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Abstract. Accurate airborne aerosol instrumentation is required to determine thespatial distribution of ambient aerosol particles, particularly when dealingwith the complex vertical profiles and horizontal variations of atmosphericaerosols. A versatile water-based condensation particle counter (vWCPC) hasbeen developed to provide aerosol concentration measurements under variousenvironments with the advantage of reducing the health and safety concernsassociated with using butanol or other chemicals as the working fluid.However, the airborne deployment of vWCPCs is relatively limited due to thelack of characterization of vWCPC performance at reduced pressures. Giventhe complex combinations of operating parameters in vWCPCs, modeling studieshave advantages in mapping vWCPC performance. In this work, we thoroughly investigated the performance of a laminar-flowvWCPC using COMSOL Multiphysics® simulation coupled withMATLAB™. We compared it against a modified vWCPC (vWCPC model 3789, TSI,Shoreview, MN, USA). Our simulation determined the performance of particleactivation and droplet growth in the vWCPC growth tube, including thesupersaturation, Dp,kel,0 (smallest size of particle that canbe activated), Dp,kel,50 (particle size activated with 50 %efficiency) profile, and final growth particle size Dd underwide operating temperatures, inlet pressures P (30–101 kPa), and growthtube geometry (diameter D and initiator length Lini). Theeffect of inlet pressure and conditioner temperature on vWCPC 3789performance was also examined and compared with laboratory experiments. TheCOMSOL simulation result showed that increasing the temperature difference(ΔT) between conditioner temperature Tcon andinitiator Tini will reduce Dp,kel,0 and thecut-off size Dp,kel,50 of the vWCPC. In addition, loweringthe temperature midpoint(Tmid=Tcon+Tini2) increasesthe supersaturation and slightly decreases the Dp,kel. Thedroplet size at the end of the growth tube is not significantly dependent onraising or lowering the temperature midpoint but significantly decreases atreduced inlet pressure, which indirectly alters the vWCPC empirical cut-offsize. Our study shows that the current simulated growth tube geometry (D=6.3 mm and Lini=30 mm) is an optimized choice forcurrent vWCPC flow and temperature settings. The current simulation can morerealistically represent the Dp,kel for 7 nm vWCPC and alsoachieved good agreement with the 2 nm setting. Using the new simulationapproach, we provide an optimized operation setting for the 7 nm setting.This study will guide further vWCPC performance optimization forapplications requiring precise particle detection and atmospheric aerosolmonitoring.