Recently,the continuously high incidence of fog and haze events has been a topic of considerable concern among the public,since such events can bring serious harm in terms of the health and daily lives of the masses.A rare fog and haze event,which was wide-ranging,long-lasting and heavily polluting,occurred during 12-17 January 2013 in the Huai'an area of Jiangsu Province.In particular,the visibility of most of the area during 13-14 January was lower than 0.1 km,which is extremely rare.The event caused considerable disruption to expressways,air transport and sea transportation.In this study,in order to reveal the formation mechanism of regional and successional fog and haze,and to provide scientific evidence for forecasting and prediction and air pollution abatement,this event was chosen as a case of typical heavy fog and haze for the analysis of pollution variation and its characteristics.The variation characteristics of PM₁₀ and PM_2.5,the relationship between visibility and meteorological variables,the circulation features at middle-low levels,and the pollution sources were explored based on the pollutant particle concentration,FNL data,conventional meteorological data,and Global Data Assimilation System data.The results indicated that the minimum value of hourly average PM₁₀ and PM_2.5 concentrations during this fog and haze event occurred at 0500-0700 BST and 1300-1700 BST,while the maximum value occurred at 2100-2300.For PM₁₀ and PM_2.5,the maximum concentrations appeared at different times.The increase inrelative humidity and the increase of PM_2.5 and PM₁₀ concentrations in total pollutant particles were the leading cause of visibility variation and persistent pollution.Several conditions,such as the slight variations inpressure gradient,sudden increase inrelative humidity,relatively low wind speed,and the increase in PM₁₀ and PM_2.5,were the main reasons for the rapid development of the fog and haze event.A good co-relationship between visibility and pressure,relative humidity,and PM_2.5concentration was observed.The overall variation trend of visibility could be well fitted by the regression equation.The representativeness of these results was analyzed and verified based on long-termsample data.The stable upper-air circulation situation,the warm advection at middle-low levels,and the stable ground pressure field provided abeneficial circulation situation for the development of the fog and haze event.During the event,varying levels of the thermal inversion layer werefound at lower layers.The deepthermal inversion layer and east wind near the surface layer wereconducivetothe low visibility and its wide distribution.The stable stratification structure,east-northeast air mass at the inversion layer and middle-low levels,local-source pollution and serious air pollution weremainly attributed to the relatively low visibility and longevity of the haze.The air quality was dependent on the concentration of pollutants.Meteorologists should identify meteorological indicesthatareadverse to the diffusion and dilution of pollutants when meteorological conditions that are adverse to pollutant diffusion appear.Early forecasting and timely warningsareessential for the prevention and treatment of air pollution.