Review of Hao et al., 2024

General Comments

The authors present simultaneous measurements of particle number size distributions, aerosol optical and hygroscopic properties, and bulk chemical composition from urban Chongqing during the extremely hot summer of 2022 to investigate the characteristics of new particle formation (NPF) events for two distinct cases: polluted and clean-heatwave event. The authors claim that heatwave(s) may induce stronger photooxidation, enhancing hygroscopic growth and thereby aerosol direct radiative forcing. Overall, the manuscript is well-written. Although the objective of this study is intriguing, I find that a single heatwave event or unusually hot summer is not necessarily sufficient to support the findings and therefore speculation or tall statements must be avoided. I would like to recommend the publication of this study after the authors carefully address all the following concerns.

Specific comments

Fig. 1c shows the time evolution of (hourly?) temperature during the study period. Air temperature (RH) steadily increased (decreased) after 8 Aug. Surprisingly, the wind speed is slightly higher during period P2 (heatwave. Fig 2i), but heatwaves are usually associated with stagnant conditions. Nairn et al. (2015) calculated the excess heat factor to identify heatwave events. Could this be explored to determine the spatial extent of this particular heatwave event, using gridded temperature data if it is available for the region? Heatwaves are anomalous events characterized by extremely high surface air temperatures, typically lasting over a week. A mere surface air temperature threshold is not the best indicator of a regional heatwave event. This is indeed critical in the context of regional NPF events and the conclusions drawn from this study. The question is – Did the heatwave event trigger the NPF event, or did relatively cleaner conditions favour the NPF event or specific dynamical weather pattern favoured NPF (high-pressure system) or a combination of everything?

Please provide statistics of NPF events and non-events for both periods. A total of 23 days is divided into 4 categories and conclusions are drawn from a mere one heatwave event. How confidently can you say heatwave(s) promote NPF (based on your results alone)?  How about NPF frequency from previous years during the same time period? I also suggest showing an averaged contour plot of particle number size distributions for all these four categories.

Air mass history plays also a critical role in new particle formation processes. Consider showing an airmass history analysis (source and altitude) using HYSPLIT or Flexpart or similar models. The wind direction during the P2 period appears to be persistently east-southeast.

How is aerosol optical enhancement factor related to particle diameter for both cases (RH<30% and RH=-85%)? You may include a figure in the supplementary if you feel relevant.

Page 16, Section 3.4: I don’t understand why Figure 4 (c & d) focuses on non-events. I suggest showing results in a similar fashion for all four categories in Fig 4c and 4d, and also in Fig 5. Lines 415-418: Are you referring to NPF and non-events during P2? the subsequent discussion appears to be for non-events during P2? Please update Figure 4 and 5, and revise this section thoroughly.

Why GR (<25nm, 25-100 and >100 nm) and FR are not reported and compared between the event types based on SMPS data.?

Technical comments

Abstract: “Heatwaves triggered NPF earlier” – please quantify. You may want to plot sunrise and sunset times in Fig. S5. Define NPF event end time and growth event end time somewhere in the text.

Consider an obvious abbreviation for event classification – relatively polluted period (P1) to be indicated as NPFpolluted and clean heatwave-induced to be indicated as NPFclean,HW

All figure captions should clearly mention what is being plotted, time resolution, time (local to UTC),  etc and they should be self-explanatory.

There is an interesting recent paper by Garmash et al., 2024, the authors should consider citing and discussing it – DOI 10.1088/1748-9326/ad10d5

Particle size distribution measurement size range (and number of bins), and time resolution may be mentioned.

How was MLH obtained? All data and methods must be explicitly stated.

Page 4, line 127 -130: consider revising. The data/event sample is too small to draw implications for climate.

Page 5, Line 147-149: If I understand correctly, the authors deployed two nephelometers, one with a humidification unit and the other without. I would suggest giving explicit details of how the measurements were conducted.

Page 7, Line 212: chemical analysis results are plotted in Fig. S2, correct it.

Page 7, Line 220: “Fig.” S4? I can not find meteorological and air quality data. Or do you mean Fig.2? Please check all figures numbering and citations in the text.

Page 18, Lines: 443-445, consider revising the sentence starting “In this sense….”  What pollution level are you referring to?

