The manuscript “Modeling Diurnal Variation of SOA Formation via Multiphase Reactions of Biogenic Hydrocarbons” presents an attempt to model SOA formation from three different VOC precursors under varies conditions and to validate those results with chamber experiments. To do so the authors have extended their own UNIPAR model with additional oxidation pathways and reactions including product lumping based on volatility. In addition, physical parameters like relative humidity, temperature, etc. from the chamber experiments have been used as conditions for the model simulations.

The paper presents some interesting science and an approach that is well worth publication in ACP after major revisions and corrections.

General remarks:

• The authors should try to improve the language of the manuscript to increase its readability and help the reader to understand it more clearly. One common mistake to be found throughout the manuscript is the use of the direct article or its omission in the wrong places. I recommend careful proof reding by a native speaker (of the revised manuscript!).
• The title suggests investigation of the complete dial cycle. The experiments for individual compounds however do not cover whole day cycles but limited periods (3-12 h total, 2-10h for VOC oxidation/chemistry). In most cases these periods even differ for different compounds/experiments. Therefore, I recommend changing the title to “… under day- and nighttime conditions …” or similar.
• The abstract is quite long and detailed and partially feels like a conclusion. It might be good to shorten it slightly to give the audience the motivation to read the paper!
• In several places the authors mention that modeling the gas phase chemistry was done using explicit mechanisms, at other places the talk about “near explicit mechanisms”. These expressions are quite ambiguous without clear statements what the authors mean by explicit at the time of use (kinetics, mechanism, et cetera). This is especially true since it is mentioned (in some parts) that the resulting products were lumped based on their volatility, preventing for example explicit modeling of 2^nd and higher generation products. I recommend to either rephrase or clarify the extent of this “explicit modelling”.
• Often the authors tend to mix introduction, background, results, and discussion throughout the whole manuscript leading to repetitive statements and elongating it unnecessarily. The manuscript would gain a lot quality wise by clearly separating these different parts (both in term of length and readability).
• Some parts of the manuscript seem to be more a review paper than an original work. Again, a more in depths presentation and discussion of the results of this work would be very beneficial for the manuscript.
•  Many of the plots and diagrams are very small, and the axes are unfortunately scaled. This makes it really difficult to follow the authors discussion of their results! The graphical presentation of the results should also be improved!

Individual comments:

P1 L9: “… intensively evaluated …”: I suggest removing “intensively”. It is an unnecessary exaggeration.

P1 L13: How can the gas mechanism implement the MCM? Please rephrase!

P1 L15: How did integration with the SAPRC gas mechanism “increase feasibility”? Please clarify or rephrase.

P1 L21: What are background NO_x levels in the simulation (and the chamber experiments)?

P1 L30: What is the meaning of “...more sensitive to the aqueous reactions...” Please clarify.

P1 L32: “Diurnal patterns” cover the whole daily cycle! Your experiments do not comply with this definition.

P2 L40: Organic aerosol is not a “well-known” factor – that is why research on this topic is so important. Please rephrase!

P2 L42: SOA is in general important number wise, but for mass mostly in the PM1 range!

P2 L47: “The SOA from ... is considerable in a global budget of SOA.” Please rephrase, there is only one global SOA budget!

P2 L50: “... HC is oxidized mainly with OH radicals ...” Is this still true in the presence of high concentrations of NO_x?

P2 L54: Ozone is normally not persistent during night times but destroyed by chemical reactions. Only when it is lifted above the boundary layer (e.g., by an inversion layer) can it survive until the next morning. Otherwise, tropospheric ozone concentration would continue to rise. Please rephrase!

P2 L64: What is “absorbing organic matter concentration”? Please clarify.

P2 L65: “The oxidation ... was approached by ...”: Please rephrase.

P2 L67: What do you mean by “were not additive”?

P2 L74: “inorganic salted aqueous phase”: An aqueous solution is by definition a solution of a chemical in water, often of an inorganic salt. I guess what you want to say is “aqueous phase of inorganic salts”. This expression “salted aqueous phase or solution” is several times used throughout the manuscript. I recommend changing it.

P2 L78: “This model has been demonstrated by ...” I believe this sentence is incomplete.

P3 L 82: “...were generated from the four different ... pathways” How were they generated from the pathways? Rephrase!

P3 L 83: “To improve the feasibility ..., providing the HC consumption by each oxidant” This sentence is in parts an exact replication of the sentence in the introduction, and again it does not make too much sense. Please clarify. Also, a citation for the SAPRC-07TC mechanism would appropriate.

