the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 20 Feb 2026
Mapping the Performance of a Small Mixing-type Condensation Particle Counter
Abstract. The performance of a small mixing-type condensation particle counter (sMCPC) was numerically evaluated. The modeling calculated the fields of turbulent flow and temperature, and species transport in the particle channel of sMCPC, and the growth of particles included the effects of Kelvin, non-continuum and latent heat. Upon the validated, the model was applied to investigate the effects of temperature difference (ΔT=Ts−Tc, where Tc and Ts are the temperature setting for working fluid saturation and sampled aerosol cooling, respectively), total flow rate (Qg), and vapor fraction (f) on the working-fluid-governed supersaturation and particle activation in the sMCPC. It is found that the supersaturation ratio is increased, and the critical activation diameter (Dp,50) is lowered by increasing ΔT; the excessive increase of Qg reduces the supersaturation ratio and shifts the ratio peak towards the downstream of carrier flow; both the supersaturation ratio and the Dp,50-slope are increased by increasing f. Under specific thermal and flow conditions, minimum activation diameters obtained in the cases with working fluids of ethylene glycol (EG), diethylene glycol (DEG), and dimethyl phthalate (DMP) is less than that in the case with n-butanol (B). Because of the particle growth after the activation, final sizes of particles exiting the particle growth tube are in micrometers in the case with n-butanol (B), and ~700 nm in case with EG; in contrast, final particle sizes in cases with DEG and DMP generally remain below the detection limit of typical optical particle counters (OPCs), i.e., ~0.3 μm.
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Status: final response (author comments only)
- RC1: 'Comment on egusphere-2025-5526', Anonymous Referee #1, 16 Mar 2026
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RC2: 'Comment on egusphere-2025-5526', Anonymous Referee #2, 18 Mar 2026
This study by Zhou et al. presents COMSOL modelling of a small mixing type CPC in order to infer its counting efficiency behavior with respect to the choices of temperature settings, flow rates, and working-fluid selection. Most of the obtained results confirm experimental results of size-dependent counting efficiency measurements of other laminar-flow and mixing type devices and I consider the article as a step forward in putting the experimental results into better context with theory. I can therefore recommend publication in AMT after the following points have been addressed:
Major comments:
- In the current manuscript it is not clear how the solved fields from the COMSOL simulations have been transformed into counting efficiency curves. In the conclusion, the authors mention that it is calculated from particle trajectories, but it remains unclear how this was done.
- The authors give a homogeneous nucleation equation which they claim to use for the calculation of the homogenous background in the CPC. They do not report any values of this later on and even more importantly, it is typically well know that homogenous nucleation rates from classical nucleation theory by far underestimate the real nucleation rates. It is therefore not clear if CPCs under the tested temperature conditions would be feasible or suffer from high background.
- It is not clear if the sMCPC which is simulated here corresponds to a real-world instrument or not. As it seems that the simulated sMCPC is not a commercially available instrument, geometric considerations would also be a big added value to this study. What happens in the simulations when the dimensions of the instrument are slightly varied? In addition, I wanted to remark that the authors cite a pending patent application from China. If so, the involved co-author should specify his conflict of interest.
Minor comments:
- Line 59: The authors cite Vanhanen et al. (2011) here, but the PSM is not a laminar-flow CPC. They cite it in the next paragraph again correctly, so I would modify the statement here.
- Line 63: I would end the paragraph with some word on the usage of other working fluids in CPCs (especially as testing different working fluids is a major result of the manuscript). There have been many new developments in that area e.g. Wlasits et al. (2024)
- Line 77: What does “offer enhanced mixing” mean?
- Line 93: Introduce the abbreviation sMCPC here.
- Line 309: “It is attributed to the elevated vapor pressure results in the enhanced vapor loss at the high temperature”. Not quite clear to me what the authors want to say here. Please try to reformulate.
- Line 309-316: This is similar o what Barmpounis et al. (2017) and Wlasits et al. (2020) found for laminar flow types. Could be mentioned here.
- Line 329: The authors speak of a broadening of the effective activation zone. This is not shown. In general, radial distributions such as in Fig. 2 could be helpful to understand better why the curves are sometimes steeper and sometimes are not.
- 7.: The steepness of the activation curves is what makes MCPCs in my opinion valuable compared to laminar flow type CPCs. This should become very clear in this manuscript and could be further emphasized (supported by the above mentioned 2D considerations) and a proper explanation on how the activation curves were obtained (see major comment).
- 6: What is Z0 on the x-axis?
- Line 402-406: The lower saturation vapor pressure of some working fluids however has the advantage that the Delta T can be even more enhanced (see the standard PSM settings), which at least for the PSM enables the detection well below 2 nm. This could be discussed here or even tested.
Citation: https://doi.org/10.5194/egusphere-2025-5526-RC2 -
CC1: 'Comment on egusphere-2025-5526', Michel Attoui, 21 Mar 2026
This study introduces a model designed to evaluate the activation efficiency of a mixing- chamber of a mixing CPC. By accounting for the mixing chamber and growth tube geometries, the model predicts the internal temperature and supersaturation profiles, as well as the resulting droplet size at the outlet. The system allows for the adjustment of input variables such as flow rates and temperatures for both aerosol and vapor, facilitating the optimized design of CPC devices using different working fluids.
