Review of "Hygroscopic growth of ambient fine particles measured at five sites of China" by Chen et al.

This study reports hygroscopic growth measurements at five sites in China, three in urban locations and two in suburban locations. Approximately one month of measurements was conducted at each site over the years of 2016 - 2020. The studies all took place between April and July, with the exception of the study in Guangzhou which occurred in November. The observations show the smaller particles at the urban sites are usually composed of externally-mixed modes of varying hygroscopicity compared to the larger particles as well as particles of all sizes at the suburban sites. The authors also characterize the observations diurnally, as a function of PM2.5, in the presence of new particle formation and compared to chemical composition. Although numerous other HTDMA observations have been reported from China, I believe that this study is still of interest due to its greater spatial coverage and various analyses. The manuscript is well-written and the conclusions easy to follow. I recommend that the manuscript be accepted for publication provided that the authors address my scientific comments listed below, with the exception of the point about Sect 3.5 which would not be necessary for a Measurement Report but I am nevertheless interested in their explanation. 

Scientific comments

Sect 2.2 - What was the residence time in the humidifying region? How often were calibrations conducted?

Can the authors comment on whether the pandemic and any ensuing lockdowns potentially affected their measurements in Shanghai and Xinzhou?

Sect 3.4 - could the fact that the GZ study occurred in November have affected these results: more rain, changing wind patterns, etc?

Sect 3.5 - I'm curious what the authors think about the source / composition of the 40 nm particles on non-NPF days

Sect 3.6 - Are the kappa values presented in this section for each Do represent all the fractions (NH, LH, MH)? Or is it for just one of the modes? Is it possible to calculate the kappa value for each of the different modes? 

Line 365 - The authors state that the more hygroscopic mode accounts for only 20-40% of number fraction at urban sites. However, does this change significantly during NPF times?

Minor and Technical comments:

Title - consider changing to "measured at five sites in China"

Line 103 - should be "field campaign"

Eq (3) - consider inserting a multiplication symbol between GF and c

Line 215 - consider changing to "the aged particles"

Figure 5 - I understand that the current emphasis is on comparing the composition of each size particles over the five sites. However, have you considered making each site a panel with the sizes on the x-axis? This would make it easier to see how the hygroscopicity changes with size.

Line 293 - Consider spelling out NCP since you only use it 3 times

Figure 8 - What do the dots in panels b and d represent? It is not explained in the caption. 

Line 345 - edit to "owing to the fact that"

Line 368 - edit to "aggregated PM2.5"

Lines 370-372 - I'm having trouble understanding this sentence. Are you trying to explain the spatial variability? Or attributing the observed differences to the spatial variability? Please clarify

Line 489 - the reference for Liu et al., 2011 is repeated

General - Replace "accumulated particles" with "accumulation mode particles"

The manuscript (Chen et al., Measurement report: Hygroscopic growth of ambient fine particles measured at five sites of China) report the HTDMA data at major cities in China (Beijing, Shanghai, Guangzhou) as well as that from suburban areas (Xinzhou and Xingtai). The data from these places, especially that from Beijing, Shanghai, and Guangzhou, have already been well documented in literature. However, the data from various places have rarely been well complied. The manuscript should be useful for researchers who would like to know the spatial distributions of hygroscopicity of aerosol particles in China. The data quality looks acceptably good. I suggest publication of the manuscript after addressing the following comments.

Major comments

I understand that the major purpose of the manuscript is to compile the HTDMA data from five locations. It seems that the authors also have the ACSM data for all the locations. However, only the campaign average ACSM data seem to be compared with the HTDMA. It might have been possible to elucidate the controlling factors of the hygroscopicity in more detail by comparing it with the highly time-resolved ACSM data, although it might not be required for a measurement report.

Specific comments

L154 ‘the non-refractory chemical compositions in PM2.5 were measured simultaneously using a quadrupole aerosol chemical speciation monitor (Q-ACSM) in real time’

I wonder if the authors used a traditional aerodynamic lens or PM_2.5 aerodynamic lens. It would be ideal to clarify it if chemical compositions were measured as PM2.5.

L170 ‘This indicates different mixing states of ambient aerosol particles between urban and non-urban regions on account of their contrasting emission sources.’

It would be helpful if the authors could provide more specific ideas on the potential differences in emission. I personally wonder if chemical aging of particles during atmospheric transport from urban areas to suburb regions could influence hygroscopicity.

L191 ‘At the non-urban sites, however, the hydrophobic modes of GF-PDFs were negligible throughout the whole measured sizes.’

Figure 3 clearly demonstrates that hydrophobic modes exist even at non-urban sites, although the fraction could be smaller than the corresponding values for urban areas. I suggest modifying the statement.

L205 ‘This could be associated with the nucleation process in the daytime, which was demonstrated that the growth of the newly formed particles is mainly contributed by hydrophilic matters’

Is the hypothesis supported by the SMPS data?