To identify the critical factors impacting the number concentration of particles with the aerodynamic diameters less than 2.5 μm (PNC2.5),the continuous measurement of PNC2.5,chemical components in PM2.5,gaseous pollutants and meteorological conditions were conducted at an urban site in Tianjin in June 2015.Results indicated that the average PNC2.5 was 2839 ± 2430 dN/dlogDp 1/cm3 during the campaign.Compared to other meteorological parameters,the relative humidity (RH) had the strongest relationship with PNC2.5,with a Pearson,s correlation coefficient of 0.53,and RH larger than 30％ influenced strongly PNC2.5.The important influence of secondary reactions on PNC2.5 was inferred due to higher correlation coefficients between PNC2.5 and SO42-,NO3-,NH4+ (r =0.78-0.89;p ＜ 0.01) and between PNC2.5 and ratios that represent the conversion of nitrogen and sulfur oxides to particulate matter (r =0.42-0.49;p ＜ 0.01).Under specific RH conditions,there were even stronger correlations between PNC2.5 and NO3-,SO42-,NH4+,while those between PNC2.5 and EC,OC were relatively weak,especially when RH exceeded 50％.Principal component analysis (PCA) and Pearson’s correlation analysis indicated that secondary sources,vehicle emission and coal combustion might be major contributors to PNC2.5.Backward trajectory and potential source contribution function (PSCF) analysis suggested that the transport of air masses originated from these regions around Tianjin (Liaoning,Hebei,Shandong and Jiangsu) influenced critically PNC2.5.The norch ofJiangsu,the west of Shandong,and the east of Hebei were distinguished as major potential source-areas of PNC2.5 by PSCF model.