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Abstract:
We measured the mixing state of refractory black carbon (rBC) particles using a single‐particle soot photometer (SP2) on 11–27 July 2013 at a polluted regional site (~60 km to the south of Beijing) of the North China Plain. To improve retrieval of rBC mixing state, we validate and optimize the optical size (Dopt,p) of ambient particles derived from SP2 and Mie calculation. By matching the retrieved Dopt,p with the measured mobility diameter, we retrieve the densities (~0.3–0.6 g/cm3) and refractive indices (from 1.21–0.21i to 1.42–0.42i) of rBC core for thinly coated BC particles at mobility size of 200–350 nm; the refractive index of non‐BC (or coating) materials is estimated as 1.42–0i. It is found that the leading‐edge‐only‐fit method of SP2 calculation can be used to retrieve Dopt,p at least up to ~600 nm for ambient BC‐containing particles, and it can successfully provide Dopt,p of ambient non‐BC particles only up to ~500 nm. During the measurement campaign, the aging rate from thinly coated (or bare) BC to thickly coated BC can reach up to ~21%/hr around noontime, indicating the fast coating of BC. The average coating thickness (CT) of BC‐containing particles (200–350 nm) is ~41–71 nm, with ~13–16 nm CT for thinly coated (or bare) BC particles and ~75–114 nm CT for thickly coated BC particles. The mixing state of rBC obtained in this study indicates fast aging process and strong light‐absorbing capacity of BC particles under the polluted conditions in the North China Plain.