This versatile tool will certainly be of great value to the mixing instrument community. It represents, to my knowledge, the first functional turbulent mixing CPC with a cold growth tube, whereas previous efforts have largely focused on laminar models. I suggest, however, that the authors cite the work of Fisenko et al. (2007): https://doi.org/10.1016/j.ijheatmasstransfer.2006.10.046
The work presents the theoretical framework of the various scientific fields involved in the heterogeneous nucleation of seed particles.
Here are few remarks:
The authors should clearly distinguish between the mixing chamber (characterized by turbulent flow) and the cooled down growth tube (laminar conditions) throughout the manuscript, rather than using the generic term 'MCPC'. As the MCPC is a combination of both components, precise terminology is essential for clarity. Furthermore, strictly speaking, a mixing CPC is not followed by a cold tube (see Wang et al.2002 cited in the paper and Wehner et al., 2011: https://doi.org/10.5194/amt-4-823-2011).
The air is assumed to be saturated upon entering the mixing chamber. The saturator schematic lacks clarity; specifically, the purpose of the central hatched area should be defined. Furthermore, unlike the sample line and growth tube, the mixing chamber appears to lack temperature control. Is it thermally isolated (adiabatic mixing) from the environment and from the other parts of the CPC?
Line 78: I would suggest using 'minimize' instead of 'suppress' regarding diffusion losses, as these losses can be reduced but rarely eliminated entirely.
Line : 40 The parameter Ractwithin the integral is difficult to interpret, define, or calculate. Could you clarify how it is determined? Further details would be greatly appreciated
Line 154 : The mathematical definition of the integral of over the chamber volume is unclear. Specifically, could you clarify the functional relationship between the nucleation rate and the spatial coordinates of the chamber? Furthermore, please distinguish between the local nucleation rate (per unit volume) and the total (integrated) nucleation rate. It would also be helpful to explicitly state the differential element (dV? ) used in the expression.
Line 162 : Dv the diffusion coefficient of the vapor molecules appears in the equation 5 but not in the list of symbols. The list of symbols gives the diffusivity.
Line 164: in the text p is the surrounding vapor pressure. In the list of symbols: P is the partial pressure of the condensing vapor [Pa]. Can you clarify?
Line 165: Same thing for pd. Please choose one name or definition in the text and in the list of symbols.
Ligne 174: Could you please clarify the purpose of Equation 8? It defines the droplet surface temperature (Td), yet this value is neither calculated nor utilized elsewhere in the text. Is it related to potential droplet evaporation? It needs some clarification. The temperature of the flow T in the same equation requires further clarification too. What is the value of T since the axial temperature profile is not constant?
Ligne 175: What is the density rv of the droplet? Is it the density of the working fluid or the density of the seed particle? What is the difference between rv and r the density of the fluid given in the symbols list? The list of symbols says rv is the density of the vapor.
Ligne 181: Could you say few words to argue why at = 1
Ligne 190: Here you have to say mixing chamber rather than MCPC
Line 207: What is F given in the equation (11) for the present model and instrument ?
Line 220: The term 'local saturation' is somewhat ambiguous in this context and requires clarification. Given that the carrier gas is assumed to be saturated upon exiting the saturator and before entering the mixing chamber, it would be more precise to refer specifically to the 'mixing chamber' rather than the 'MCPC'. Indeed, the MCPC encompasses the entire system, including the saturator, mixing chamber, and growth tube.
Line 226: qturb is missing in the symbols list.
Line 283: Could you please provide more details regarding the temperature measurement method? Specifically, was a thermocouple inserted from the outlet of the growth tube, or were measurements taken through sealed ports at various positions along the tube?
Line 302 : Fig 6 not Fig 5.
Line 618: vm the molecular volume of the water has nothing to do in the paper since you have not used water.
Line 612: Ts and Tc are inverted in the list of symbols. You should correct.
Line 632: Could you please give the numerical values of : 𝜎𝑘, 𝜎𝜔, 𝛽∗, 𝛾, and 𝛽0?
Citation: https://doi.org/10.5194/egusphere-2025-5526-CC1
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- 1
Jitong Zhou and colleagues present the model description and evaluation of a mixing-type condensation particle counter using the COMSOL multiphysics software suite. The physical processes within the device are described, and the sensitivity to using different operational parameters (flow rates, temperatures, operating fluid) are explored. The study contributes to a better understanding of the properties within such a device, provides quantitative guidance abut the influence of parameters in the specific instrument, and more generally intuition about general CPC processes. I consider the manuscript to have an appropriate scope, the methods to be adequate, and the results to be properly presented. I further believe the results could be useful to the readership of AMT. Overall, I would be happy to see this manuscript published.
I am still unclear about how the multiphysics modeling in COMSOL was complemented by MATLAB . There is not just one trajectory of particles entering and leaving the CPC but the particle flux density into the CPC follows the flow velocity profile. Was MATLAB used for an ensemble of released particles? Was it done in a different way?
In understand COMSOL is capable to represent particles that change their physical properties during their travel through the system. Why were the particles not modeled fully within COMSOL.
This modeling step is quite fundamental, please ensure it is clearly communicated in the manuscript.
Figure 2 has a lot of white space at the moment, the data are only a detail. Please explore showing the results more clearly. A panel that stretches the information horizontally (a zoom in horizontal direction only) could be a viable option. The labels of the color legend should be enlarged.
I appreciate the model verification regarding the mesh quality and the external corroboration with measured temperatures. While further comparison to experimental data would be welcome, I am convinced about the general value of the developed model.