Page 18, Lines:445, Remove “Meanwhile”

Avoid  unnecessary use of “pretty” (page 14, line 351), “relatively”, “meanwhile” , “In this sense” as above, etc. throughout the manuscript. Also Page 3, line 97 “there have been a great many studies” – looks unnecessary and no study is cited either. Simply say “Previous studies showed….” and cite relevant studies.

How was the aerosol effective radius calculated? Figures S4c1, c2, and c3 are unclear to me. Further, authors should show how the particle mode diameter behaved during P1 and P2 (averaged diurnal variation) for both event types and the condensation sink

There are several linguistic errors or issues with sentence phrasing. As I am not a native English speaker, I prefer not to correct them for the authors. I kindly urge authors to thoroughly proofread the manuscript to ensure clarity before submission. This will greatly enhance the readability and overall impact of the work.

References

Nairn J R and Fawcett R J B 2015 The excess heat factor: A metric for heatwave intensity and its use in classifying heatwave severity; Int. J. Environ. Res. Public Health 12(1) 227–253.

This manuscript focus on the aerosol optical hygroscopicity in Chongqing during three weeks’ field campaign using a combination of a home-built humidified nephelometer system and a scanning mobility particle sizer (SMPS), the total suspended particle (TSP) filter sampling and following chemical analysis, as well as the air pollutants data and meteorological data from available sources.

The measured aerosol scattering coefficients, aerosol optical hygroscopicity f(RH), and particle number size distribution are reported. Based on the temperature and aerosol scattering data, the measurement period from July 19 to August 19, 2022 was divided into P1 period and P2 period (heatwaves-dominated). The authors discussed the characteristics of NPF events during P1 and P2 periods, the characteristics of the aerosol optical and hygroscopic properties during P1 NPF and P2 NPF events, and the effects of f(RH) on aerosol direct radiative forcing.

Major comment #1:

Section 3.3 Characteristics of the aerosol optical and hygroscopic properties during NPF events

As can be seen from Figure S7, Size-dependent light scattering, backscattering and HBF efficiencies showed the particles with diameter less than 100nm have insignificant contribution to aerosol scattering. If so, why the authors pay more attention on the aerosol optical and hygroscopic properties during NPF events? What the exact meaning of NPF events here? Since there are 4 and 7 NPF days during P1 and P2 period, it is a little bit hard to investigate the influence of heatwaves on NPF events and subsequent impacts on aerosol optical and hygroscopic properties.

 Major comment #2:

Section 3.5 f(RH)-induced changes in aerosol direct radiative forcing

The effect of aerosol hygroscopicity on aerosol direct radiative forcing depends on f(RH) and the ratio of HBF_{525, RH} to HBF₅₂₅ ratio which were measured in this study.

The authors stated the mean HBF_{525, RH} was generally larger than HBF₅₂₅ with the ratios centered around 1.8 and even approached 2.5 on P2 NPF event days (Fig. 6c, Table S2). This result is in contrast with previous results such as from Titos et al. 2021, Xia et al 2023 and so on. The study by Titos et al., 2021 showed the fb(RH=85%) were lower than f(RH=85%) based on the data from 22 different sites covering a wide range of site types (Arctic, marine, rural, mountain, urban, and desert). The study of Xia et al 2023 showed the backscatter hygroscopic growth factor was lower than scattering hygroscopic growth factor (Figure 3) based on 2-year measurement in Beijing. This ratio is critical for this conclusion of this section. The authors need to explain why the ratio is so different? Please give more information about the humidified nephelometer operation information such as the time series of the temperature and relative humidity variation inside the nephelometer, the background variation etc.

 Comments in details:

The authors divided the NPF into two classes, one is relatively polluted period and clean cases during heatwave-dominated period

Some sentences are really hard to follow. Such as “Heatwaves triggered NPF earlier and prolonged the subsequent growth.” NPF events usually occurred in clean environment with low RH and high sunshine. What evidence does support NPF is triggered by heatwave?

Line 34-36 This sentence is not supported by the data in Table S2 where the f(RH) is almost the same for NPF events and non-event days.

Line 90-93 NPF could alter the size distribution thereby aerosol optical properties, nonetheless, there is currently limited research on the impact of NPF on aerosol optical hygroscopicity (Ma et al., 2016; Ren et al., 2021). Since the particles with diameter less than 100nm have insignificant contribution to aerosol scattering just like showed in Figure S7.

Line 142-144 As can be seen from Figure 1, the hourly temperature during P2 period (August 7-19) are not always above 40℃ which in not consistent with these sentence. In addition, why do you choose the hourly total scattering coefficient at 525 nm of 100 Mm-1 as criteria?

