Characterization of Atmospheric PM_2.5 Inorganic Aerosols Using the Semi-Continuous PPWD-PILS-IC System and the ISORROPIA-II

Round 1

Reviewer 1 Report

Overall:

 

            The paper is interesting and has scientific importance in terms of knowledge on aerosol chemical constituents and their phases in atmospheric PM2.5.  The present study uses the real-time measurement of these components through PPWD-PILS-IC which is a viable and accurate method in terms of the water-soluble component of atmospheric aerosols and metal denuder sampler for precursor gases. The computational efficiency of ISORROPIA II is also used here for the better comparison of measured aerosol/gas components. The study adds knowledge to the field in terms of aerosol thermodynamic partitioning of nitrate species and the importance of including nitrous acid in the total nitrate component of aerosols.

The manuscript is well written apart from a few minor spell-corrections which is mentioned in the minor comments below.  I recommend the paper to be published after addressing some of the specific comments I have given below.

 

 

 Major Comments:

 

1. In the introduction part, it would be better to include some basic references on focusing the point rather than back referencing.

 

2. Line 149-159: Mention the measurement site (state or country) and maybe add a line or two about the site. As it was a bit difficult to infer the sources of the region while reading the observation part.

 

3. Line 317: Do you mean daytime T high and RH low?  As from the figure I can see the temperature is high during day time and RH low.

 

4. Line 283: The previous study mentioned in the reference accounted for the lower observation value of HNO3 to the reaction of the same with carbonate (Ca) species. How the present study takes this observation here? As from Figure S1, it shows a mass concentration peak around 2-5µm for NO3 and Ca as well, both in the coarse mode.

 

5. In figure 9 both NO and NO3 (observed as well as predicted) show a peak at 6 AM. Is it because of the fumigation effect? Or traffic related?

 

6. Also, does the site observes any breeze conditions (land or sea) as most of the pollutants peaks starts after 6 PM. Is it due to the transport of aerosol to the study location or is it only due to the reactions mentioned?

 

 

 

Minor Comments:

1. Line 39: hygroscopic?
2. Line 63: The word “overestimate” can be used instead of “under measure”. And the consistency can be maintained throughout the manuscript
3. Line 63: acid/basic, Use either acidic/basic or acid/base and maintain consistency throughout.
4. Line 101: daily mean concentration (I guess it’s the averaged-out value but can be mentioned here or somewhere else)
5. Line 290: Would be better if you could add some reference for the measurement of the coarse mode of nitrate and other components (measured over other locations).
6. Figure 3: Correct the spelling porous
7. Figure 8: Correct the y-axis label
8. Line 377: change to were correlated well

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Reviewer 2 Report

Overall this manuscript suggests that heterogeneous reactions of nitrate particles, which are not incorporated into ISOROPPIA-II, are important for accurately predicting HNO3 and NOx gas-particle partitioning. Including more nitrogenous species by increasing the total nitrate parameter improved model-observation agreement, but not appreciably for nitric acid. The authors attribute the overprediction of HNO3 in ISORROPIA-II to the lack of key interfacial reactions in the model.

 

The manuscript is thorough, scientifically sound, and well written. I recommend publication with very minor edits.

The full particle distribution could be discussed in greater detail. As the authors suggest surface reactions are so critical to predicting nitrogen speciation, one would expect a correlation with particle surface area. 

 

Please improve clarity on the source of predicted results, sometimes they are clearly indicated to be from ISORROPIA-II and other times not. In particular the figures clearly indicate ISORROPIA results, while the text may just make mention to "predicted" results.

 

324 and 346: Wording should not be so strong, change “implying that” and “it means that” to “this suggests that” because a causal relationship has not been clearly determined, only a correlation.

 

Author Response

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Author Response File: Author Response.pdf

Reviewer 3 Report

Le et al. developed a porous metal denuder sampler (PDS) to use with a PILS-IC system. The experimental design was well thought of.  The field verification and data analysis are sound. The comparison between field observations and ISORROPIA-II model output further verifies that the performance of the overall PPWD-PILS-IC system is improved. The paper is well written and does not have any obvious flaws in terms of experimental design, execution, and analysis. I recommend acceptance pending minor revisions.

 

Specific comments

1. The SI plots are very interesting, the size distribution seems to suggest ammonium sulfate, sodium nitrate, MgCl2 and CaCl2 in the observed PILS-IC data. The authors should comment on these, because they would enhance the discussions in the main text.
2. In addition, the source of K+ seems to be more complicated. This brings up an interesting point as well. The authors should add some discussions to talk about possible sources of K+ in their observation surface site.
3. What is the efficiency and break-through time of the denuders? Are they reusable? How often should the denuders be changed in field measurements?
4. There should be more details in the denuder preparation and relevant citations of denuder preparation should be added.

Author Response

Please see the attachment.

Author Response File: Author Response.pdf