Abstract: During July 2003，a comprehensive field measurements on aerosol chemical, physical and radiative properties at Lin'an regional background monitoring station in Yangtze delta region of China were conducted by the project “An observational study on the continental baseline air over China”.Only size resolved ionic characteristics of aerosols were discussed. Particles were sampled using Andersen multi stage impactor sampler. Afterthe sampling finished, samples were immediately shipped to the laboratory in Chinese Academy of Meteorological Science(CAMS) in Beijing for mass and chemical analysis. The mass was measured with gravimetric method using Sartorius microbalance of 0.01 mg readability, which was installed in a temperature and humidity controlled clean room. The anions were analyzed using Dionex DX500, and the cations were analyzed using Hitachi 180-70 Atom Absoption Spectrometry(AAS). 9 inorganic ions were analyzed, which were: F-,Cl,NO ,SO ,NH ,K+,Na+,Ca2+,Mg2+.In average, the mean mass concentration of particles summing over all size ranges (TSP) during the experiment was 69.6 μg/m3，with the minimum of 45.2 μg/m3 and maximum 90.8 μg/m3，which was much lower than those obtained in the period from August to October, 1991，and also lower than that measured in October 1994 at the same site. The calculated PM11 (<11 μm) mean mass concentration (standard deviation) was 64.0（14.9）μg/m3，and PM2.1 (<2.1 μm) was 46.2(13.2)μg/m3，which was about half of mass of PM10 and PM2.5 measured in November 1999. The investigation of weather and the emission of aerosol and its precursors indicated that the lower concentration of aerosols during this measurement were not caused by emission change, the summer dry and hot weather in that region could be the main reason. Dry and hot weather reduced the conversion process of SO2 to SO in liquid phase.Base on the size distribution of particle mass and ionic composition, which showed the high percentage of concentrations in the size range with less than 0.65μm, and ionic balance analysis, it could be inferred that the gas to particle process might play an important role in the aerosol formation during the experiment period. The statistics of aerosol data indicated that: The mass of aerosol with size less than 2.1μm accounting for 66% of total mass, in which about 50% of the mass was the particles with size less than 0.65μm. Similar to the size distribution of aerosol mass, the water soluble ions mainly concentrated on the size range of <0.65 μm, accounting for about 77％ of the sum of the analyzed ions, and the ions within the range of <2.1μm reached 88%.The sulfate, ammonium and potassium were the dominant ionic components in fine particles (particle size <2.1μm). Ion balances implied that the sulfates in fine particles were mostly consisted of (NH 4) 2SO 4,Na 2SO4 and K2SO4, but in submicron particles the non seal salt sulfate (NSS - SO ) was produced through gas-particle conversion process, and in the form of (NH4) 2SO4.