|The revised paper has improved, and it offers a useful look at the potentially more important processes in the evolution of the components of aerosol particles in this region. The authors enhanced the discussion of OC and EC, and they now include some comparisons with the results of WRF-Chem simulations. However, I find the comparisons and their discussion to be a little disappointing for the following reasons: |
• The comparisons are not shown in the main text.
• There are no comparisons with OC, EC or NaCl.
• Other than to say that “it might not be well simulated”, there is no attempt to assess the model’s simulation of cloud with cloud observations (e.g. Figure S11), which is fundamental to their argument for cloud processing.
• Overall, the discussion seems hurried. It does not carefully consider all the major issues that may be associated with the comparison to their measurements and the use of the model to develop the conceptual picture they present in Figure 9.
The authors state that model evaluation “is beyond the scope of this study”, yet they use the model results (e.g. Figure 7) to build their concept of the processes affecting haze formation in this region (Figure 9). This is not evaluation for the model. It should be evaluation to support your results, in other words, “does the model support their portrayal of the processes?”. I suggest addressing that broader question by addressing the following more specific questions:
• Is it reasonable to expect that these data represent the grid box(es) that are being compared?
• How do the WRF-Chem simulations of OC, EC and NaCl compare to the observations? This will help to further identify model limitations.
• If WRF-Chem does not simulate sulphate in a reasonable way, then might it be because the temperature structure and winds of the lower levels are incorrect (as suggested by their statement that SO2 was underestimated “possibly due to the insufficient upward transport of SO2”), because SO2 emissions are underestimated or because of a deficiency in the aqueous phase conversion of SO2 to sulphate (e.g. representation of cloud; underestimation of oxidant concentrations; etc.)?
• Based on RH, it would seem that the model gets the clouds about right in the winter. Is that correct? What about autumn?
• If upward transport is the problem, then is it valid to use the profiles in Figure 7 to develop the concept shown in Figure 9?
1) Page 1, line 21 – I find the statement starting with “Great progress has recently been made” needs to be referenced. I don’t understand how great progress can be made at ground level, without making it aloft. Ground level and the air above are connected. If you match things well at the ground, but not above, then it is not progress. I suggest the authors think more about their opening statement.
2) Page 2, line 4 - "droplet mode" is a less commonly used term. It has basis, but you need to discuss it first before using it. Also, it presumes the process that you are trying to establish. Use it in your discussion, but I suggest removing 'droplet' here and on line 7 as in my next comment.
3) Page 2, lines 7-10 – I suggest "Our results suggest that much of the sulfate and nitrate are formed from aqueous-phase reactions, and we attribute coarse mode nitrate formation at the measurement site to the heterogeneous reactions of gaseous nitric acid on existing sea-derived coarse particles in autumn.
4) Page 2, lines 10-11 - This sentence is repetitious. Reduce to "Case studies show that in combination with stagnant weather conditions, sulfate and nitrate from aqueous-phase and heterogeneous reactions contribute to haze formation during autumn and winter in the PRD region." Also, define 'PRD'.
5) Page 3, lines 8-12 – I believe that ‘droplet mode’ resulted from "fog" studies. Often, there are significant differences between fog and cloud, and those differences are reflected in residual size distributions. It seems that you are knowledgeable about this, but you need to make it clear in this paragraph.
6) Page 4, line 15 – Change “the vertical size-resolved chemical composition” to “size-resolved chemical composition in the vertical”
7) Page 4-5, lines 28-2 - “Additionally, more consistent evidences of aerosol formation through heterogeneous reactions are needed from field measurements, laboratory experiments and model simulations.” Is awkward. Perhaps “Additionally, more studies of aerosol formation resulting from heterogeneous reactions are needed from field measurements, laboratory experiments and model simulations.”
8) Page 5, lines 17-20 – There is no mention of modelling here. Evaluation of the model is not one of your objectives, but the use of modelling in evaluating your observations is part of your work, and I think it should be mentioned.
9) Page 8, line 25 – page 9, line 4 – I don't see the need for this classification. It adds unnecessary words, and you never use it beyond here. You only have 3 measurement altitudes, and you do not use “Type…” in any of the figures. Just start with ay something like, “We classify the ... components into those peaking at the ground, those peaking at 118 m and those peaking at 488 m”, and modify the remainder of the paragraph accordingly. Also, I think these statistics should be in the main text, if you are going to discuss them here.
10) Page 9, line 15 – “likely”; line 18 – “cleanER”
11) Page 12, lines 15-18 – The proposition you refer to here was put forward decades ago. Re-write as "The coarse-mode nitrate was likely formed ... coarse particles (e.g. Anlauf ...)."
12) Page 12, line 20 - 6 - This equation isn't necessary. I suggest removing it and adding the references to the list beginning with Anlauf et al.
13) Page 12, lines 21-22 – This sentence repeats what is said four lines above. Remove it, and add the references to the “(Anlauf et al….)” list.
14) Page 12-13, lines 28-1 - At best, "activation" is misleading in this context. Substantial water uptake by coarse NaCl particles can, and more likely occurs without true 'activation'. Replace with something like "Sea salt particles can grow by water uptake in fogs and clouds." I think this entire discussion could be better integrated and shortened, since the preference of HNO3 for NaCl over Calcium compounds that you mention is likely related to water solubility. I would make this point on line 25, and then refer to deliquesced particles, fog droplets and cloud droplets. As it stands now, it is as if the aqueous-phase processes are in addition to whatever drives your finding, stated on lines 22-24, and as further demonstrated by the repetitious statement on line 5 of page 13.
15) Page 14, line 2 – “were slightly higher”
16) Page 14, line 11 – “demonstrating more aged aerosol” - Are you saying this simply because the aerosol was sampled at a higher elevation?
17) Page 14, lines 12-15 – Might some of the winter salt particles be attributed to dry salt lakes in the interior of China?
18) Page 14, line 18 – “smallER”
19) Page 14, lines 21-22 – But the profiles of SO4=and OC differ. Secondary contributions to SO4= appear to be more elevated than OC, at least in autumn.
20) Page 14, line 24 - The ratio is determined by 2 quantities. A change in the ratio doesn't necessarily result from a change in just one of those quantities. Maybe the ratio at the source differs, or maybe precipitation near the source remove some of the particles, including EC, after which the sulphate, nitrate and OC are replenished by secondary reactions. The statement, “indicating the presence of secondary organic aerosols”, needs more justification.
21) Page 15, lines 24-25 – What about radiative cooling?
22) Page 16, line 1 – Should be Fig. 7a?
24) Page 16, lines 14-15 – "Our results suggest that aqueous-phase and heterogeneous reactions contributed significantly to the sulfate and nitrate in the PRD region during this episode.
25) Page 16, line 2 – As in my first review, this is NOT "strong convection". I suggest "Low-level cloud was observed during this period, associated with weak convection simulated by WRF.”
26) Page 17, line 3 – What is meant by “aggravated”?
27) Page 17, discussion at end of Section 3.3 – There is no discussion of precipitation in the paper. Was there precipitation? If so, might it have played a role?
28) Page 17, line 23 – Figures S13 and S14 should be in the main text.
29) Page 18, line 21 – “Vertical characteristics and potential formation processes of…”