The chemical characteristics (water-soluble ions and carbonaceous species) of PM2.5 in Guangzhou were measured during a typical haze episode.Most of the chemical species in PM2.5 showed significant difference between normal and haze days.The highest contributors to PM2.5 were organic carbon (OC),nitrate,and sulfate in haze days and were OC,sulfate,and elemental carbon (EC) in normal days.The concentrations of secondary species such as,NO3-,SO42-,and NH4+ in haze days were 6.5,3.9 and 5.3 times higher than those in normal days,respectively,while primary species (EC,Ca2+,K+) show similar increase from normal to haze days by a factor about 2.2-2.4.OC/EC ratio ranged from 2.8 to 6.2 with an average of 4.7 and the estimation on a minimum OC/EC ratio showed that SOC (secondary organic carbon) accounted more than 36.6% for the total organic carbon in haze days.The significantly increase in the secondary species (SOC,NO3-,SO42-,and NH4+),especially in NO3-,caused the worst air quality in this region.Simultaneously,the result illustrated that the serious air pollution in haze episodes was strongly correlated with the meteorological conditions.During the sampling periods,air pollution and visibility have a good relationship with the air mass transport distance;the shorter air masses transport distance,the worse air quality and visibility in Guangzhou,indicating the strong domination of local sources contributing to haze formation.High concentration of the secondary aerosol in haze episodes was likely due to the higher oxidation rates of sulfur and nitrogen species.