the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 24 Jul 2026
An Extended Modal Aerosol Dynamics Model (ExMADv1.0) for Simulating Early Nanoparticle Growth
Abstract. New Particle Formation (NPF) is one of the important sources of aerosol particles in the atmosphere and plays a significant role in global climate change and air quality. Accurately simulating the early growth of nanoparticles from NPF remains critical yet challenging. Specifically, the trimodal framework in current modal schemes often fails to resolve the distinct evolution of nucleation-mode particles and their separation from the background Aitken population. To address this, we developed an Extended Modal Aerosol Dynamics Model (ExMADv1.0) that explicitly incorporates a nucleation mode. This model can be completely driven by observational data for process analysis. The ExMAD model was evaluated against observational data from two representative NPF events in Nanjing, China. A nucleation-resolved decomposition of observed particle size distributions confirms that newly formed particles constitute a distinct sub-25 nm mode during NPF events. Compared with the conventional modal configuration, which captures only ~45 % of the observed sub-25 nm particle number concentrations, ExMAD reproduces ~80 % of the observed magnitude during the NPF events. ExMAD also reproduces the observed geometric mean diameter (Dg) of the nucleation mode within ~2 nm, a key metric reflecting early-stage growth dynamics. The extended scheme also improves the simulation of particle growth into Cloud Condensation Nuclei (CCN)-relevant size ranges, reducing the overestimation of >50 nm particle concentrations by nearly half relative to the conventional configuration. Sensitivity simulations further show that the Kelvin effect suppresses early-stage condensational growth, delays the transfer of particles from the nucleation mode to larger modes, and limits the uptake of low-volatility organics by the smallest particles. Compared with sectional simulations, the extended modal scheme provides a more realistic representation of sub-25 nm variability while requiring only a ~2.4 % runtime increase relative to the default modal configuration, far lower than the cost of sectional schemes. These results demonstrate that the extended scheme offers an optimal balance between physical realism and computational efficiency, supporting its future application in three-dimensional aerosol-climate simulations.
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Status: open (until 18 Sep 2026)
- RC1: 'Comment on egusphere-2026-3636', Anonymous Referee #1, 28 Aug 2026 reply
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Lin et al. present a modal aerosol dynamics model framework ExMAD that incorporates a nucleation mode to represent ~sub-25 nm particles originating from new-particle formation (NPF). In conventional modal models, the nucleation mode is merged with larger particles in the Aitken mode. This distorts the modeled size distribution and affects the particle number predictions. The new model is evaluated against two observed NPF events in Nanjing, China, and also compared to a sectional aerosol dynamics model for the same events.
Adding a nucleation mode is expected to improve the representation of NPF by modal models with a minor computational cost, and the work is thus relevant to modeling of aerosol concentrations and their effects. The model shows improved performance compared to a modal framework without a nucleation-resolving mode especially in the sub-25 nm size range. The manuscript is very clearly written, and the results are well presented and suitable for the journal. I find, however, the model evaluation quite limited as detailed below.
Main comments:
(1) I find the evaluation of the model inadequate: the performance is only compared to two 1-day NPF events, primarily to measured PNSD and additionally to the same events modeled by a single sectional model. This doesn’t give a proper view of model performance. Numerical GDE models are normally evaluated against more accurate benchmark model(s) and/or analytical solution when available, at various ambient conditions.
Specifically, the two evaluation cases are very similar, showing strong NPF typical in polluted environments in e.g. Chinese megacities. The model needs to be evaluated in different types of environments, especially if it is to be applied in regional and global large-scale models, including less polluted and pristine environments (e.g. marine, rural, less polluted background). Extensive case studies may be out of the scope of the current work, but some comparison to e.g a sectional benchmark model would be needed, including:
i. representative cleaner environments, and/or
ii. NPF with different nucleation and growth rates, not only intense NPF and rapid growth.
(2) Regarding the comparison of ExMAD and a sectional model (Section 3.4): I wonder if ExMAD is “better” for right reasons? It doesn’t sound correct that a parameterized modal representation is “more accurate”. Evaluating the models by comparing both of them to a given field measurement doesn’t necessarily give information of their physical goodness, as the observation is likely affected by factors that are not included in the models (e.g. emissions and transport, as also noted in the manuscript). It’s even discussed in the Intro that sectional models are suitable for NPF and UFP modeling. Please comment on this.
(3) I’d also suggest some more discussion on the advantages and disadvantages of the modal and sectional GDE model approaches for NPF modeling. For example, modal models have typically issues with NPF as the averaging “pulls back” the size distribution at high concentrations of nucleated particles. This effect should remain also when the nucleation mode is added, even if its magnitude is decreased. I suggest to summarize in the Introduction the types of biases that can be expected for modal vs. sectional models. Also note that there are different types of sectional models with different advantages (e.g. fixed grid, hybrid bin).
(4) There are some references to previous works that have incorporated a nucleation mode in a modal model (L83-84). How do these models and results compare to the current work?
(5) The effect of the Kelvin factor for evaporation is assessed by comparing ExMAD results with and without the factor. This doesn’t give info on how well the Kelvin effect is captured by the weighted average Kelvin factor in the nucleation mode, applied in the model. The size-dependence of the factor is very strong at the smallest sizes. How does the Kelvin factor affect model results in a sectional framework, in which its size-dependence may be more accurately described?
Other comments:
(1) Discussion on Fig. 5 on p. 15: The low bias of ExtMAD for N(25-100) is assumed to be due to missing primary emissions and transport. Can it also be contributed by the modeled growth rates? In Fig. 4 it looks like the modeled growth of nucleated particles is slower than what is observed.
(2) The sentence about Fig. 6c-d on L442-L443 is not fully justifiable, it’s not obvious from the figure that ExMAD would capture the temporal variability (even considering a possible temporal mismatch). The modeled and observed mean diameters in the Aitken mode look very different.
(3) Regarding modeling of larger CCN-relevant sizes N(>100) (Fig. S3), it’s stated that the model overestimates N(>100). It should also be noted that the observed N(>100) seems to be approximately unaffected by the NPF events, while the model predicts a notable (up to over ~3-fold) increase in N(>100).
Technical:
(1) In Figs. 2 and S1, panels (c) (3-modal fit) look much worse than panels (b) (2-modal fit); should the panels be the other way around?
(2) Figs. 3 and S2: What are the units of the axes?
(3) Fig. 5: The explanations for the line styles are opposite in the figure panels and in the caption.
(4) L569: There is some error message in the text.