Referee Comment for “New Particle Formation Events Observed during the COALA- 1 2020 Campaign” by Ramirez-Gamboa et al.

The manuscript reports measurements of New Particle Formation in South-Eastern Australia in a site that is influenced by isoprene and monoterpenes as well as NOx and SO2. The manuscript focusses on New particle formation and growth up to Aitken mode sizes and fits the scope of the journal. Such measurements are certainly very interesting and important, especially in the Southern Hemisphere, however, there are several major issues with the current manuscript that need more discussion and in-depths analysis. After properly addressing these concerns and improving the manuscript as well as the interpretation and contextualization of the data, I would however recommend publication.

General comments:

My biggest concern is that while the data itself are certainly interesting, the results are not properly discussed and put into context of the developments in the field over the last several years. Even if no measurements with a nitrate CIMS are available to measure the nucleating species directly, also the existing data allows for a more thorough discussion. The main message of the manuscript is that NPF events are correlated with SO2 and NOx, but the given explanation for why these gases should lead to NPF is not adequate and in one case even wrong as discussed below. Also in the biogenic case it is mentioned that VOC oxidation plays a role for growth, but this role is not really specified. Currently the analysis is mainly pointing out correlation, but not investigating causation adequately.

In the case of SO2, we know that it can be oxidized to sulfuric acid, which is a prominent driver of nucleation, however, you do not really mention sulfuric acid in your discussion or interpretation. While sulfuric acid production via OH produced by photolysis is plausible for the early-morning event in Fig 2, this is not as straight forward for the nighttime event in Fig 7. Both events have elevated SO2, but one has photochemistry and one not, so the question is where would the OH be coming from in the nighttime case? It could be OH production via ozonolysis of monoterpenes, but you do not discuss this.

In order to get one step closer to the nucleation process, I would recommend to estimate the sulfuric acid concentration via proxies as e.g. described in Dada et al 2020 (https://doi.org/10.5194/acp-20-11747-2020), for which you have measured all necessary quantities (SO2, global radiation, condensation sink, RH). With these proxy values, you could then estimate whether sulfuric acid alone or in combination with ammonia leads to roughly the nucleation rates that you find in the field and in how far this would be comparable to other sites with sulfuric acid driven NPF and laboratory measurements.

The discussion of SO2 driven nucleation in the section between lines 264 and 271 is not correct. You suggest that SO2 will produce SO₄^2- radicals, which then trigger NPF. The SO₄^2-  time trace you use for that statement was however derived from the ACSM which measures particle composition of large (you state >40 nm) particles and not gas phase radicals. So the chain of events is flipped in your statement compared to what would actually happen in case of SO2/H2SO4 driven NPF (Oxidation of SO2 to H2SO4, then nucleation and growth, then, once the particles are larger than the cutoff of the ACSM, the SO₄^2-  signal in the ACSM would increase). Figure 4 and your statement in Line 264-266 also points to that direction: First you have SO2, then CN3 increases after 120 minutes due to conversion of SO2 to H2SO4 and nucleation and then another hour later the ACSM SO42- signal increases as the particles have grown to large sizes. So everything fits together and flipping the order does not make any sense.

The studies of Wang et al 2019 and 2020b that you refer to in the section starting in line 291 to my understanding do refer to oxidation of SO2 to sulphate ions inside the aerosol phase in the context of haze formation and are thus not directly applicable to NPF that you study here.

In the case of NOx, you again give no clear explanation how, on a chemical level NO or NO2 are expected to trigger nucleation. While in the SO2 to sulfuric acid case there is a clearly establishes path for nucleation, for NOx it is much less straight forward. In the biogenic case NOx typically reduces nucleation rates compared to the no-NOx case. One reason for this would be that nitrate functional groups typically lead to higher saturation vapor pressures than e.g hydroperoxide groups.

If NOx emissions together with SO2 emissions do indeed contribute to NPF, then a potential mechanism could be the SA-HNO3-NH3 growth enhancement described by Wang et al 2020 (https://doi.org/10.1038/s41586-020-2270-4). The nitric acid could form via NO2 oxidation by OH.

