This manuscript presents a modelling investigation into the role of anthropogenic pollution from Manaus, Brazil, on CCN, clouds and precipitation downwind in Amazonia. They do this based on 5 days of simulations compared to measurement data collected from aircraft during the GoAmazon campaign in March 2014/15 (wet season). The setup includes a regional model embedded in a global model with horizontal resolution of 3 km, making it suitable for explicit representation of deep convection. They present sensitivity studies and their implications for particle number concentration, droplet and ice crystal concentrations and rainfall. Overall, they conclude that the impact of regional emissions from Manaus is moderate in terms of both aerosol number and cloud droplet number concentration and low for cloud water mass and rain. The paper is mostly well written, interesting and it is well within the scope of ACP.  

I have two more general comments and then detailed comments below:

• Overall, the analysis in the paper focuses on high sulfur regions. I would expect NPF to have a larger impact in the regions where the plume is diluted somewhat into the cleaner surroundings and I am wondering why the research question is limited to the in-plume conditions? I am not suggesting that this needs to be changed, but rather that this could be discussed.
• This model description section (2.2.) doesn’t read very well in my opinion. There’s quite a bit of repetition and the information is not well structured. For the aerosol description specifically, I would suggest having a separate paragraph on BVOC oxidation products and yields, then introduce the different nucleation rates (is it correct that there is no sulfuric acid-water nucleation rate btw?), then say something about the overestimation in the models and where you turn off the nucleation rate.

The cloud microphysics scheme also deserves a bit more detail. How is rain initiated for example? Ice formation is mentioned in the discussion/results, but not in the description.

See detailed comments below.

L16: I think it would help if you specify what is meant by non-linear here.

L58-59: The meaning is somewhat unclear to me here. Consider rewording.

L105-107: The sentence does not flow well; the order of information seems slightly off. I suggest splitting it in two with the first containing information about the height and the second about the number concentrations.

L127: Is 3-km resolution enough to resolve all types of convection? What about shallow convection?

L136-137: This is a bit unclear: “i.e. collision and coalescence of aerosol-containing cloud droplets with some of the aerosol assumed to be deposited to the surface.”

L154: What does "that include vegetation" mean? Also, why are these not interactive? What about diurnal variability?

L155: Are all primary aerosols emitted at the same size?

L171-172: “The ability of biogenic vapour to nucleate depends on vapour volatility.” I feel like this should be followed by a statement about how the model treats the formation of ELVOCs/HOMs.

L173: “NPF in the UKCA model produces aerosol particles up to 3 nm in diameter.” Unclear what the implication is. Also, below you only mention parameterizations giving 1.7 nm particles. Do you mean that you parameterize growth up to 3 nm in diameter? Which parameterization do you use for this?

L175: Why below 100 m altitude? It’s only mentioned earlier that the model overestimates NPF in the free and upper troposphere?

L178: I don’t quite follow here. You find that even when you turn it off above 1 km and below 100 km, NPF is still the dominant source?

L181: This sentence seems a bit out of place. Move to earlier?

L184: The yields for HOM1 should be mentioned here?

L184: “Here, monoterpene is a type…” Everywhere, not just here :) monoterpene is a class of BVOCs, and I guess it is treated as one tracer by the model?

L195: “HOM2 is oxidized […]” maybe instead “HOM2 is the oxidation product of monoterpene by OH and O3.”

L195: “a steady-state approximation”. Please explain.

L197: Repetition.

L204-208: Do you mean that you include this in your survival probability calculation to the smallest nucleation mode? Do you use Kerminen and Kulmala (2002, https://linkinghub.elsevier.com/retrieve/pii/S002185020100194X) here? Maybe state explicitly.

L255-256: I would say maybe that for N100 the model mostly misses the rapid variability though? I.e. it only captures the large drops towards e.g. the end of the days some days.

L270: Same as above.

L272: I don’t think you describe the CTL-Bn simulations and H2SO4-H20 nucleation above in the description. It’s a bit unclear to me as well if H2SO4-H20 nucleation is only turned on in this simulation and if you have the same restrictions in the vertical (i.e. turned off above 1 km).

