Particular matter (PM) is one of the major pollutants in China.The radiation balance,visibility and human health can be strongly influenced by high concentrations of PM.Also,roduction rates of relevant heterogeneous reactions can be dramatically enhanced by significant levels of PM.Due to local anthropogenic emissions and the effect of regional transport,haze events occur frequently in the Yangtze River Delta (YRD),severely impacting upon the everyday and working lives of citizens.In this study,an intensive field campaign was conducted in December 2013 at a regional background site in Nanjing referred to as SORPES (Station for Observing Regional Processes of the Earth System) and,based on the data collected,the formation mechanism of haze pollution in wintertime was discussed and analyzed with a focus on the aerosol chemical composition and synoptic pattern.An online composition analysis instrument named MARGA (Monitoring Instrument for Aerosols and Gases) was used to measure water-soluble ions such as NO₃^-,SO₄^2-,NH⁺₄,Cl^-,K⁺,Na⁺,Ca²⁺ and Mg²⁺ in the fine PM (PM_2.5) fraction.Meanwhile,the gases SO₂,NO_x and O₃ were measured by a trace gas instrument manufactured by Thermo Fisher Scientific (Massachusetts,USA).In order to study the synoptic pattern during hazy days,weather systems were analyzed using data from NECP.Moreover,the HYSPLIT model was used to simulate the atmospheric transport and dispersion of pollutant plumes originating from various air mass sources.HYSPLIT has capabilities ranging from simple trajectories to complex dispersion and deposition calculations for both puffs and particles.It can simulate dispersion based on input variables (wind,temperature,pressure etc.) from various meteorological models.These fields can then be integrated both in space and time to with respect to particles' positions in order to simulate the dispersive nature of the atmosphere.The results showed that the monthly average concentration of PM_2.5 was 142 μg·m^-3 and the water soluble ions (NO₃^-,SO₄^2-,NH₄⁺,Cl^-,K⁺,Na⁺,Ca²⁺and Mg²⁺) contributed about 62% to the total mass concentration of PM_2.5.Two heavy haze events were observed during the campagin.The concentration of PM_2.5 reached 267 μg·m^-3 in the first event and 150 μg·m^-3in the second event,during which three major inorganic ions (sulfate,nitrate and ammonium)accounted for 63.6% and 66.4%,respectively.Both haze events lasted for several continuous days with very low visibility-only 1.9 km in the first event and 4.7 km in the second.Aerosols in the first event were mostly secondarily formed,with a stable weather system and lower mixing layer.The higher temperature and humidity compared to the second event provided appropriate conditions for the heterogeneous transformation of particles in the surface layer.HYSPLIT showed that the air masses of the first event mainly originated from local Nanjing and the surrounding YRD.However,the second event was largely influenced by long-range transport from the North China Plain.The formation of sulfate and nitrate in daytime was observed during both haze events,which mainly came from homogeneous reactions.Meanwhile,nighttime heterogeneous or aqueous-phase reactions contributed to the first event only.The catalytic oxidation of metal ions or liquid-phase oxidation of NO₂ may have been important mechanisms through which sulfate formed at night in first haze event.