In this study,a four-season field campaign was conducted from December 2017 to November 2018 to investigate the optical properties of aerosols in the northern suburbs of Nanjing.The seasonal variations of aerosol absorption and scattering coefficients were observed to be highest in winter,followed by spring and autumn,and lowest in summer.The daily variations exhibited a bimodal pattern,with the early peak of absorption coefficient occurring earlier in summer than in winter,which can be attributed to seasonal changes in boundary layer heights and human activity patterns.The average Absorption Ångström Exponent (AAE) was higher in summer and autumn,indicating a more prominent contribution of brown carbon to aerosol absorption,while black carbon dominated in spring and winter.Conversely,the Scattering Ångström Exponent (SAE) was higher in spring and winter than in other seasons,with fine-mode aerosols prevailing throughout the year and smaller aerosol particle sizes observed in spring and winter.During winter,the transport of pollutants from the northwest direction under the influence of northwest wind led to high values of aerosol extinction coefficients.In spring and summer,the diffusion of local pollutant emissions from the east and southeast directions was the primary factor contributing to the elevated aerosol extinction coefficients.Compared with the air masses originating from inland,the transport of maritime air masses reduced the aerosol extinction coefficients.When the relative humidity was less than 90%,the aerosol extinction coefficients were inversely proportional to the visibility.However,when the relative humidity exceeded 90%,the visibility was generally less than 10 km and had minimal correlation with the aerosol extinction coefficients.In spring and winter,the water-soluble components of PM_2.5 were primarily secondary source ions,with a stronger correlation between aerosol extinction coefficients and nitrate ions than that of sulfate ions,indicating that transportation-related emission sources were the main contributor to aerosol extinction coefficients.In contrast,during summer and autumn,the pollutants affecting the aerosol extinction coefficients were mainly from stationary sources such as coal combustion.