This could potentially be relevant to the event shown in Figure S7 at around 10 am. There is an initial rise in particle concentration both in CN3 and the SMPS and then at around 10:15 suddenly large particles appear. The important question is, is this an airmass change or really such a large growth inside the same air mass, which would then very nicely resemble the rapid growth event described in Wang et al 2020. I would recommend to investigate this further and check whether this potential rapid growth could be mostly inorganic, organic or both. (This is a prime example of very interesting data, but currently too little discussion)

The authors do not report ammonia levels in this study, however the particles are containing ammonium as measured by the ACSM and another study to which some of the authors of the current study did also contribute (Phillips et al. 2019) showed high ammonia levels >5ppb for a site not too far away from the measurement site in this study. Wang et al report their mechanism for colder temperatures than are reported in the current manuscript. Maybe with higher concentrations than in Wang et al. you could overcome the effect of the higher temperatures. I am not saying that this is necessarily what is going on, just that more discussion of what is going on is needed.

Another important question is, where are the SO2 and NOx emissions actually coming from. You discuss this briefly and mention the Appin road and traffic emissions. Can you add wind direction and speed to the time series plots and indicate (eg with colors) when an airmass was influenced by road traffic. This would make the plots easier to interpret. Especially to distinguish between changes in chemistry and real growth on the one hand and simple air mass changes on the other hand. (In Figure S7 it looks like the sudden growth event at around 10:10 is within an airmass, but the sudden appearance of particles at around 22:40 might be caused by airmass change. Also at the vertical black line at 13:40, indicating the end of the event, it simply looks like the airmass has changed to a more isoprene dominated and less polluted air mass). Related to this, you could also look whether you have any known ammonia sources along the way, if you have high SO2/NOx.

Another comment is that the manuscript tends to use imprecise or overly generalizing language. One example is from the introduction: “Secondary aerosols are produced via gas-to-particle transition, where reactive compounds in the atmosphere are oxidised to become low volatility organic compounds (LVOC). These compounds, along with sulfuric acid vapour are often involved in the nucleation process promoting clustering (e.g., Yu and Luo, 2009).”

As it is currently formulated it would mean that LVOCs are involved in the nucleation process, i.e. initial cluster formation. This is however not the case, as ELVOCs or ULVOCs are typically needed for this, which were described only after 2009. In this context I would recommend to more clearly separate the process of SOA formation, which in traditional understanding needs preexisting seed particles, and New-Particle-Formation (which means initial cluster nucleation and early growth), as both processes have very different requirements for the involved molecules in terms of saturation vapor pressure. Since the title of the manuscript contains New-Particle Formation, I would recommend to focus especially on this vs SOA formation (which you do not mention in the later part of the manuscript anyway).

The field of biogenic driven New Particle formation is rapidly evolving, however many of the references that are cited in the introduction regarding this topic are from the 2000s, which was before the discovery of ELVOCs/HOMs and pure biogenic nucleation. I would recommend to use newer literature to make your point in a more precise way, eg Kirkby et al 2023 (https://doi.org/10.1038/s41561-023-01305-0) or Zhao et al 2024 (https://doi.org/10.1038/s41586-024-07547-1), Bianchi et al 2019 (https://doi.org/10.1021/acs.chemrev.8b00395), or others.   

Another example of imprecise or out-of-date wording is the section from line 62-70: The first sentence implies in a sense that multi-generation oxidation is key to low saturation vapor pressure and refers to Kiendler-Scharr et al. 2009, which was before the discovery of autoxidation for atmospheric terpenes, as well as OH recycling pathways (eg Taraborelli et al. 2012). HOM formation from monoterpenes and the related autoxidation is a 1^st generation process with only one ozone or OH attack and one RO2 termination reaction. Only isoprene with its two double bonds is capable of 2^nd generation chemistry and this is an open question how autoxidation and 2^nd generation chemistry balances in this case, but you only very broadly speak of VOC oxidation.  

The main reason why isoprene scavenges nucleation is not competition for OH, as stated by Kiendler-Scharr 2009, but interference in monoterpene-RO2 termination reactions with isoprene RO2 radicals that suppress C20 production in favor of C15 production, which are weaker nucleators. Since isoprene chemistry is OH dominated and monotperene HOM formation ozone dominated, higher OH leads to a higher isoprene suppression effect.

The last sentence in this paragraph about high levels of SO2 and VOCs is somewhat isolated and unspecific.

In general, many of the discoveries related to HOM/ELVOC formation and their implications for NPF are missing from the introduction and discussion, even if this directly affects the main topic of the manuscript.

Overall I would suggest to improve the figures with more traces that are relevant to NPF (temperature, H2SO4 and HOM proxies, wind direction, etc) and based on these figures arrive at a more thorough interpretation and chemical classification of the events.

