Visibility is defined as the greatest distance in a given direction at which it is just possible to see and identify a prominent dark object against the skyline with the unaided eye in daytime, visibility degradation is a major problem in atmospheric pollution in many mega cities around the world. The object of this study was to investigate the correlation of visibility with physical and chemical composition of PM2.5 . The field experiments were conducted in four seasons from November 2017 to July 2018 in Qianjin and Qiaotou. Integrating Nephelometer and AE33 was used to measure the aerosol optical properties and compared with the calculated values based upon the measurements of aerosol concentration, size distribution and chemical composition (NH4NO3, (NH4)2SO4, EC and OC). The result indicated the relative humidity was above 80%, the particles usually grew into larger size range in fall and summer, but particle concentration and extinction coefficient in fall was higher than that in summer. The extinction coefficient was the sum of scattering coefficient and absorption coefficient. The ratio of scattering coefficient and absorption coefficient were 90% - 95% and 5% - 10% respectively. The results showed that surface area and volume concentration had a great relationship with extinction coefficient. The RSQr of surface area and volume concentration were more than 0.87. (NH4)2SO4 and NH4NO3 measured in size range of 0.46 µm - 0.7 µm (Qianjin) and 0.3 µm - 0.45 µm (Qiaotou) respectively. The concentration of (NH4)2SO4 and NH4NO3 in Qianjin was higher than in Qiaotou, and the size range were in the visible region between 0.4 µm and 0.7 µm, thus the visibility in Qianjin was lower than in Qiaotou.

Extinction coefficient, Particle concentration, Particle size distribution, Water-soluble ionic species