Your revisions to your manuscript have addressed some of the referees’ questions, but it is not clear if the referee questions and concerned have been adequately satisfied by your response and revisions. The referees are asked to please consider the revised manuscript and comment on if the manuscript is now ready for publication, requires further revisions, or does not warrant publication in ACP.

Much of the manuscript is still difficult to follow, and seems to rely on a great number of assumptions, such as the SOA factor having a constant kappa of 0.1 to estimate kappa_org. The analysis of how hygroscopicity varies with aerosol composition is rather weak, especially considering that only changes in O:C were considered for the organic component. The AMS data can and should be analyzed in much greater detail to better understand the causes for the measured variations in hygroscopic growth factor, and why this does not seem to depend on O:C ratio here.

The new science findings that were obtained from this study are still not presented very clearly.

A few additional comments to consider:

It appears that no size-resolved AMS measurements were made? This would have provided a lot of valuable insight into how hygroscopicity varies with particle size. If no PToF measurements were made the authors should state this and explain why.

In a reply to one of the referee’s questions the authors refer to measurements of black carbon size distributions in Beijing from summer 2012. The measurements presented here are from 2014. I do not see how measurements from a completely different time period can be reliably used to indicate what the size distribution of black carbon was during these measurements. The authors must be much more careful in the assumptions they apply to their analysis and interpretation.

To estimate particle density the authors compared the AMS’s mass concentration measurement with the SMPS volume concentration measurement. It is possible to estimate density this way, see for example:
Kostenidou, E.; Pathak, R. K.; Pandis, S. N. An Algorithm for the Calculation of Secondary Organic Aerosol Density Combining AMS and SMPS Data. Aerosol Sci. Technol. 2007, 41, 1002–1010, doi:10.1080/02786820701666270.
But it must be done carefully. First, the collection efficiency of the AMS during this study has to be determined or otherwise estimated. Only a fraction of particles sampled by the AMS are vaporized in the instrument, and this collection efficiency is known to change with changes in particle size and composition. What was done regarding the AMS’s collection efficiency here? Also, the particle size transmission efficiency of the AMS must be considered, as the len inlet’s efficiency decreases quickly above ~500 nm, and there can be significant variations between aerodynamic lenses on different instruments. The method used here of just dividing the AMS mass concentration by the SMPS volume concentration is too simplistic.

The Referee’s question does not appear to have been answered here:

“Page 11505, Line 27: Were there other changes in aerosol characteristics (i.e. chemical composition, size distribution or κ) during high PM events?

Response:
The chemical composition and size distribution of particles should vary with PM concentration. In this study, the particle hygroscopicity is concerned. Other parameters will not be discussed in detail.”


How was the PAX instrument calibrated, using what black carbon standard?

Please give the flow rates used for the two SMPS instruments. Measuring up to 800 nm with an SMPS requires a very low sheath flow rate. This significantly broadens the width of the mobility selected aerosol.


The grammar and language use is still not correct in many places. Careful proofreading and editing by a person fluent in English is necessary. A few examples: “degree of scatter point of view”, “relatively dominated”.

The authors state that dust and sea salt are only present in supermicron sizes but this is a gross oversimplification. Mineral dust and sea spray aerosol can and often do extend into the submicron range, even below 350 nm. The authors cannot just assume these components were not present, though they may offer some justification for why assuming they made a small contribution is reasonable.

While the revisions you have made to your manuscript have addressed some of the questions and concerns raised by the Referees and myself, another round of revisions is required to address some important remaining issues.

An important issue to address in your revisions is the lack of detailed AMS data analysis and spectra in your manuscript. Only the O:C ratio is used to explain the observed trends. This is not a sufficient use of the rich dataset that can be obtained by AMS and other co-located measurements.
The following section of your manuscript highlights this issue, though it is not the only instance of this deficiency in your analysis:
“We should also note that the different aerosol formation ways in different environments could also be an explanation for the different relationship between κorg and O:C.”

Other studies also suggest that O:C does not encompass changes in detailed chemical composition that are responsible for changes in hygroscopicity (e.g. Mei et al., 2013; Rickards et al., 2013; Suda et al., 2014). However, AMS spectra obtained in this study are not discussed aside from the O:C ratio. This claim should be supported by presentation of representative AMS spectra. Furthermore, it would be useful to discuss what unique features of the Beijing aerosol chemical composition lead to the different observed hygroscopic behavior relative to other related studies.


Please also address the questions raised by Referee #2:

While the authors have submitted an improved manuscript, including a number of additional references, there are still several key issues that make this manuscript not yet ready for publication:

- The grammar in the manuscript must be further improved before it can be published. It is possible that Copernicus offers editorial services.

