Round 1

Reviewer 1 Report

General comments

This is a really interesting paper because it compiles many ground based monitoring networks, satellite data sets and modeling studies in one neat package. This will be a useful reference tool for everyone studying ammonia, particularly for new graduate students. 

It would be helpful if a table was created for ground based monitoring networks and satellite data sets.

A table summarizing the different emission inventories would also be useful.

Overall I think this paper is very well written. However, be sure to go through and check for grammatical corrections.

Specific comments

lines 193-197 - what were the results of these validation studies?

Table 2/Table 3 - put these tables at the end of the paragraph, they are really awkward in the middle of the lines 225 and 226.

 

 

 

Author Response

Response to Reviewer 1

We are grateful to Anonymous Reviewer 1 for their constructive comments and suggestions, which have helped us revise and improve the clarity of this manuscript. Please find below our replies (bold) addressing the reviewer’s comments; corresponding revisions (italicized) have been made and highlighted (in red color) in the revised manuscript.

 

General comments

This is a really interesting paper because it compiles many ground-based monitoring networks, satellite data sets and modeling studies in one neat package. This will be a useful reference tool for everyone studying ammonia, particularly for new graduate students. 

We thank Anonymous Reviewer 1 for reviewing this manuscript and affirmation of the value of this review paper. Their following questions and suggestions were helpful in the revision of this manuscript.

 

It would be helpful if a table was created for ground-based monitoring networks and satellite data sets.

Agreed. We have added Table 1 (Page 4) for ground-based monitoring networks and extended Table 2 (Page 6) with the satellite datasets in the revised manuscript.

 

A table summarizing the different emission inventories would also be useful.

Agreed. We have added Table 5 (Page 9) in the revised manuscript and associated Figure 3 (Page 10).

 

Overall, I think this paper is very well written. However, be sure to go through and check for grammatical corrections.

Thank you. We have proofread the document and minor changes have been made and highlighted in the revised manuscript.

 

Specific comments

lines 193-197 - what were the results of these validation studies?

Added the following line (202–204):

These studies show that despite the aforementioned limitations, satellite-based remote-sensing inference of [NH₃] is an invaluable method of quantification, tropospheric vertical profiler, and source apportioner of atmospheric ammonia.

 

Table 2/Table 3 - put these tables at the end of the paragraph, they are really awkward in the middle of the lines 225 and 226.

Fixed.

Author Response File: Author Response.pdf

Reviewer 2 Report

Reviewer General Comments:

This paper made an excellent review of recent understanding of NH3 for measurement, satellite retrieval and numerical modeling perspectives. This review paper includes many of important information for the reader of this journal. Reading of this paper is a fun for many of NH3 related researchers and should be published. One thing I want to mention is that the base of this review is a “character” and almost no graphical figures for comparison of measurement technique, satellite and model results. This paper will become more important by including several figures as pointed out below.

 

Comments:

1. 37-39: ‘Secondly, NH3 can have direct negative impacts on the ecosystem’ Please explain through what mechanisms? How may they directlyaffect vegetation or livestock or humans. And what ‘they’ mean here?
2. 56-57: May be better to mention the typical atmospheric lifetimes of NH3 here.
3. Table 2 & 3: CBM-IV appears in ‘Equilibrium Model’. I think CBM is a series of chemical reaction scheme not an Equilibrium Model.
4. 4. Model–Observation comparisons: The authors only mentioned about US, and may be better to add discussion for other areas.
5. 13: As mentioned by the authors that NH3 deposited 10-100 times faster than NH4+ (L.339). So, why slower rate of conversion from NH3 to NH4+ can reduce the dry deposition velocity? More explanations are necessary.
6. 663 ‘dry deposition velocity is reduced due to reduced acidic precursor gases’ need correction. dry deposition velocity is independent from concentration.
7. Section 3.1: It is better to show the emission map over the strong emission regions (like China, India, Europe and US) to compare the differences from different inventory estimates. (This can be added in Section 4)
8. Section 3.2: It is better to demonstrate by figures how much results become different by different ‘gas-particle partitioning’ scheme.
9. Section 5: it might be better to add comparison or reviewing figures for diurnal and seasonal variation from different studies, regions and different methods.

Author Response

Response to Reviewer 2

We are grateful to Anonymous Reviewer 2 for their constructive comments and suggestions, which have helped us revise and improve the clarity of this manuscript. Please find below our replies (bold) addressing the reviewer’s comments; corresponding revisions (italicized) have been made and highlighted (in red color) in the revised manuscript.

