The relationship between relative humidity (RH) and extinction properties isof widespread concern. In this study, a hygroscopic parameter () andthe volume fraction of elemental carbon (EC) were used to characterize thechemical characteristics of particles, and a core-shell model was built basedon these characteristics. The size distribution, chemical composition, and RHwere measured in Nanjing from 15 October to 13 November 2013. Themodel-derived extinction coefficients of particles were fit with the program of coated spheres according to Bohren and Huffman (2008) (BHCOAT), and the modeledvalues correlated well with the measurement-derived extinction coefficients( = 0. 81), which suggested that the core-shell model produced reasonable results. The results show that more than 81 % of the extinctioncoefficient in Nanjing was due to particles in the 0.2–1.0 µm size range. Under dry conditions, the higher mass fraction of particles in the 0.2–1.0 µm size range caused the higher extinction coefficient. An increase in RH led to a significant increase in the extinction coefficient, although the increases differed among the different size segments. For = 550 nm, the extinction coefficient from the 0.01–0.2, 0.2–0.5, and 1.0–2.0 µm size ranges increased significantly with the increase in RH, whereas the extinction contributions from the 0.5–1.0 and 2.0–10.0 µm size ranges to the extinction coefficient decreased slightly.
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