This study presents measurements of isoprene and its SOA products in Shanghai across summer and wintertime in 2015, 2019, 2021. The study examines isoprene SOA response to emissions reductions, particularly the hydroperoxy pathway (IEPOX-SOA) and NOx-dominated pathway (HMML, MAE).  CMAQ modeling captures some trends in experimental data, including a decreasing IEPOX-SOA over the study period.  The study concludes that emissions reductions can decrease biogenic SOA in urban areas, and that a deeper understanding of isoprene-derived SOA is needed.

I have major technical concerns about the manuscript that should be addressed prior to reconsideration for publication. Additionally, there are numerous opportunities for more thorough comparisons of the modeled and experimental data.  Similarly, there is opportunity for thorough and meaningful comparisons of the current study to literature more specific to urban areas. 

1. I have major technical concerns about the potential for matrix effects and interferences in the selected analytical measurements, and how these may influence the results of the current study.  
   
   a.    Recent work demonstrates that use of reversed phase liquid chromatography (RPLC) with mass spectrometry (MS) can caused significant underestimation of some isoprene-derived organosulfates (OS) with little retention on reversed phase LC columns (Liang et al. 2025, pubs.acs.org/doi/10.1021/acs.est.5c01846). OS co-elute from the RPLC column with sulfate and other inorganic ions, which can suppress OS signal.  Changes to the aerosol matrix can thus be misinterpreted as changes in OS concentrations.  Because the author’s OS measurement method is likely subject to matrix suppression, there is concern about the validity of the OS concentrations and relative ratios presented in this study.  
   
   b.    There are additional concerns about C5 alkene-triols being artifacts of gas chromatography (GC) MS analysis (Frauenheim, et al. doi/10.1021/acs.estlett.2c00548).  The extent to which these may be artifacts in the current study should be considered, especially following the result that they are the dominant product observed by GCMS.
   
   c.    Taken together, the suppression of OS and potential artifact formation of C5 alkene triols raises questions to the validity of the paragraph that discusses “the dominance of iSOA polyol tracers over OS tracers…”  With one signal being enhanced and the other suppressed, such comparisons have very large uncertainties.
   
   d.    The potential for bias in measurements mentioned should be thoroughly considered and discussed in downstream calculations and comparisons, including SOA estimates, relative ratios of isoprene SOA products, correlation analysis, model comparisons, etc. 
   
   e.    The current treatment of matrix effects is insufficient at lines 221-225. While the expected extent of matrix effects may be informative, correction factors are not valid across studies. The authors must discuss the relevance of the sample matrix in the study by Bryant et al. (2021) and their work.  Similarly, it is not a valid approach to extrapolate relative ionization efficiencies observed by others (i.e. Bryant et al. (2021, line 169)) across studies, because ionization changes day-to-day within an instrument, and depends upon specific instrumental conditions and mobile phase composition. Ratios of relative responses are reported to 2-3 significant figures at lines 169-173, while in reality these estimates are known with much less certainty.
2. There are also opportunities for more thorough comparisons of the modeled and experimental data.  These are mentioned in the text, but should be integrated into figures.  These additions would strengthen the conclusions of the paper and improve clarity.  
   
   a.    Extend Figure 2 be expanded to also show PM2.5 mass, OC (or OM), sulfate, nitrate, and other relevant PM component or atmospheric parameters (NOx, O3).  This would be a useful way to provide context for understanding changes in isoprene SOA that are discussed subsequently.  
   
   b.    In figures 6 and 7, include experimental data be for comparison to modeled values.  
   
   c.    Also in figures 6 and 7  – the x-axis labels are confusing.  Can they be simplified?
3. In many places, the authors compare the current study to select literature references.  Many of these comparisons are to background or rural locations (i.e. central Amazonia and rural sites in the Southeastern United States), raising question as to their relevance to Shanghai.  To better understand the urban influence (and the emissions reductions) on isoprene SOA chemistry, the authors should more thoroughly compare and discuss their work in relation to prior studies in urban locations in Asia and elsewhere. 
4. When comparing to prior studies, the authors need to consider how similar or different methods may influence the comparison.
5. The notation “HMML/MAE” implies the ratio of HHML / MAE.  Is this intended?  Or could this be HHML, MAE, or HHML & MAE?
6. At least one of the studies mentioned at line 166 (Hettiyadura et al. 2015) did not use camphorsulfonic acid as a surrogate standard.  Please check and revise this thoroughly.
7. At line 38, it is it a realistic recommendation to “regulate atmospheric oxidizing capacity”?  Typically, regulations are either on emission sources or on ambient concentrations of hazardous pollutants.  Please reconsider this closing statement in the abstract.
8. Line 67, a symbol before 45 is appearing as a box.
9. The logic at lines 98-104 needs improvement.  The approach that “previous studies have mainly focused on the characterization of the particle-phase abundance of the iSOA tracers” is justified in that for iSOA to form it must be in the particle phase.  Additionally, this paragraph implies that gas phase concentrations of tracers were measured in this study, whereas they appear only to be estimated by calculation and not measured.
10. Can the authors validate their estimates of gas-particle distributions using experimental data?
11. Table S2, it appears that matrix effects were considered in only six samples, not eight as suggested by the caption.
12. In Figure 1, is wind speed data missing from 2021 and 2022?  It seems not to appear in the figure shown.  In general, the resolution and the quality of this figure should be improved prior to publication.

