Abstract: Using the conventional meteorological observations and the NCEP reanalysis data,a persistent haze process from December 1 to 8,2013 in Changzhou was analyzed comprehensively in this study.The results show that zonal circulation at high levels in the middle-high latitudes is straight during the haze event,and meanwhile the low levels are mostly controlled by the warm and moist southwesterly airflows,with very weak cold-air activities.The reasons for the haze outbreak are mainly due to the transport of upstream contaminants by westerly airflows caused by an "L"-type high pressure locates at Changzhou on November 30,and the existence of a thermal inversion layer is in the early morning of December 1.The continuous uniform pressure field and the frequent thermal inversion layer are the maintaining mechanism of the haze.The strong cold air front leads to the dissipation of the haze on December 9.The haze process is accompanied by decreasing dew point depression,the increasing relative humidity,the southerly and westerly weak winds,and the ineffective upward motion near the surface,which provides a stable atmospheric layer structure and appropriate water vapor for the haze maintenance.The main pollutants during the haze event are particulate matters (PM_2.5,and PM₁₀).The gaseous pollutants such as SO₂,NO₂,and O₃ generate particulate matters through the cooperative transformation effect,and lead to the dramatic increase in pollutant concentrations,which is an important internal cause for the haze formation.The backward trajectory analysis illustrates that the transport of pollutants from the upstream region of Jiangsu province also contributes to the haze event.

Key words: Haze, Circulation pattern, Temperature inversion, Particulate matter, Backward trajectory simulation