One thing that could also help is to sum up the smps channels to get a total particle count above 14 nm. You can then subtract CN3-CN14 and derive a time trace of nucleation mode only particles. This should give you a nice indication of when NPF is happening exactly, as the definition purely based on smps data tells you only when particles where growing above 14 nm, which depending on growth rates could take several hours. (I am not suggesting to get rid of the current estimation, but to add the CN3-CN14 method to it)

The ordering of panels in the time series plots is unintuitive, as you go from gas phase precursors to chemical composition of large (>40 nm diameter) particles back to small particles (CN3). I would suggest to put the ACSM panels at the bottom.

The time series plots have a large amount of white space between the panels, which makes the space available for actual data very small. I would suggest to place all panels directly on top of each other and only have x-axis labels on the lowermost panel to save space. I would also suggest to include two panels (one for Temp and RH and one for wind speed and direction). You might also include global radiation, ozone and sulfuric acid and HOM proxies, in order to make the story for the NPF event as comprehensible as possible.

How is it possible to sustain 1-2 ppb of NO during the night (e.g in Fig 2 or S7), when no NO2 to NO photolysis is taking place, traffic emissions might be lower than during the day and 5 ppb of ozone should oxidize NO to NO2 within 7 minutes? Are these values background corrected?

Specific comments:

line 60: can you mention the key condensing species?

line 182: You state that you measure at a rural area with relatively low anthropogenic influence, but a large number of the NPF events are influenced by NOx and SO2, so I would not call it low anthropogenic influence.

line 197: The term “density of particles” is misleading. Density refers to the mass to volume ratio of the solid or liquid particles in units of kg/m3, you seem to refer to the number concentration of particles, as you point to the CN3 measurements.

In Figure S2 you show an interesting event starting at 11 am. Here the ozone trace would be very helpful, as it looks like you have strong biogenically driven or enhanced NPF in addition to H2SO4 driven NPF. Can you provide a proxy for HOMs like monoterpene*ozone timetrace as well as the sulfuric acid proxy. More than 1 ppb of monoterpenes should lead to strong nucleation with typical levels of ozone. Can you derive a nucleation rate for this event and compare them with laboratory values, eg Kirkby et al 2016? (At least to get a sense for the order of magnitude)

Figure 8: here again the ozone trace and the HOM proxy monoterpene*ozone would be interesting, as it looks like as isoprene is decreasing monoterpene driven NPF is appearing. This would be actually confirm the isoprene suppression effect on NPF. Additionally to ozone, in this case it looks like NO3 driven oxidation of monoterpenes might be important, as NO2 is very high, so NO3 will certainly also be high after sunset. You can compare this to Yan et al 2020 (https://doi.org/10.1021/acs.est.3c07958), which report a suppression effect of NO3 on NPF.

Figure 9: Do you know why the CN3 levels are relatively low during the NPF event, lower than before, whereas the SMPS clearly shows an increase in particle number. It looks like the actual nucleation starts already well before the green line and the growth rate is rather slow, so the small particles might have coagulated away already. You could check this with the CN3-CN14 method outlined above.

line 288: Unclear if you mean that isoprene or SO2 are promoting NPF. You further state that “This event shows how even when VOCs available if there is no 𝑆𝑂2 in the atmosphere (13:00) the particle formation will substantially decrease”. Not all VOCs have the same NPF potential, as pointed out above, this needs a more in-depths discussion.

line 387f: I would not expect NPF to significantly increase PM1 mass, as the mass is dominated by the larger particles. You can still have significant NPF with a strong increase in ultrafine number concentration without affecting the overall mass concentration much.

Technical comments:

Figure S4: You mention MACR+MVK in the figure caption, but you do not show the data in the plot. You also mention how VOCs are consumed at around 22:00 in line 339, but you do not show VOC data.

Figure S6: Can you spell out PAR (PPFD) in the caption?

In summary, the manuscript describes a really nice dataset, but more analysis regarding causations and chemical pathways for NPF is needed.

This manuscript describes field observations of gas vapours, particle number distribution and composition in an Australian site at Cataract Scout camp. These measurements are utilised to find correlations of SO2, organic vapours with aerosol formation events. While such measurements are indeed valuable, since new particle formation measurements are rare in the southern hemisphere, and therefore potentially warrant publication.

