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

To identify the critical factors impacting the number concentration of particles with the aerodynamic diameters less than 2.5μm (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 dN/dlogDp 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 SO₄^2-, NO₃^-, NH₄⁺ (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 NO₃^-, SO₄^2-, NH₄⁺, 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.