₂/NO_x/CO and PM_2.5 indicated that coal burning and vehicleemission were the major sources. Large-scale regions with high AOD (aerosol opticaldepths) and low?ngstr?m exponent were detected by remote-sensing observation duringthe dust pollution episode, and this episode corresponded to coarseparticles rich in mineral components such as Al and Ca contributing 76.8%to TSP. The relatively low Ca/Al ratio of 0.75 along with the air massbackward trajectory analysis suggested the dust source was from Gobi Desert.Typical tracers for biomass burning from satellite observation (column COand HCHO) and from ground measurement (CO, particulate K+, OC, and EC)were greatly enhanced during the biomass burning pollution episode. Theexclusive linear correlation between CO and PM_2.5 corroborated thatorganic aerosol dominated aerosol chemistry during biomass burning, and thehigh concentration and enrichment degree of arsenic (As) could be alsopartly derived from biomass burning. Aerosol optical profile observed bylidar demonstrated that aerosol was mainly constrained below the boundarylayer and comprised of spheric aerosol (depolarization ratio <5%)during the secondary inorganic and biomass burning episodes, while thickdust layer distributed at altitudes from near surface to 1.4 km (averagedepolarization ratio = 0.122 ± 0.023) with dust accounting for44–55% of the total aerosol extinction coefficient during the dustepisode. This study portrayed a good picture of the typical haze types andproposed that identification of the complicated emission sources isimportant for the air quality improvement in megacities in China.