However, the manuscript itself is very poorly prepared. It is clear that the authors did not follow the recent proceedings of new particle formation, isoprene oxidation and relevant topics. The discussions, references provided in this manuscript are rather out-of-date, and lots of them are even wrong. There appears many grammar and structural issues with the writing of this manuscript which make it extremely difficult to grasp useful information from this manuscript. The connections of the paragraphs are rather poor - it often occurs that the discussions jump from one point to another without any explanation. The same also applies internally to the same paragraph.

Scientifically, there are many flaws in the discussions of this manuscript.

1) The authors frequently mix up basic concepts, such as suggesting: 1) aerosols only have three modes and VOC oxidation by nitrate anion (NO3-) instead of nitrate radical (NO3), etc.

2) From their measurements, the authors suggest that SO2/NOx have strong impacts on their observed NPF events. While the impact of SO2 is understandable from the literatures, since it leads to the formation of H2SO4, which subsequently leads to particle formation, the role of NOx is mere speculation. The authors propose that under low NO conditions that organic oxidation might be enhanced (Nie, 2023) but their measured NOx concentrations are rather high compared to clear environments. Therefore, it is less likely the case. I should also note here that the H2SO4-HNO3-NH3 mechanism proposed by the reviewer #1 might not be relevant here since this mechanism is primarily important for cold environments, and there is no evidence that this will be important for boundary layer conditions.

3) The authors propose in this manuscript that isoprene is important for particle formation at their site but clear evidence is lacking. There were sufficient literatures suggesting that isoprene might inhibit aerosols formation under boundary layer conditions and the mere correlation provided in this study does not in any way challenge that. I suggest the authors to read these recent literatures and base their discussion on these studies instead of proposing correlation as the cause-and-effect. It should also be mentioned that the authors consistently measure high levels of monoterpenes in their measurements, which by themselves might be sufficient for triggering aerosol formation from monoterpene and H2SO4 system. I recommend the authors to read, for instance, Lehtipalo et al 2018 (10.1126/sciadv.aau5363).

4) the nighttime event presented in this study is very interesting. However, it should be noted that during these events no clear gas transitions, e.g., VOC, SO2, NOx, are consistently observable in all cases. In fact, in cases presented by Figure 8 and 9, these vapours did not change at all. This is rather surprising since it either suggest 1) the particle formation is not a particle formation but the meteorological conditions change at that time or 2) some other vapours are contributing to the particle formation events. To answer this question, the authors have to do more through investigations which include meteorological data, such as wind directions, back trajectory etc. to help them to interpret their observations.

Finally, the quality of the present manuscript prevents me from giving detailed comments which I hope the authors can address in their next version. While I have concerns about whether the authors can sufficiently address my suggestions and improve the quality of their manuscript to an ACP standard, I would recommend giving them a chance to improve their manuscript because of their valuable and rare measurements. It should also be noted that this manuscript should be categorized as a "measurement report" instead of an ACP article.

Major comments:

There is no clear storyline in the introduction. Many paragraphs are discussing their own independent evidence with very little connections between them. Even worse, within each paragraph, it reads like pileup of unconnected evidence and occasionally rather out-of-date, even wrong statements. This is very clearly seen from e.g., the lines 44 to 70, which I will give example as below:

L44-52: This paragraph mainly discusses the secondary aerosol formation processes, CCN and common knowledge of size distribution definitions. This paragraph contains multiple essential and basic errors which led me to believe that either the authors did not take their time to read the recent literatures. First of all, Yu and Luo didn't mechanistically find nucleation of sulfuric and/or organic vapours; therefore the references to their paper to show the involvement in NPF is inappropriate. Better references are needed here. Second, clusters and ultrafine particles are not the same thing, so lease remove "(ultrafine particles)" as surrogate of clusters. Additionally, aerosols are commonly discussed in four modes and not three (coarse mode, >1000 nm) which is a common knowledge in the field. I should also mention that I do not understand why one has to define the aerosol modes in this paper? Since the paper itself has nothing to do with classifying size distributions and also because the potential readers of this manuscript would surely know the four modes.

