Comment on "Retrieval of Atmospheric Fine Particulate Density Based on Merging Particle Size Distribution Measurements: Multi‐instrument Observation and Quality Control at Shouxian" by Li et al.

This work raises questions about the omission of Cunningham Slip Correction Factor (Cc(D)) in geometric equivalent diameter (Dp) conversions from the electric mobility diameter (Dm) and the aerodynamic diameter (Da) in relation to the calculation of effective density (ρe) as defined by Li et al. (2018; https://doi.org/10.1029/2018JD028956). They reported that ρe could be calculated using the formula "(Da/Dm)2," in which Cc(D) was not introduced into the conversion between two diameters involving Dm and Dp or Da and Dp. However, these conversions, which ignored Cc(D) in the transition regime, are not supported by current studies; nor did the group demonstrate their rationality. This comment shows that an error of up to 24.7% could be caused by ignoring the Cc(D) during conversions between the Dp and Dm for particles with a dynamic shape factor of 2.50 and a Dm of 550 nm; an error of 5.6% could be caused by ignoring the Cc(D) during conversions between Dp and Da for particles having a dynamic shape factor of 1.12, a density of 2.20 g/cm3, and a Da of 550 nm. The deviation of ρe could be amplified by 55.5% and 11.2% due to the squared term in the formula. Therefore, this comment argues that it is irrational to ignore Cc(D) when calculating Dp and ρe by using Dm and Da. Key Points: Cunningham Slip Correction Factor must be introduced in the conversion of geometric equivalent diameter in the transition regime [ABSTRACT FROM AUTHOR]