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This work raises questions about the omission of Cunningham Slip Correction Factor (C-c(D)) in geometric equivalent diameter (D-p) conversions from the electric mobility diameter (D-m) and the aerodynamic diameter (D-a) in relation to the calculation of effective density (rho(e)) as defined by Li et al. (2018; https://doi.org/10.1029/2018JD028956). They reported that rho(e) could be calculated using the formula "(D-a/D-m)(2)," in which C-c(D) was not introduced into the conversion between two diameters involving Dm and D-p or D-a and D-p. However, these conversions, which ignored C-c(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 C-c(D) during conversions between the D-p and D-m for particles with a dynamic shape factor of 2.50 and a D-m of 550 nm; an error of 5.6% could be caused by ignoring the C-c(D) during conversions between D-p and D-a for particles having a dynamic shape factor of 1.12, a density of 2.20 g/cm(3), and a D-a of 550 nm. The deviation of.e 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.e by using D-m and D-a.