Abstract
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 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, Dp,kel,0 (smallest size of particle that can
be activated), Dp,kel,50 (particle size activated with 50 %
efficiency) profile, and final growth particle size Dd under
wide operating temperatures, inlet pressures P (30–101 kPa), and growth
tube geometry (diameter D and initiator length Lini). 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 Tcon and
initiator Tini will reduce Dp,kel,0 and the
cut-off size Dp,kel,50 of the vWCPC. In addition, lowering
the temperature midpoint
(Tmid=Tcon+Tini2) increases
the supersaturation and slightly decreases the Dp,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 Lini=30 mm) is an optimized choice for
current vWCPC flow and temperature settings. The current simulation can more
realistically represent the Dp,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.