Haze episodes occur frequently in Beijing in recent years and its characteristics and formation mechanisms remain unclear. In this work, a newly-developed instrument, single particle aerosol mass spectrometry (SPAMS) was used to characterize size distribution and chemical species of single particles (200-2000 nm) with 5-min time resolution during December 10 to 23, 2011 in Beijing. Another two online instruments including aerosol chemical speciation monitor (ACSM) and fast mobility particle sizer (FMPS) were simultaneously deployed at two different sites in Beijing to study the detailed chemical composition and size distribution of fine particles. During the campaign, a pollution episode was observed, which lasted about 12h and was characterized by high PM2. 5 mass concentration and low visibility (5. 3 km). Our results indicated that particle characteristics between haze and non-haze days were different in (1) Size distribution: based on FMPS data (measuring size range of 5. 6-560 nm), particles peaked in 10-40 nm in the clean day and shifted to larger size (about 100 nm) during the haze episodic day. However, from SPAMS (measuring size range of 200-2000 nm) data, highest particle counts occurred around 400-800 nm. Particles peaked around 500-600 nm in the morning and shifted to a wider range of 400-800 nm after noon time during the haze day, indicating hygroscopic growth under high relative humidity. Particles in these size ranges could contribute significantly to light extinction. However, in the clean day, particles peaked around a narrow range (500-550 nm); (2) Chemical species: SPAMS data indicated that the winter haze episodic day included a higher fraction of sulfate, ammonium and lead particles, but a lower fraction of nitrate compared to clean day. Chemical speciation analysis using ACSM showed that both organic matter and secondary ions increased significantly during the haze episode; (3) Sources: Several major sources for particles were distinguished by SPAMS including dust, industry (enriched with heavy metals), biomass burning, and other sources for carbonaceous aerosols. Organic matter and heavy metal exhibited obvious increase during the haze episode, but dust contribution decreased at the same time. In addition, meteorological conditions (e. g. high relative humidity, low temperature and low wind speed) were found to play an important role on haze formation and particle properties.