Chemical absorption of CO2 with a droplet of aqueous NH3 was numerically analyzed, and an empirical formula was derived to describe the instantaneous absorption mass flux as a function of instantaneous concentration conditions and other constant operating parameters. The proposed formula fit the numerical results excellently over a wide variation of initial or control parameter values and throughout the absorption process at different instants. The formula will be very useful to analyze the efficiency of an absorption reactor in which distribution of poly-dispersed droplets is nonuniform, and is applicable to various liquid absorption processes after appropriate modification of parameter values. (C) 2015 Elsevier Ltd. All rights reserved.