P3 L86: “The potential SOA yield(s) ... were applied” Applied to what? Please rephrase!

P3 L97: “... injected from a 2% NO cylinder under the air flow ...” What do you mean by under the airflow? Please clarify!

P3 L99: How did you ascertain that the sulfuric acid seeds where not neutralized during injection? It is quite a task to prevent its neutralization by ammonia which is very abundant.

P3 L102: What do you mean with “NO_x condition was controlled by NO₂”?

P3 L113: “The inorganic ion ... concentrations were in situ monitoring by ...” should be “... in situ monitored by a Particle-Into-Liquid-Sampler ...”

P3 L115: The use of butanol-based CPCs in chamber studies is quite controversial because of potential contamination. Did you check for this?

P3 L119: “An Aerosol Chemical Speciation Monitor ... to compare with data obtained from OC/EC and PILS-IC for accurate measurements.” Each of these methods have their own uncertainties; therefore, I would avoid the term “accurate measurements”.

P3 L119: “The relative humidity and temperature were monitored ...” How was this done (at how many different locations)? Since your chambers are quite large there could be a temperature (and humidity) gradient inside of it.

P3 L120 (also P1 L9): “... sunlight intensity was measured by Total Ultraviolet Radiometer.”? The results of these measurements are not presented in Table1 but could be important for the chemistry and the SOA mass yield!

P3 L120: Why are the results of the aerosol acidity measurements not presented in Table 1? This could also help to answer the question regarding sulfuric acid neutralization.

Follow up: In the beginning the authors speak of sulfuric acid seed particles, in the later parts they only mention ammonium bisulfate (which is to be expected). This should be clarified and correctly be mentioned right from the beginning.

P4 L124:  Again, explicit to which degree? Which mechanisms were used (reference)?

P4 L127: “… were determined by the near-explicit gas mechanism.” Which mechanism?

P4 L134: “… salted aqueous solutions …” How were they salted? Sodium chloride, ammonium sulfate, or something different?

P4 L135: What is the relation between liquid-liquid-phase-separation and isoprene SOA formation and condensation on inorganic seed particles, and why did you exclude it? An explanation (and maybe a citation?) for the general reader would be nice.

P4 L117: What do you mean by “was included to fulfill the oxidation mechanism in the current regional model”?

P4 L157: “a_i ... was estimated by the predetermined mathematical equation originated from the explicit mechanism as a function of ...” This sentence is difficult to understand. What is “a_i”? Are these the elements of the unified array calculated for each hydrocarbon oxidation pathway? Please elaborate!

P5 L169: OM_T is only introduced on P6 L222.

P5 L 171: “... is also calculated as the traditional ...” should be “... according to the ...” or similar.

P5 L209: Many phrases and statements are repeated again and again throughout the manuscript with nearly the exact wording, for instance here the “predetermined mathematical equations”, and “lumping species generated from the explicit mechanism”. The manuscript would gain a lot if those repetitions would be minimized, and the description of the mechanism development would be much more comprehensible for the reader.

P6 L230: “Thus, the seed effects observed in the presence of NO_x ...” This sentence is really difficult to understand. Do you want to say that the presence of NO_x had no influence on the SOA formation in the presence of seeds? What are those “seed effects”?

P7 L243: Which large molecules and how did you measure them? And even if they have a poor solubility, couldn’t the aerosol acidity still be important in a liquid organic phase?

P7 L245: Why do you mention that isoprene products can be “mixed” with aqueous solutions of inorganic salts, when you did exclude this, as mentioned in the previous sentence?

P7 L270: First you mention that your results show that for isoprene OH is the most important oxidant and that this agrees with previous studies, then you write that reaction with ozone is only minor, only to conclude (again) that your results suggest that a sizable fraction (whatever that is) of isoprene SOM is formed via the OH pathway.

P7 L263: The whole paragraph about the α-pinene simulations has only one sentence that discusses your results. All remaining parts are statements and knowledge from references, which belong rather into the introduction and not in a result/evaluation section if not put into a direct context with your simulations. What is missing here would be for example a discussion why for most cases there is no significant difference between low and high humidity (besides the ammonium bisulfate/acidic seed cases).

P8 L272: Have you investigated the amount of photo degradation of β-caryophyllene and the resulting products in your simulation to be able to calculate their volatility or is this a speculation or scientific knowledge (citation!)?