 Line 171-173 What kind of assumption are behind this calculation of ALWC? What kind of data are used to estimate dry aerosol volume concentration (Vdry) by a machine learning method?

The authors mentioned the Nafion dryer are used to dry the ambient air in S1 section in the supplement, what’s the total flow for online measurements? Both humidified nephelometers and SMPS share the same PM2.5 impactor?

More information of SMPS should be given, such as the sheath flow and aerosol flow, the sheath flow control mode, data retrieval, etc. In supplement S5, is the neutralizer model right?

Line213-215 This sentence is not supported by the Figure S2, where the sum of the measured chemical composition mass concentration is higher than PM2.5 mass concentration. It is really hard to understand to use TSP results for the characterization of NPF.

Line 228-229, Line 235-236 The two sentences are not consistent.

Line 239-241 Are the mean values of 1.6 ± 0.1 and 1.7 ± 0.2 during the P1 and P2 periods different significantly？

Line 241-243 Do you think the results is dependent on the algorithm of ALWC? Please clarify it.

Line 278-279 relatively polluted?

Line 284-285 the upper detection limit of 30 km? please clarify it

Line 291-292 why the authors emphasize that “sulfuric acid concentration was a critical factor for the occurrence of P1 NPF events.”? Do you mean P1 NPF and P2 NPF different? Figure2f show the diurnal variation of H2SO4 during different periods with minor difference.

Line 296-297 This might suggest that meteorological factors might not be the predominant determining factor of NPF occurrence? In this study, the measurement period is so short, more caution should be paid to reach this conclusion.

Line 309-311 “…the occurrence and subsequent growth of NPF during non-event days…”, the sentence should be clarified. 

Line 319 NPF could occur worldwide, what’s the temperature threshold of NPF events?

Line 359-360 during P2 heatwave-dominated NPF events? The meaning is not clear.

The PVSDs in Figure S3 a2-c2 are strange above ~ 500nm, why?

Section 3.3 Characteristics of the aerosol optical and hygroscopic properties during NPF events？

Line 338-339 What the meaning of NPF events in this study? Refer to the whole day?

Line 403-405 This opinion could not supported by the data in Table S2, where the fw during P1 and P2 NPF are 0.47 ± 0.04, 0.48 ± 0.05, while 0.46 ± 0.04 and 0.46 ± 0.06 during P1 and P2 non-event days.

Line 407-410, The authors mentioned “data mainly within the time window of 08:00-22:00 were utilized for the following discussion”, but, Figure 4a and 4b included other data, please clarify it.

Why only a few data in Figure 4d is with the temperature above 40°C

Figure 5 Why the authors labeled polluted and relatively polluted in Figure 5a and 5b for P1 NPF and P2 NPF，respectively? which is not consistent “in clean environment” mentioned above in the manuscript?

Line 447-449 Such a positive (negative) correlation of f(RH) with SAE (CS) was more pronounced in heatwave-induced high temperature days during P2 period. Which is not supported by the correlation R=0.58 during P2 NPF in Figure 5b1, while R=0.65 during P1 NPF in Figure 5a1.

Line 463-465 It is worth noting that f(RH) did not show a consistently higher level after the NPF occurrence during P2 period, and it was slightly higher within the first few hours of NPF occurrence during P1 NPF events (Fig. 3b). Which is hard to see from Figure 3b.

Line 474-476 The critical sizes corresponding to the cumulative frequency of 50% in σsca, 525 were 358.7 nm and 333.8 nm on P1 and P2 NPF event days, respectively. Have you seen the particles grow to this particles during NPF events?

Line 485-486  “…leading to a reduced enhancement in aerosol light scattering…”, please make it clear

Line 540-542 It should be noted that the reported fRF(RH) for the UGR site (Spain) was even higher, likely due to the relatively larger HBF in that area (Titos et al., 2014; 2021). This is not supported by the data in black dots (black dots for urban sites, UGR is an Urban site) Figure 2 in Titos et al 2021, although it is really hard to see which black dot is for UGR.

Line 565-566  “the new particles of higher hygroscopicity could contribute more to the activation of CCN,” this opinion is not supported by aerosol optical hygroscopicity measurement, however, could be supported by HTDMA hygroscopicity measurement.

 Figure S8, It seems the fitting equation wrong.