Mapping the performance of a versatile water-based condensation particle counter (vWCPC) with numerical simulation and experimental study

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 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, inline-formula $M1inlinescrollmathml{D}_{\mathrm{normal\; p},\mathrm{normal\; kel},\mathrm{normal\; 0}}$ 33pt14ptsvg-formulamathimg1c5dcc5d967086cd9ae3e9faa0e73e78 amt-16-3973-2023-ie00001.svg33pt14ptamt-16-3973-2023-ie00001.png (smallest size of particle that can be activated), inline-formula $M2inlinescrollmathml{D}_{\mathrm{normal\; p},\mathrm{normal\; kel},\mathrm{normal\; 50}}$ 38pt14ptsvg-formulamathimgaa9be6825186b4b3598062785f8d6a6d amt-16-3973-2023-ie00002.svg38pt14ptamt-16-3973-2023-ie00002.png (particle size activated with 50 % efficiency) profile, and final growth particle size inline-formulaD_d under wide operating temperatures, inlet pressures inline-formulaP (30–101 kPa), and growth tube geometry (diameter inline-formulaD and initiator length inline-formulaL_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 (inline-formulaΔT) between conditioner temperature inline-formulaT_con and initiator inline-formulaT_ini will reduce inline-formula $M10inlinescrollmathml{D}_{\mathrm{normal\; p},\mathrm{normal\; kel},\mathrm{normal\; 0}}$ 33pt14ptsvg-formulamathimg353731010cb80698c56323bbfd86abe5 amt-16-3973-2023-ie00003.svg33pt14ptamt-16-3973-2023-ie00003.png and the cut-off size inline-formula $M11inlinescrollmathml{D}_{\mathrm{normal\; p},\mathrm{normal\; kel},\mathrm{normal\; 50}}$ 38pt14ptsvg-formulamathimg8c4cc191fa964d63d23b13424f66e57e amt-16-3973-2023-ie00004.svg38pt14ptamt-16-3973-2023-ie00004.png of the vWCPC. In addition, lowering the temperature midpoint (inline-formula $M12inlinescrollmathml{T}_{\mathrm{normal\; mid}}=false\frac{T_{\mathrm{normal\; con}}+T_{\mathrm{normal\; ini}}}{\mathrm{normal\; 2}})$ 69pt18ptsvg-formulamathimg4249d4fcdb790d8afec245e53ef909bf amt-16-3973-2023-ie00005.svg69pt18ptamt-16-3973-2023-ie00005.png increases the supersaturation and slightly decreases the inline-formulaD_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 (inline-formulaD=6.3 mm and inline-formulaL_ini=30 mm) is an optimized choice for current vWCPC flow and temperature settings. The current simulation can more realistically represent the inline-formulaD_p,kel for 7 nm vWCPC and also achieved 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.