P8 L 279: Throughout the document you often equate NO_x with the NO₃ radical, which can be done in parts at nighttime in the presence of ozone. However, during daytime jNO₂ might play an important role which could even terminate certain oxidation pathways (of VOCs) resulting in higher volatility organic nitrates in the gas phase and thus less SOA. Again, this paragraph consists in large parts of repetition and citation of literature without direct context to your results and does not really discuss the results of the simulation.

 P8 L303: “The simulation of ... is performed with ... In the presence of the chamber wall” – Do you mean to say that wall effects were included in the model? Please rephrase!

P8 L306: “Overall, the effect ...” Any suggestions why that could be the case?

P9 L317: Why did you choose those VOC concentrations? They seem quite high compared to typical values, especially in the urban atmosphere.

P9 L318: “The sensitivity of SOA mass ... is simulated ...” should be “... is/was investigated by simulating ...”. Why are the results of this simulation only shown in the supplementing material and not discussed here?

P9 L321: This paragraph repeats just the results presented in 4.2.

P9 L324: “In addition, OH radical’s contribution ... is positively correlated to ozonolysis, which produces OH radicals as a byproduct.” This is a recursive statement and should be removed or rewritten.

P9 L 342-345: This is nearly a 1:1 repetition of L316-319 ...

P9 L347: “In nighttime, the simulation suggests ... at the high NO_x zone due to the high SOA potential ...” What is a NO_x zone? Please rephrase sentence!

P10 L360: What kind of gasoline fuel was used? And why this concentration? Is this relevant for the real atmosphere or is it just a simulation experiment. If it is relevant, what are common concentrations of gasoline fuel in the (urban?) atmosphere?

P10 L365: How was the oxidation of the gasoline fuel implemented? If it was not implemented comparable to for example the α-pinene (tegarding mechanisms and chemistry) it could also explain why it did not contribute to the SOA formation in the simulation.

P10 L370: It has long been known that aromatic compounds can be efficient OH scavengers

P10 L381ff: The discussion of results from section 4.5 is effectively missing. It is hinted, that α-pinene SOA formation showed the strongest reaction to the changes of the model parameters, and that the change of the partitioning coefficient had the largest impact on the three biogenic SOA formation systems, but nothing more. Therefore, this section should be either extended or removed.

P10 L386: Why was a different relative humidity used in these simulations?

P11 L394ff: Maybe “Conclusions” would be more fitting to this section!

P11 L411: “... under high NO_x zones ...” should be “concentrations” or “levels”.

P11 L412: “Under the rural environment” should be “In rural environments” or “In a rural environment”.

P11 L415: Nearly all soluble inorganic and organic salts are electrolytes.

P11 L419ff: “In this study ... showed a weak seed impact ...” How is this related to the previous statement about acidic seeds in other studies? Rephrase!

P11 L428: “... showing the higher biogenic HC emission” should be “showing higher biogenic HC emission(s)”.

P11 L429: Why is the "concentration of NO₂ generally high" during daytime? If there is no source of NO and no ozone, where is it supposed to come from? Rephrase the sentence!

P12 L435: “The model uncertainties ...mainly originate from gas mechanisms and aerosol phase reactions.” What other factors are important for SOA formation besides gas and aerosol phase? Please rephrase.

P12 L440ff: “Neither ... nor ... were not fully considered” In this context a double negation does not affirm but resolves to a positive. Rephrase!

P12 L447: The manuscript ends abruptly. There should be some final remarks or conclusions.

P17 Table 1: Why were such huge isoprene concentrations used for the experiments? I do not believe isoprene concentrations of 700 ppbC (140 ppbV) and above are of any atmospheric relevance. And even under such high hydrocarbon concentrations I would not consider 50 ppbV NOx to be a low NOx case!

P17 Table 1: I am quite surprised by mass yields as high as 157 µg per cubic meter from 84 ppbC (8.4 ppbV) a-pinene, especially without any seed (AP01).

P20 Figure 2: The caption claims to present OM_T and OM_AR (dotted line), while the legend annotates the dotted line as OM_p

P22 Figure 4: It is difficult to distinguish between O(3P) and OH

P23 Figure 5: All plots are too small. In addition, the axis of (a) suppresses the view of the dynamics in the plot. While it is in general a good idea to scale plots similar this should not hinder the interpretation of the diagrams.

