Mapping the Performance of a Versatile Water-based Condensation Particle Counter (vWCPC) with COMSOL Simulation and Experimental Study

WEIXING HAO, Fan Mei, Susanne Hering, Steven Spielman, Beat Schmid, Jason Tomlinson, Yang Wang, University of Miami

     Abstract Number: 79
     Working Group: Instrumentation and Methods

Abstract
Accurate airborne aerosol instrumentation is required to determine the spatial distribution of ambient aerosol particles, particularly when dealing with the complex vertical profiles and horizontal variations of atmospheric aerosols. A versatile water-based condensation particle counter (vWCPC) has been developed to provide aerosol concentration measurements under various environments with the advantage of reducing the health and safety concerns associated with using butanol or other chemicals as the working fluid. However, the airborne deployment of vWCPCs is relatively limited due to the lack of characterization of vWCPC performance at reduced pressures. Given the complex combinations of operating parameters in vWCPCs, modeling studies have advantages in mapping vWCPC performance.

In this work, we thoroughly investigated the performance of a laminar flow vWCPC using COMSOL Multiphysics® simulation coupled with MATLAB. We compared it against a modified commercial vWCPC (vWCPC Model 3789, TSI, Shoreview, MN, USA). Our simulation determined the performance of particle activation and droplet growth in the vWCPC growth tube, including the supersaturation, D_p,kel,0 (smallest size of particle that can be activated), D_p,kel,50 (particle size activated with 50% efficiency) profile, and final growth particle size D_d under wide operating temperatures, inlet pressures P (0.3 – 1 atm), and growth tube geometry (diameter D and initiator length L_ini). The effect of inlet pressure and conditioner temperature on vWCPC 3789 performance was also examined and compared with laboratory experiments. The COMSOL simulation result showed that increasing the temperature difference (ΔT) between conditioner temperature T_con and initiator temperature T_ini will reduce D_p,kel,0 and the cut-off size D_p,kel,50 of the vWCPC. In addition, lowering the temperature midpoint T_mid =(T_con + T_ini)/2) increases the supersaturation and slightly decreases the D_p,kel. The droplet size at the end of the growth tube is not significantly dependent on raising or lowering the temperature midpoint but significantly decreases at reduced inlet pressure, which indirectly alters the vWCPC empirical cut-off size. Our study shows that the current simulated growth tube geometry (D = 6.3 mm and L_ini = 30 mm) is an optimized choice for current vWCPC flow and temperature settings. The current simulation can more realistically represent the D_p,kel for 7 nm vWCPC and also achieved a good agreement with the 2 nm setting. Using the new simulation approach, we provide an optimized operation setting for the 7 nm setting. This study will guide further vWCPC performance optimization for applications requiring precise particle detection and atmospheric aerosol monitoring.

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