Insight into the critical factors determining the particle number concentrations during summer at a megacity in China.

Abstract To identify the critical factors impacting the number concentration of particles with the aerodynamic diameters less than 2.5 μm (PNC 2.5), the continuous measurement of PNC 2.5 , chemical components in PM 2.5 , gaseous pollutants and meteorological conditions were conducted at an urban site in Tianjin in June 2015. Results indicated that the average PNC 2.5 was 2839 ± 2430 d N /dlog D p 1/cm3 during the campaign. Compared to other meteorological parameters, the relative humidity (RH) had the strongest relationship with PNC 2.5 , with a Pearson's correlation coefficient of 0.53, and RH larger than 30% influenced strongly PNC 2.5. The important influence of secondary reactions on PNC 2.5 was inferred due to higher correlation coefficients between PNC 2.5 and SO 4 2 −, NO 3 −, NH 4 + (r = 0.78–0.89; p < 0.01) and between PNC 2.5 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 PNC 2.5 and NO 3 −, SO 4 2 −, NH 4 +, while those between PNC 2.5 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 PNC 2.5. 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 PNC 2.5. The north of Jiangsu, the west of Shandong, and the east of Hebei were distinguished as major potential source-areas of PNC 2.5 by PSCF model. Graphical abstract Unlabelled Image [ABSTRACT FROM AUTHOR]