P23 Caption: What does “the reference sunlight intensity” mean? Is this the total intensity of the sunlight at that specific day, or is it a wavelength dependent intensity (plot), and why did you choose this specific date? How does it compare to the average sunlight intensity at this place, and at other places? This is a very arbitrary measure which modern science tries to and should avoid. Please reason why you chose this day and intensity spectrum.

P24 Figure 6: Like Figure 5 all diagrams are way too small. Again, the scaling of the y-axis of the isoprene plot makes it impossible to clearly see any dynamic factors.

General Comments

Han et al present a series of experiments conducted in a rooftop chamber examining the oxidation of three biogenic hydrocarbons (isoprene, a-pinene, b-caryophyllene) during both daytime and nighttime conditions.  They examine the role of four different oxidants (OH, O₃, NO₃, O(3P)) and a series of environmental conditions, including hydrocarbon to NO_x levels, relative humidity, temperature and particle seed composition.

The major emphasis of the paper is on a gas-particle partitioning model, UNIPAR, that is first fit to the experimental data and then used to make predictions about the variation in SOA yields with different parameters.  Major conclusions are that there is a strong positive NO_x dependence to SOA yield during nighttime conditions and a weaker negative NO_x dependence during daytime, and that there is a modest negative temperature dependence.

Overall the paper is in line with other studies of these systems in the recent literature, but offers some new insights based on the explicit gas-particle partitioning model.  Some aspects of the presentation should be clarified prior to publication, however, as outlined in the more specific comments below.

Specific Comments

Line 43: Give the total SOA budget for reference.  Also add the caveat in this line that these are models of global SOA, and that the cited work is just one of several estimates of this quantity.

Line 54: Not clear what is meant by a sustainable NO₃ radical – perhaps this refers to production of NO₃ radicals being sustained?

Line 58: There are more recent references to the organic nitrate yield from NO₃ + isoprene.  See for example:

Brownwood, B., A. Turdziladze, T. Hohaus, R. Wu, T.F. Mentel, P.T.M. Carlsson, E. Tsiligiannis, M. Hallquist, S. Andres, L. Hantschke, D. Reimer, F. Rohrer, R. Tillmann, B. Winter, J. Liebmann, S.S. Brown, A. Kiendler-Scharr, A. Novelli, H. Fuchs, and J.L. Fry, Gas-Particle Partitioning and SOA Yields of Organonitrate Products from NO3-Initiated Oxidation of Isoprene under Varied Chemical Regimes. ACS Earth and Space Chemistry, 2021. 5(4): p. 785-800.

Perring, A.E., A. Wisthaler, M. Graus, P.J. Wooldridge, A.L. Lockwood, L.H. Mielke, P.B. Shepson, A. Hansel, and R.C. Cohen, A product study of the isoprene+NO₃ reaction. Atmos. Chem. Phys., 2009. 9(1): p. 4945-4946.

Line 97: The definitions of high and low NO_x seem arbitrary and as though they might both be high NO_x.  Was the fate of RO₂ radicals considered in defining the high and low NO_x conditions – i.e., the rate of RO₂ + NO compared to other RO₂ losses?

Line 145: Inclusion of O(3P) is relatively unusual and not normally important in the lower atmosphere (also a conclusion of this study).  What motivated the inclusion of this oxidant rather than other minor oxidants such as chlorine radicals or Criegee intermediates?

Line 214: At what rate was N₂O₅ hydrolysis included, and how efficiently does this compete with gas phase NO₃reactions?

Figure 2: The abbreviations NS, SA, wAS, etc. are not defined in the figure or the caption and not easy to find in the text.  Clarify the meaning of these abbreviations in the figure.

Line 230: Conclusion not clear in this sentence.  Is this stating that in the presence of aerosol there is no NO₃ reaction with the biogenic hydrocarbons?

Line 248: The biogenic mixing ratios used in the simulations are unrealistically large – does this also bias the SOA yields high?

Line 254: SOA yields from NO₃ said to be low during daytime, but Figure 4 shows them to be larger than OH?  Is this correct?  The description of isoprene SOA beginning in this line does not appear consistent with what appears in the figure.

Line 279: This paragraph contains a series of qualitative statements about the roles of different mechanistic pathways in forming SOA.  Presumably, all of these could be quantified with the model and shown as a figure?  

Line 362: What is the chemical composition of gasoline fuel?  Presumably this is in the gas phase?  Is the high mixing ratio used here realistic to ambient conditions?

Line 403: Suggest removing the reference to “government agency” and referring instead to NO_x control measures.

Line 415: The term “electrolytic” appears out of place here.