The manuscript by Hu et al. presents interesting new results on isoprene-derived SOA in a megacity in China and the response to the on-going reductions in anthropogenic emissions. In general the results are well presented and discussed. The study includes both measurements and modelling results. One limitation, which the authors should keep in mind, is that the field measurements are conducted on a limited number of samples. The modelling approach balances this, but the text sometimes overstates the conclusions that can be drawn.

Specific comments:

Line 21: profoundly is a somewhat strange word to use here.

L28: Mass spectrometry – please be more specific.

L38: “regulating atmospheric oxidizing capacity and aerosol reactivity to mitigate biogenic SOA formation” Is biogenic SOA formation really the major source of air pollution? How can atmospheric oxidizing capacity and aerosol reactivity be regulated?

L46: Heald et al., 2008 -is there a newer reference.

L58-59: “The gaseous IEPOX can be taken up into aqueous aerosol and undergo acid-catalyzed reactions to form polyols, organosulfates (OSs), and oligomers.” This sentence needs references.

L61: “abundantly measured” it is unclear what you mean – where they measured a lot or in high concentrations?

L67: The sign before 45% is missing.

L100: Which iSOA tracers?

L108: cation ions -> cations

L109: The text is about recent changes but one of the references is from 2014.

L113: later -> latter

L119: comprehend -> understand

L149. What is KPA?

L151. Please provide information about sources of authentic standards.

L171-172: Can this depend on the vendor of the ESI inlet and MS?

L181: Please be a bit more specific about the distance and the surrounding areas i.e. emissions.

L194-195: Please discuss the uncertainty associated with this approach.

L224: It is nice that the authors bring forward this uncertainty, but it is unclear how it affects the results presented here and whether it should have been tested if the magnitude of the effect is the same in the current study.

L299: Are the values statistically significantly different?

L302: Dramatic is not a correct scientific word to use here. It would be nice if the authors could list some numbers.

L303: Are there other types of nitrate than aerosol nitrate?

L305: Please also state the average values.

L308: Please avoid the use of the word dramatic. Aerosol pH – are the differences statistically significantly different?

Figure 1: The figure is very small and it is difficult to see details.

L334: Please state standard deviations.

L361: What do you mean by median?

L365-367: Please clarify this sentence.

L368: Is it consistent or different?

L396-398: It seems like a bold statement to say that the concentrations in winter decrease when the values are 0.8 ± 0.3, 0.8 ± 0.2, and 0.7 ± 0.4.

L400-403: These sentences need clarification.

Figure 4: It is not possible to read the numbers in the heatmap.

L429-431: The correlation is quite low, which should be reflected better in the text.

L434-436: The correlation is quite low to moderate, which should be reflected better in the text.

L439: The change is quite small. Was a statistical test performed to check this?

L451: What happens to isoprene when the oxidation capacity is lower? Does it not just take longer for the oxidation to occur?

L496-498: Unclear sentence.

Figure 6: Describe better what the numbers/years for the cases are.

L551: This line needs editing.

L554: “Field studies have demonstrated” – this is a bold statement as there are differences between urban areas.

L564: underappreciated is a loaded word not suitable here.

L567: three- year measurement gives the impression that samples for three complete years were studied. Please correct.

L568: During the period 2015-2021, however only periods during three years were studied. Please correct.

L574-575: “while HMML/MAE-SOA species (4.3, 4.2, and 4.3 ng m-3 in 2015, 2019, and 2021,

575 respectively) did not decrease significantly”. No they did not seem to decrease at all! Please correct.

L579-581: Please check that this can be concluded based on the current study.

L582-583: This sentence is not clear.

L599: Are biogenic emissions pollution?