An online analyzer for Monitoring for AeRosols and Gases (MARGA) was employed to measure major water-soluble (WS) inorganic ions in PM10 at 1-h time resolution in Shanghai from May 27 to June 16, 2009. During the field campaign, hazy days were encountered over which atmospheric visibility was commonly less than 10 km, and hourly average PM10 reached peaks of exceeding 150 μg m−3. Based on WS ions and pollution gases, the haze events were classified as biomass burning induced, complicated and secondary aerosol pollutions depending on their distinct formation schemes. During the biomass burning induced pollution, which was aroused from biomass burning plume, the concentration of K+ was increasing up to maximum about 19 times higher than the average of clear days, and K+ behaved a strong positive linear correlation with Cl−. Because of sulfate and nitrate significantly enhanced by secondary production, in which precursor gases of SO2 and NO2 were converted into SO42− and NO3− on the surface of pre-existing KCl particles, the complicated pollution was responsible for a combining contribution of aerosols directly transported from biomass burning sources and known secondary aerosols linked to local emission. Under high atmospheric oxidation ability and steady atmosphere condition, the secondary pollution resulted from a significant increase of sulfate and nitrate aerosols which were oxidized from large amounts of anthropogenic gases of SO2 and NO2 in the urban atmosphere.Highlights► The data at 1-h resolution is help to characterizing haze pollution more details. ► The haze events were classified as biomass burning, complicated and secondary pollutions. ► Biomass burning plumes could cause complicated pollution over downwind regions.