- The authors use a density of POA of 1400 kg/m3. However, the studies that they cite to back up this value were specifically conducted for secondary organic aerosol. A more realistic density for POA would be closer to 1000 kg/m3, since POA is thought to be similar to lubricating oil. This would affect the overall density of the aerosol enough that a study-long average of 1500 kg/m3 is inappropriate.

- In the chemical closure part of the manuscript, a K_org of 0.1 is used. It is only mentioned in passing in the response to the reviewer that this value was chosen because it resulted in a good fit. More details should be provided on how the fitting was determined and how other values affect the final fit. In addition, this should be described clearly in Section 3.2.

- The top panel of Figure 3, which shows the trajectory cluster based on circle size, is not meaningful since adjacent trajectories are not always related. For example Cluster 3 is from the opposite direction as Cluster 4. But it is difficult to distinguish a circle of size 3 vs 4. Perhaps you could replace it with a bar that corresponds to the colours of your clusters instead.

- Page 9 line 29 to page 10 line 1, add ``in an urban environment'' to the end of the sentence. I would argue that a 33% enhancement is not insignificant, although in absolute terms it is not as much.

- Page 13 line 5, the authors say that the hygroscopicity parameter of organic aerosols are typically lower than 0.1 but a quick look at Petters and Kreidenweiss (2007) shows that several organic compounds have a kappa greater than 0.1. Perhaps the authors were referring to ambient organic aerosol? The sentence should be clarified.

- Page 15 line 5, it should be stated at the beginning of this section that this analysis is only for 150, 250 and 350 nm aerosol.

References:
Petters, M. D. and Kreidenweis, S. M.: A single parameter representation of hygroscopic growth and cloud condensation nucleus activity, Atmos. Chem. Phys., 7, 1961-1971, doi:10.5194/acp-7-1961-2007, 2007.


And be sure to also address my prior questions and comments:

Much of the manuscript is still difficult to follow, and seems to rely on a great number of assumptions, such as the SOA factor having a constant kappa of 0.1 to estimate kappa_org. The analysis of how hygroscopicity varies with aerosol composition is rather weak, especially considering that only changes in O:C were considered for the organic component. The AMS data can and should be analyzed in much greater detail to better understand the causes for the measured variations in hygroscopic growth factor, and why this does not seem to depend on O:C ratio here.

The new science findings that were obtained from this study are still not presented very clearly.

A few additional comments to consider:

It appears that no size-resolved AMS measurements were made? This would have provided a lot of valuable insight into how hygroscopicity varies with particle size. If no PToF measurements were made the authors should state this and explain why. [Based on Sect. 4.3 it appears that PToF measurements were made, so these should be discussed.]

In a reply to one of the referee’s questions the authors refer to measurements of black carbon size distributions in Beijing from summer 2012. The measurements presented here are from 2014. I do not see how measurements from a completely different time period can be reliably used to indicate what the size distribution of black carbon was during these measurements. The authors must be much more careful in the assumptions they apply to their analysis and interpretation.

To estimate particle density the authors compared the AMS’s mass concentration measurement with the SMPS volume concentration measurement. It is possible to estimate density this way, see for example:
Kostenidou, E.; Pathak, R. K.; Pandis, S. N. An Algorithm for the Calculation of Secondary Organic Aerosol Density Combining AMS and SMPS Data. Aerosol Sci. Technol. 2007, 41, 1002–1010, doi:10.1080/02786820701666270.
But it must be done carefully. First, the collection efficiency of the AMS during this study has to be determined or otherwise estimated. Only a fraction of particles sampled by the AMS are vaporized in the instrument, and this collection efficiency is known to change with changes in particle size and composition. What was done regarding the AMS’s collection efficiency here? Also, the particle size transmission efficiency of the AMS must be considered, as the len inlet’s efficiency decreases quickly above ~500 nm, and there can be significant variations between aerodynamic lenses on different instruments. The method used here of just dividing the AMS mass concentration by the SMPS volume concentration is too simplistic.

The Referee’s question does not appear to have been answered here:

“Page 11505, Line 27: Were there other changes in aerosol characteristics (i.e. chemical composition, size distribution or κ) during high PM events?

Response:
The chemical composition and size distribution of particles should vary with PM concentration. In this study, the particle hygroscopicity is concerned. Other parameters will not be discussed in detail.”


How was the PAX instrument calibrated, using what black carbon standard?

Please give the flow rates used for the two SMPS instruments. Measuring up to 800 nm with an SMPS requires a very low sheath flow rate. This significantly broadens the width of the mobility selected aerosol.


The grammar and language use is still not correct in many places. Careful proofreading and editing by a person fluent in English is necessary. A few examples: “degree of scatter point of view”, “relatively dominated”.

The authors state that dust and sea salt are only present in supermicron sizes but this is a gross oversimplification. Mineral dust and sea spray aerosol can and often do extend into the submicron range, even below 350 nm. The authors cannot just assume these components were not present, though they may offer some justification for why assuming they made a small contribution is reasonable.