L285: I assume this is gas phase only? In the sentence before you write that these regions are defined “define according to the total sulfur species” which could be thought to be gas plus particle phase. I suggest clarifying this.

L336-339: I have a hard time following this. I suggest including an equation to make it clear.

L381-382: In addition to H2SO4 and nucleation rate going down, condensation sink goes down as well. I would have thought that the H2SO4 and nucleation rate went down due to high condensation sink, but this cannot be the explanation here?

L402-403: Why are the responses to the 0.25xaero and the 4xaero so symmetrical? Would you not expect some leveling off of the impact at high aerosol concentrations due to entering the updraft limited regime?

Fig. 8: The resolution of the arrows is quite bad.

L417-419: I suggest checking if these differences are significant and if not potentially removing this discussion.

Fig. 10: I suggest hatching insignificant areas here.

L438-440: How is rain initiated in the model? I think this is not covered in the model description?

L474: Is this out of the total or of the ones added regionally (i.e. CTL-offREG)?

L479: This sentence seems a bit unmotivated. Consider revising.

L498: Again, if this is the reason, why does also the condensation sink decrease then?

L510: Check this sentence, there’s repetition.

L516: Why would heterogeneous ice nucleation not being interactive weaken the “relationship” between ice and droplets? Is “interactively correlated” the right term here? I would say connected for example?

L527-528: Could you specify how a lack of prognostic supersaturation lead to a weaker effect?

L528-530: I don't understand these two sentences or what the implications are for your result. In the last sentence you seem to contradict yourself?

L531: Is this is just because your changes in emissions change the CCN concentration much much less than a factor of 4? Do you think it would be interesting to see the change in Nd plotted against the CCN concentration (or N100) in each of the simulations to see if it's completely linear?

L554: Again, I am not sure correlation is the right word here.

L557-558: “[…] of background aerosols even without anthropogenic emissions in the regional domain and the small perturbations of Nd as a result of small changes in aerosols.” The end of this sentence seems strange to me. Consider revising. 

Review for ACP manuscript EGUSPHERE-2025-132.

The influence of Amazonian anthropogenic emissions on new particle formation, aerosol, cloud, and surface rain. From Xuemei Wang1,2, Ken S. Carslaw1, Daniel P. Grosvenor1,3, and Hamish Gordon2

This manuscript, EGUSPHERE-2025-132, addresses crucial processes that connect particle formation in the remote tropical atmosphere, particle growth, and CCN concentrations in Amazonia. This manuscript investigates the impact of anthropogenic emissions from Manaus, Brazil, on new particle formation (NPF), aerosol concentrations, cloud properties, and rainfall in the Amazon region. The authors used the HadGEM3 climate model as a nested regional climate model to simulate various emission scenarios and analyze the effects of these emissions. The use of a high-resolution nested regional domain (3 km resolution) in the HadGEM3 model enables a highly precise representation of local atmospheric processes and their interactions with anthropogenic emissions. The study highlights the complex interactions between anthropogenic emissions, aerosol formation, and cloud processes in this relatively pristine environment, but very significantly, regionally and globally.

In terms of methodology, the use of a high-resolution regional model nested within a global model allows for detailed simulations of aerosol and cloud processes. The study design, with its various emission scenarios, is effective in isolating the impact of anthropogenic emissions.  The inclusion of observational data from the GoAmazon2014/5 campaign provides a strong basis for model validation. Comparing model results with aircraft measurements and radar data enhances the credibility of the findings. The paper is generally well-structured, with a clear introduction, detailed methods section, well-organized results, and a comprehensive discussion. The figures and tables are effective in presenting the data.

The authors acknowledge several limitations in their model, including the exclusion of upper tropospheric NPF and simplifications in cloud microphysics. These limitations may affect the accuracy of the simulated aerosol-cloud interactions. However, it is well recognized that NPF in high altitudes is a very recent finding. In all climate and regional models, cloud microphysics must be simplified to a great extent.

