Peroxyacetyl nitrate (PAN),as a major secondary pollutant,has gained increasing worldwide attentions,but relevant studies in China are still quite limited.During winter of 2015 to summer of 2016,the ambient levels of PAN were measured continuously by an automatic gas chromatograph equipped with an electron capture detector (GC-ECD) analyzer at an urban site in Jinan (China),with related parameters including concentrations of O3,NO,NO2,PM2.5,HONO,the photolysis rate constant of NO2 and meteorological factors observed concurrently.The mean and maximum values of PAN concentration were (1.89 ± 1.42) and 9.61 ppbv respectively in winter,and (2.54 ± 1.44) and 13.47 ppbv respectively in summer.Unusually high levels of PAN were observed during severe haze episodes in winter,and the formation mechanisms of them were emphatically discussed.Study showed that high levels of PAN in winter were mainly caused by local accumulation and strong photochemical reactions during haze episodes,while mass transport played only a minor role.Accelerated photochemical reactions (compared to winter days without haze) during haze episodes were deduced by the higher concentrations but shorter lifetimes of PAN,which was further supported by the sufficient solar radiation in the photolysis band along with the high concentrations of precursors (NO2,VOCs) and HONO during haze episodes.In addition,significant PAN accumulation during calm weather of haze episodes was verified by meteorological data.