Organic Acids in Sequential Volume-Based Rainwater Samples in Shanghai: Seasonal Variations and Origins

Round 1

Reviewer 1 Report

Manuscript Number: Atmosphere-1880454

Full Title: Organic acids in the sequential volume-spaced rainwater samples in Shanghai: seasonal variations and origins

The manuscript reports an environmental study where rainfall events were sequentially sampled in a Chinese site and analyzed for organic acids and water-soluble ions. The study allows to obtain valuable information about the organic acids compositions and the sources in the rain samples. The manuscript provides valuable results about the organic acids and is therefore suggested for publication. However, there are some critical points, especially the terminologies used in the manuscript, to address in order to be suitable for a final publication.

Abstract:

·         Sequential rain samples have been collected mainly as volume-based or time-based. In this work, the authors preferred a volume-based approach. However, instead of using “volume-spaced” terminology please use “volume-based” as in literature. And please replace them all in the whole manuscript.

·         Authors used a homemade sequential rain sampler (Lines 9-10). That’s fine but it may be called; an automatic volume-based sequential rain sampler designed ……

·         Line 11. “the sequential volume-spaced rainwater” please replace with “the rain sequence”

·         Line 15. “ forepart rainwater” please replace with “early sequences of rainfall event”

·         Please use “rainfall event” instead of using “total rainwater”

·         Add an additional keyword like “sequential rain” for example, to emphasize sample type.

Introduction:

·         Generally well documented and enough citations were addressed. Again, however, do not use “total rain” but use “rainfall samples” or just “rainwater”(Line 73).

·         Line 74: In this sentence “cloud water” is not wrong but can be called “in-cloud” or may be referred to as “rainout” as mostly termed in literature.

·         Lines 78-79: What do the authors mean by saying “cloud water and rainwater”? Are they rainout and cumulative concentration in the rainfall event, or “rainout and washout (below cloud scavenging of pollutants or PMs)”? Please rewrite clearly.

Materials and Methods:

·         Line 93: “The sampling equipment used here is an automatically continuously quantitative rainwater collection device” authors may change this part as “A home-made automatic sequential rainfall event sampler”

·         Line 111. Please mention the filter material (cellulose acetate or cellulose nitrate etc.)

·         If possible, please add a table showing rainfall events and their corresponding sequences with mean and ranges of pH values. The authors may have presented in the Supplementary Materials section however I could not reach this section. If the authors presented there, please do not consider this comment.

Backward trajectory analysis:

·         In rain studies, for the trajectories both 500 and 1500 m (AGL or AMSL) trajectories have been calculated in literature. 500 m is generally considered for local contributions and 1500 m for long-range transports, additionally, 1500 m covers most of the boundary layer at the sampling site. Therefore, it is better to use or add 1500 m altitude also in back trajectory calculations that will provide more information on the regional sources.

Results:

·         “3.1. General variation patterns of chemical compositions in the sequential volume-spaced 166 rainwater” Subtopic may be changed as “ 3.1. General variation patterns of chemical compositions in the sequential rainwater samples”

·         Lines 174-175: “in-cloud scavenging and sub-cloud scavenging” may be changed as “in-cloud (rainout) and below-cloud (washout) scavenging processes”

·         Line 177 and in whole manuscript, please change “sub-cloud” by “below-cloud” or “washout”

·         Lines 201-212: Another possibility can be the presence of local emission sources at the sampling site. Therefore, authors should also account for local or regional background pollutant levels. Otherwise, according to the paragraph, it can be understood that there is almost no local contribution to organic acids or inorganic acids but this is impossible since there must be contributions at least from traffic or point emission sources.

·         Line 247: Please delete “kinds of”

·         Line 250: leave a space “to63.4 μeq L–1,”

·         Authors should mention the possible sources of ammonium ion (or ammonia) in the samples. It is actually higher than the well-known abundant ions in the rainwaters (e.g., calcium, sodium, chloride, and sulfate ions).

·         Table 1: Please add headings to the columns (names of organic acids and date).

·         Lines 314-315: The lower concentration of oxalic acid in summer may also be explained by the neutralizations in the rain droplets by the calcium carbonate, the final product is calcium oxalate which is insoluble in water.

Author Response

Please see the attachment

Author Response File: Author Response.docx

Reviewer 2 Report

The manuscript contains new and extremely important data on the content of organic acids in the atmospheric precipitation of a large city of Shanghai. A significant contribution has been made to the study of the sources of organic acids in precipitation, the genesis of which is associated with in-cloud and below-cloud processes. Unfortunately, the file with Supplementary Materials is not attached to the manuscript, so it is difficult to assess the completeness of the materials. The moderate grammar and spell checks are required. There are also a number of issues that need to be resolved before the paper is accepted for publication.