Some of the emission scenarios, particularly those involving very strong reductions in aerosol number concentrations, are considered unrealistic. While these scenarios help to explore the system's sensitivity, their relevance to real-world conditions is limited. The complex nature of aerosol-cloud-precipitation interactions leads to some difficulties in interpreting the model results. The authors acknowledge the non-linearities and buffering effects in the system, which can make it challenging to draw definitive conclusions.  

Future work should focus on incorporating more comprehensive representations of cloud microphysics and aerosol-cloud interactions. Including upper tropospheric NPF would provide a more complete picture of aerosol formation and transport. : The study could benefit from a more in-depth analysis of the mechanisms driving the observed relationships between anthropogenic emissions, aerosols, and cloud properties. For example, a more detailed examination of the role of specific chemical species and microphysical processes would be valuable.

The finding that even drastic reductions in aerosol concentrations lead to only a 4% increase in rainfall raises questions about the model's sensitivity. This outcome may suggest limitations in the model's ability to capture the complexities of cloud and precipitation processes in a non-linear convective environment. I think a more comprehensive discussion on this aspect could be good for the manuscript.

Overall Recommendation:

This is a well-executed and valuable study that contributes to our understanding of the complex interactions between anthropogenic emissions and the Amazonian “natural” atmosphere. The authors use appropriate methods and present their data clearly. While the model has some limitations and challenges in interpreting the results, the study offers valuable insights into the complex interactions between anthropogenic emissions, aerosols, and cloud processes in the Amazon. I believe that the most valuable finding in this study is that the Amazonian climate-aerosol-CCN system is highly resilient to change (see the first specific comment). I recommend that the title could include the high resilience of the system.

I recommend that ACP accept the manuscript for publication after responding to the specific questions listed below.

Some specific comments.

Abstract: It focuses on how resilient the Amazonian system is: “The 7-day simulations show that, in the areas that are affected by anthropogenic emissions, when aerosol and precursor gas emissions are doubled 10 from the baseline emission inventories, aerosol number concentrations increase by 13 %.”. “We also found that doubling the anthropogenic emission can increase the cloud droplet number concentrations (Nd) by 9 %”. “Even extreme reductions in aerosol number concentrations by a factor of 4, which is an unrealistic condition, cause only 4 % increase in rain over the domain.” This could be somewhat included in the article’s title.

Introduction line 20: Are you sure that you need 14 references at the same point for this statement? Maybe choosing one or two as the best reference would be better. This is also true for lines 34, 52, and others. There are far too many citations that are not relevant to the manuscript.

Section 2.2 - Line 150: “CMIP6 emission inventories provide CH4, and monoterpene, isoprene and natural SO2 are from CMIP5 inventories.” We need more details on the isoprene emission inventory since it plays a critical role in natural aerosol production in Amazonia.

Section 2.2 - The manuscript does not mention the natural primary biogenic aerosol particles. This is a crucial component of the Amazonian background aerosol, and it must be considered. How was it treated in the manuscript?

Section 2.2 – Anthropogenic SO2 is obtained from Edgar. What about the sulfate precursors from DMS and sulfur compounds from flooded areas?

Section 2.3 – line 180-185: Monoterpenes are the main VOCs for boreal forests, but NOT for Amazonia. Isoprene chemistry is the primary particle precursor in Amazonia, as reported by numerous studies spanning the lower troposphere to the high troposphere, as shown in the Curtius et al. paper from 2024.

Section 2.3 - What about NPF driven by isoprene-NOx system? The GoAmazon papers show that the increase in ozone is driven by NOx emissions, which have significant consequences for aerosol production.

Results line 255: It is not true that the model represents N > 100 well, as can be seen on 20140312. These are logarithmic plots, so there are differences by a factor of 10. In the legend, make clear that the plots are on a logarithmic scale.

Section 4 discussions and conclusions - Line 570 – Your strong statement “Thus, this study may not fully represent the response of cloud to changes in aerosol”. Better qualifications are needed, as this was one of the study's primary objectives. On line 84, you explicitly mention the objective: “(2) What are the mechanisms that drive changes in aerosol and cloud properties?”.