From the journal Environmental Science: Atmospheres Peer review history

14-Feb-2022

Dear Dr Kulmala:

Manuscript ID: EA-ART-11-2021-000096
TITLE: The contribution of new particle formation and subsequent growth to haze formation

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Dr Nønne Prisle
Associate Editor, Environmental Sciences: Atmospheres

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Reviewer 1

This manuscript provides insights into the atmospheric new particle formation and growth rate of particles. This type of research is the need of the hour and the authors have done a great job. I recommend this manuscript for publication in Environmental Science: Atmospheres once authors take into account all the following suggestions:
1. In the abstract, line no. 19 please replace “tight connections” with more scientific word.
2. In section 2.1, particle number size distributions were measured between 2.5 nm and 42nm size particles. Why this range is adopted? Please explain it. I request to the authors to provide more information about the instruments and their operating conditions.
3. In line no. 113-114: “We selected this size as 5 nm in most of our later simulations.” Why the authors have adopted this size? Please explain it more in the revised version.
4. I suggest that the authors can combine figure 6,7,8 into one figure with different parts.
5. Also, the authors should elaborate the discussion of each model simulation so that the readers can easily understand the findings.
6. The grammar mistakes should be taken care of throughout the manuscript.

Reviewer 2

This is an important modeling study elucidating the secondary formation of particles in China, considering experimental data and modeling.
Some questions which I would like to raise.
In the model description two considerations could be more discussed if they are pertinent to Beijing Atmosphere and particles:
First the dependency of the coagulation sink (-1.7) and the particle density (1400kg/m3)

One question is the effect not only of the background concentration of particles but also of the new particles formed during the 48 hours of simulation. How the effect of the increase in the background concentration due to the new particles impact the results (mainly showed at Figure 9)?

Other aspect is how the relative humidity impact the secondary formation. Does this result apply to other particles precursors than HOMs and sulphuric acid?

Dear Editor,

Manuscript ID: EA-ART-11-2021-00009

We thank the two reviewers for their comments. Our responses to each of these comments are written in bold.

REVIEWER REPORT(S):
Referee: 1

Comments to the Author
This manuscript provides insights into the atmospheric new particle formation and growth rate of particles. This type of research is the need of the hour and the authors have done a great job. I recommend this manuscript for publication in Environmental Science: Atmospheres once authors take into account all the following suggestions:

1. In the abstract, line no. 19 please replace “tight connections” with more scientific word.
We reformulated this sentence into the following form:
“Our simulations reveal that the PM2.5 mass concentration originating from NPF, strength of NPF, particle growth rate and pre-existing background particle population are all connected with each other.”

2. In section 2.1, particle number size distributions were measured between 2.5 nm and 42nm size particles. Why this range is adopted? Please explain it. I request to the authors to provide more information about the instruments and their operating conditions.
This size range refers to that measured by the NAIS; however the overall size range considered here is 1.5‒1000 nm as indicated in the text. To avoid possible confusion, we simplified this sentence into the following form:
“Particle number size distributions were measured using a DEG-SMPS combined with conventional SMPS systems to cover the size-range from 1.5‒1000 nm. The DEG-SMPS was equipped with a specially designed DMA for classifying sub 10 nm particles and a core sampling inlet for improving their sampling efficiency22-24. The ion number size distributions of both naturally charged positive and negative ions for the size-range between 0.8-42 nm was measured using a Neutral cluster and Air Ion Spectrometer (NAIS)21… The configuration and calibration of this these instrument was described in detail in our previous studies25-26.”

3. In line no. 113-114: “We selected this size as 5 nm in most of our later simulations.” Why the authors have adopted this size? Please explain it more in the revised version.
The reasoning for this choice is essentially given in the previous two sentences (“This kind of behavior is mainly due to the unrealistically low survival probability of newly formed clusters during their initial growth up to a few nm, a yet unexplained feature identified already by Kulmala et al.19. Therefore, we decided to start our simulations from sizes at which dynamics of the growing particle mode is more reliably constrained from atmospheric observations.”). Note further that the sensitivity of our results to this choice has been investigated later in the paper (Figure 7, or Figure 6b in the revised manuscript).

4. I suggest that the authors can combine figure 6,7,8 into one figure with different parts.
As suggested by the referee, we combined these figures into figures 6a, 6b and 6c, and updated the figure caption accordingly. The updated figure caption now reads:
“Figure 6. Number, surface area and mass concentrations of the growing mode after 48 h as function of a) CSbackground, b) the initial diameter of the growing mode, and c) the growth rate, GR. Most of the particles survive with high GR and are scavenged with low GR. Note: the ending diameter is the same (200 nm) for different GRs, while the ending time varies (48 h when GR = 4 nm/h).The other model inputs are: J5 = 10 cm‒3 s‒1, duration of NPF = 5 h and σgeo = 1.7; GR = 4 nm/h in (a) and (b), initial diameter of the growing mode = 5 nm in (a) and (c), and CSbackground = 0.010 s‒1 in (b) and (c).”