Abstract: Specify the reason for the differences in acid concentrations by season. Specify the reason for the differences in acid concentrations between total rainwater, cloud water, and forepart rainwater. What is the environmental hazard of the studied acids, are their concentrations so high as to create a potential hazard? These questions are not clear from the abstract, briefly clarify them.

Keywords: please do not repeat words from the title.

Introduction.

Please describe very briefly in the manuscript what you mean by "cloud water" in samples that reach the land surface.

Materials and Methods.

Line 84: “no pollution sources near this sampling point” – please indicate how far away the nearest pollution sources are.

A device developed by the authors for sampling rainwater was used. Each successive container only began to fill with water after the previous container was full, so explain how the containers were protected from rain and dust until they started sampling rainwater?

Have there been cases of precipitation when there were not enough flasks (more precipitation fell than the total capacity of all the flasks of the device)?

How was the preservation of samples carried out before the analysis of the content of ammonium cations? How long were samples stored prior to analysis for ammonium cations?

What is the reason for the lack of data on the content of hydrocarbonate or carbonate ions in precipitation that could be present in solution, since the pH of some samples was above 5 (according to Figure 2)?

There are no data on the calculation of the balance of ions, how much is the concentration of anions less/more than the concentrations of cations and what is the reason?

Line 137: What is TFA? What is T[A–]? Please check all equations.

Line 158: What does [F/A] mean at the end of the equation?

Section 3 should be called “Results and Discussion”.

Figure 2: What parameters were used to select the representative rainfall whose properties are shown in the figure? Why was this sample chosen? Why is Figure 2 showing a precipitation intensity scale if there are only two intensities (blue and green circles) in the figure? Specify units for all parameters and add vertical axis labels.

Figure 3: Why are only three of the six studied organic acids shown? For acids not shown, are the identified trends stable?

Table 1: The first line of the table is missing, it is not clear which acid concentrations are given in the table.

Lines 298-304: What is the reason for the differences in acid concentrations in precipitation in Shanghai relative to other areas?

Figure 4: Are the concentrations shown in total rainwater or in cloud water?

Lines 314-315: What is the reason for low traffic emissions in summer compared to other seasons?

Figure 5: Check the caption, the links to the figures are incorrect (should be a-b, c-d, e-f, g-h instead of a, b, c, d). The legend can only be shown once, as it is the same in all four figures on the right. The pictures on the left are of poor quality, increase their resolution.

Figure S6 is recommended to be moved to the main text, as the description of the figure is given.

Figure 6: What is labeled as “TOA” and “TIC”? Decipher in the caption. The data is for total rainwater?

Lines 408-409: Why is the contribution of vehicle exhaust and secondary emission so high in summer when it was previously said that vehicle emissions are low in summer (Lines 314-315)?

Figure 7: The data is for total rainwater? How different are the seasonal contributions of sources in cloud water and forepart water? The legend can only be shown once, as it is the same in all figures. Low quality pictures, increase their resolution.

Figure 8: The legend can only be shown once, as it is the same in all figures. Check the correctness of the captions: figure (a) says that the data is shown for forepart water, and the figure itself says that the data is for cloud water.

Lines 457-461: This can also be associated with the partial neutralization of organic acids by cations, for example, Ca2+, whose concentrations increase during below-cloud washout.

References.

It must be compiled according to the rules of the journal. Be sure to add doi.

Are such a large number of references (110) necessary?

Thus, the manuscript requires major revisions.

Author Response

Please see the attachment

Author Response File: Author Response.docx

Reviewer 3 Report

1. It will be more informative if the authors will give a general idea of all acid components for the acid rain along with organic acids they discussed in the Introduction section.

2. Can you provide a site map with latitude and longitude for a better understanding of the region?

3. why -48 hours back trajectories? why not -72 hours or -120 hours?

4. What method was used for vertical velocity in calculation of back trajectories? what version of HYSPLIT was used? more details would be better.

5.  Did Figure 2 and 3 concludes that after 2.4 mm of rainfall, the acidity reaches to minimum?

6. The present study used data for 2020-2021, where the pollution was supposed to be in its minimum range due to various constraints related to COVID-19 pandemic. I couldn't find the reference to such lockdown resulted reduced pollution levels over Shanghai in the present study. Do authors believe that this acidic analysis would have been totally different in another year say 2022? Kindly discuss it and include in the revision.

7. Legends in Figure 5 are not readable. Increase the axis font size.

8. There are possibilities of Synoptic weather patterns influencing the variability of organic acids in the present study. Lot of work has been published related to synoptic weather patterns over China. I suggest to refer appropriate papers and modify the results part.

9. conclusion needs a revisit. Kindly improve the section.

Author Response

Please see the attachment

Author Response File: Author Response.docx

Round 2

Reviewer 2 Report

The authors fully responded to my comments and made the required corrections to the text. Therefore, the manuscript can be accepted for publication.