Reply to Comment by Peng and Bi on “Retrieval of Atmospheric Fine Particulate Density Based on Merging Particle Size Distribution Measurements: Multi‐instrument Observation and Quality Control at Shouxian”

This is a reply to the comment of Peng and Bi (2019) on the paper of Li et al. (2018, https://doi.org/10.1029/2018JD028956 ), which introduced a method to estimate aerosol effective density ( ρ e ) based on a joint use of measurements of the scanning mobility particle sizer (SMPS) and the aerodynamic particle sizer (APS). In this reply, we describe the extension of the Cunningham slip correction factor ( C c ) to the expression of the ρ e and analyzed the useful but incomplete expression of ρ e equation on C c in Peng and Bi (2019) resulting from neglecting aerodynamic diameter. Meanwhile, in order to estimate C c ‐related error on ρ e , Peng and Bi (2019) used an unreasonable scheme and an overestimated shape factor, which leads to a large overestimation (55.5%) of the error. We modify the error budget scheme and perform a theoretical error analysis on neglecting C c during the ρ e measurement in practice. The results suggest an average bias of 17.68% on ρ e resulting from C c . Together with other error sources presented in Li et al. (2018, https://doi.org/10.1029/2018JD028956), the typical uncertainty on ρ e based on this “SMPS‐APS” joint‐use method is 21.06%, which is still reasonable in practice.