L53-60: It is rather disturbing to read through this paragraph. EVERY SINGLE SENTENCE in this paragraph is hardly connected to one another. I give a summary of each sentence in their respective order as below: 1) BVOC is important, 2) Monoterpene has higher SOA yield than isoprene, but there is more isoprene, 3) isoprene - NO3 has higher SOA yield than isoprene - OH, 4) isoprene SOA yield in low NOx conditions is higher than expected. First, there lacks connection with the previous paragraph, and it feels that this paragraph comes from nowhere. In the second sentence, the monoterpene and isoprene have not been properly introduced yet and it directly arrives at the point that their yields and emissions are compared. In the third sentence, main channels of chemical reactions of isoprene have not been explained yet and the product yield of NO3 is compared with OH. Additionally, the sentence does not make any connection with the previous sentences and the information provided is rather outdated. Isoprene oxidation primarily occurs during the daytime through OH oxidation channel, why the higher yield at night from NO3 than OH matters at all in this discussion? In the fourth sentence, why it suddenly jumps into discussing isoprene SOA yield at low NOx conditions, especially considering the previous sentences discusses higher SOA yield from isoprene - NO3 channel? Last but not the least, what this paragraph eventually wants to tell the reader? What is the conclusion of this paragraph and the association with this manuscript?

Lines 61-70: Again, no connection with the previous paragraph. The statement "To form key condensing species, multiple oxidation steps must happen to the original VOC molecule" is quite out-of-date and ignores the auto-oxidation channels that have been discovered in the gas phase since more than 10 years ago. The final sentence is detached from the rest of discussion.

Minor comments:

Figure 1: are the authors sure that they are free to share Google Earth's pictures as they are here?

L63: define OVOC

L181-183: how the frequency alone tells you about the level of low anthropogenic influence? In the least case, some back trajectory analysis would be required to even remotely conclude on this.

L192-194: it is not clear what this sentence is describing, please rephrase.

L194-195: refer to these "several other events" in the SI.

L196-198: rewrite the sentence.  

L206: were --> where

L208: it is clearly missing something. I do not understand how one jumps from L199 to L208 without further evidence presented. The authors should show their results which lead to " SO 2 appears to only affect daytime events, while NO x seems to have a shared role in both daytime and nighttime events" first.

L219: what plumes the authors refer to?

L233: nitrate radicals (NO₃) not nitrate anions.

L233-236: rewrite the sentence.

L243-245: rewrite the sentence. Also note that the bracket ")" is missing.

L268: SO4(2-) is not a radical

L269-271: It is not clear to me what the authors are talking about. SO2 forms H2SO4 first, then nucleation and eventually forms SO4(2-). How do they derive their order of chemical processes otherwise?

L291-302: the authors clearly ignored the recent proceedings of direct condensation of oxidised organic molecules and tried to refer to a nuance channel to explain why their measured NO2 should be related to NPF.

L349-350: the authors clearly do not know what they are talking about. They first propose that monoterpene oxidation products may contribute to the slow growth and then explains it that MACR (from isoprene) needs to be oxidized to produce aerosols.

L352: the authors should check what the low NO concentrations Nie(2023) refers to. Does that align with the constant ppb levels of NOx in Figure 7?

L363-364: what does "Like isoprene, the availability of monoterpene in the morning may determine how fast a NPF event can occur after SO 2reaches the site." mean? The previous paragraph was mentioning monoterpene oxidation etc, except the false reference to MACR. Can the authors be more specific with what they are describing?

L365-367: the authors should compare the OVOC yield of monoterpene from the OH and O3 channels and isoprene oxidation from OH channel before they derive such conclusions. These discussions are not quantitative, difficult to understand and potentially even qualitatively wrong. They have measured the needed vapour concentrations and should not stop at discussing qualitative phenomenon.

L370-379: 1) how do the authors conclude: "In the absence of monoterpenes but presence isoprene the particle formation may be of smaller magnitude and the formation may occur at a slower rate."

2) how do the authors conclude: "MACR is oxidised to heavier OVOCs that eventually condense.". In more recent isoprene oxidation studies, this statement is clearly wrong.

L398-399: the authors hypothesize that "Another factor possibly influencing the NPF events at night may include the early night VOC accumulation in the residual planetary boundary layer." without evidence. In fact, their measurements in Figure 8 clearly see decreasing trend of VOCs. Can the authors elaborate on this?

L440-441 and L449-450: how the authors derive that isoprene is enhancing the aerosol number while Heinritzi (2020) suggested otherwise. The evidence in this study is rather a correlation at maximum and does not pinpoint the underlying mechanism. It should also be noted that the monoterpene concentrations measured in this study is itself enough for triggering particle formation events.