5. Also, the authors should elaborate the discussion of each model simulation so that the readers can easily understand the findings.
All the model simulations were conducted in the same way, as described in section 2.3 (with a few additions in the revised manuscript in response to other reviewer comments). The only difference between the simulations illustrated in Figures 3 to 9 (Figures 3 to 7 in the revised manuscript) are different initial and boundary conditions. For each different set of simulations, the relevant initial and boundary conditions are provided in the corresponding figure captions. In our opinion, no other information is needed to understand the results.
Concerning discussion of the simulation results, we would prefer keeping this part of the text as compact as possible, concentrating on the most essential findings obtained from each set of simulations.

6. The grammar mistakes should be taken care of throughout the manuscript.

We carefully checked out the grammar and corrected it accordingly.


Referee: 2

Comments to the Author
This is an important modeling study elucidating the secondary formation of particles in China, considering experimental data and modeling.
Some questions which I would like to raise.
In the model description two considerations could be more discussed if they are pertinent to Beijing Atmosphere and particles:

First the dependency of the coagulation sink (-1.7) and the particle density (1400kg/m3).
The exact value of the dependency of the coagulation sink depends on the pre-existing particle number size distribution, more specifically the mean diameter of the mode giving the largest contribution to CoagS (Figure 1 in ref 30), and the value chosen in our simulations (-1.7) reflects that obtained long-term observations made at the SMEAR II station in Finland. We found that in the Beijing atmosphere, the value of this parameter is close to that at SMEAR II and it is not sensitive to the degree of pollution. As shown in ref 30 (their Figure 3), particle survival properties (and hence our results) are only weakly dependent on the exact value of this parameter. We modified the text:
“The dependency of CoagS on the new particle size, dlnCoagS/dlndp, is assumed to be ‒1.7 in line with our earlier observations30. The same investigation30 showed that particle scavenging rates are only weakly dependent on the exact value of this parameter”
We do not have data needed to estimate the density of nanoparticles in Beijing but, based on the typical mixture of organic and inorganic compounds in the aerosol phase in Beijing, the value chosen here (1400kg/m3) is not expected to be very far from the real, yet variable, particle density there. For example, Hu et al. reported the density of PM1 in Beijing was 1600 kg/m3. Further, our simulation results are not strongly dependent on this parameter.


One question is the effect not only of the background concentration of particles but also of the new particles formed during the 48 hours of simulation. How the effect of the increase in the background concentration due to the new particles impact the results (mainly showed at Figure 9)?
During their growth to larger sizes, newly formed particles contribute to their coagulation sink. Our simulations include this effect via self-coagulation of the growing mode, as explained in the original manuscript (lines 74-75, line 77 in the revised manuscript). Similar to coagulation sink, newly formed particle will also contribute to condensation sink of low-volatile vapors. This does not affect our simulation results, as we constrained simulated particle growth rates (GR) by observations, not by the availability of condensable vapors.

Other aspect is how the relative humidity impact the secondary formation. Does this result apply to other particles precursors than HOMs and sulphuric acid?
This is a relevant point. Relative humidity is definitely an important factor in enhancing the production of secondary aerosol particulate matter (especially sulfate but also nitrate and secondary organic matter) during hazy conditions. This issue has been investigated in a number of modelling papers as well as using observations. Relative humidity may also influence the growth of newly formed particles by facilitating multi-phase chemistry, as mentioned briefly in our manuscript (lines 96-97, 101 and 105 in the revised manuscript), but this process is currently very poorly known. Our model simulations cannot provide information on the potential role of relative humidity in this respect, since we do not explicitly simulate gas-to-particle transfer but rather constrain the simulated particles growth using observations.

References
1. Hu, M, Peng, J, Sun, K, et al. Estimation of size-resolved ambient particle density based on the measurement of aerosol number, mass, and chemical size distributions in the winter in Beijing. Environmental science & technology, 2012, 46, 9941-9947.

21-Mar-2022

Dear Dr Kulmala:

Manuscript ID: EA-ART-11-2021-000096.R1
TITLE: The contribution of new particle formation and subsequent growth to haze formation

Thank you for submitting your revised manuscript to Environmental Science: Atmospheres. I am pleased to accept your manuscript for publication in its current form. I have copied any final comments from the reviewer(s) below.

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With best wishes,

Dr Nønne Prisle
Associate Editor, Environmental Sciences: Atmospheres

Reviewer 1

The authors have addressed all the queries properly. In fig. 6, labels b and c is missing in the graph.