Analysis of extinction properties as a function of relative humidity using a κ-EC-Mie model in Nanjing

The relationship between relative humidity (RH) and extinction properties is of widespread 10 concern. In this study, a hygroscopic parameter (κ) and the volume fraction of elemental carbon (EC) were used to characterize the chemical characteristics of particles, and a core-shell model was built based on these characteristics. The number distribution, chemical compositions and RH were measured in Nanjing from 15/10/2013 to 13/11/2013. The model-derived extinction coefficients of particles were fitted with the program of coated spheres form Bohren & Huffman (BHCOAT), and the values 15 correlated well with the measurement-derived extinction coefficients (r 2 =0.81), which suggested that the core-shell model produced reasonable results. The results show that more than 81% of the extinction coefficient in Nanjing was due to particles in the 0.2-1.0μm size range. Under dry conditions, the higher mass fraction of particles in the 0.2-1.0μm size range caused the higher extinction coefficient. An increase in RH led to a significant increase in the extinction coefficient, although the increases differed 20 among the different size segments. The corresponding functions are given in this study. For λ=550 nm, the extinction coefficient contributions of the 0.01-0.2μm, 0.2-0.5μm, and 1.0-2.0μm size ranges increased significantly with the increase in RH, whereas the extinction coefficient contributions of the 0.5-1.0μm and 2.0-10.0μm size ranges decreased slightly.