 

Reviewer General Comments:

This paper made an excellent review of recent understanding of NH₃ for measurement, satellite retrieval and numerical modeling perspectives. This review paper includes many of important information for the reader of this journal. Reading of this paper is a fun for many of NH₃ related researchers and should be published. One thing I want to mention is that the base of this review is a “character” and almost no graphical figures for comparison of measurement technique, satellite, and model results. This paper will become more important by including several figures as pointed out below.

We thank Anonymous Reviewer 2 for reviewing this manuscript and affirmation of the value of this review paper. The reviewer makes a great suggestion of illustrating key points, some of which we have included in the revised manuscript, which has also benefitted from addressing their following questions and suggestions.

 

Comments:

1. 37-39: ‘Secondly, NH₃ can have direct negative impacts on the ecosystem’ Please explain through what mechanisms? How may they directly affect vegetation or livestock or humans. And what ‘they’ mean here?

Lines 37–43 are now clarified with description of the mechanisms and impacts.

Secondly, NH₃ can have negative impacts on the ecosystem by direct toxicity, expected above a critical threshold of ∼4 ppbv of [NH₃] [22], which is frequently the case in several countries. These impacts can be on vegetation [12,23] through direct damage of stomata, leaves, and other plant surfaces, and biodiversity changes/loss owing to preferential direct uptake of NH₃ by certain plant species, as well as affecting plant resilience to abiotic and biotic stressors. Direct impacts on livestock [24–26] and humans [27,28] can be through exothermic and alkaline effects on epithelial tissues of the eyes and the respiratory tract.

 

2. 56-57: May be better to mention the typical atmospheric lifetimes of NH₃ here.

Agreed. Lines 60–63 now include this information.

While ammonia itself has a very short atmospheric lifetime of a few hours to a day owing to rapid deposition and particle uptake, its particulate forms too have relatively short atmospheric lifetimes of under a week [45–47]; this further implies that there may be more significant regional climate impacts [48].

 

3. Table 2 & 3: CBM-IV appears in ‘Equilibrium Model’. I think CBM is a series of chemical reaction scheme not an Equilibrium Model.

To clarify this, added the following footnotes to Tables 3&4 (Pages 9&10):

*Modification of the Carbon Bond Mechanism IV reaction mechanism with a scheme for NH₄NO₃ and (NH₄)₂SO₄ by Zlatev (1995).

 

4. Model–Observation comparisons: The authors only mentioned about US, and may be better to add discussion for other areas.

 We have modified the first paragraph (Lines 382–386) of the section to address this:

Review of literature indicates that such model–observation comparisons have been limited in spatiotemporal coverage. While there have been some comparisons of modeled [NH₃] with observations over other regions, such as Hungary [117], Denmark [121,139], and Canada [149], majority of the more comprehensive studies have been made over the United States using the GEOS-Chem model.

 

5. 613: As mentioned by the authors that NH₃ deposited 10-100 times faster than NH₄⁺ (L.339). So, why slower rate of conversion from NH₃ to NH₄⁺ can reduce the dry deposition velocity? More explanations are necessary.
6. 653: ‘dry deposition velocity is reduced due to reduced acidic precursor gases’ need correction. dry deposition velocity is independent from concentration.

 To clarify Reviewer 2’s points 5&6, Lines 638–643 now provide more details:

Sutton et al. [278] posited interactions with SO₂, which has shown decreasing concentration, mask the expected decrease in [NH₃] due to slower rate of conversion from NH₃ to NH₄⁺ leading to longer atmospheric residence time of the gaseous NH₃. Additionally, reduced acidic species, through increase of cuticular resistance for NH₃ uptake (Adema, 1986), can limit the potential for co-deposition (with SO₂) and therefore reduce the dry deposition velocity, which leads to increased [NH₃].

Line 679 (earlier 653) now requires no correction/clarification.

 

7. Section 3.1: It is better to show the emission map over the strong emission regions (like China, India, Europe and US) to compare the differences from different inventory estimates. (This can be added in Section 4)

This is a good suggestion, which we have incorporated as a time-series, rather than emission map as we feel the temporal evolution is more important to illustrate than the spatial patterns, which as discussed will be expectedly a strong reflector of agricultural/human footprint. This has been added as Figure 3 (Page 10) in association with (new) Table 5 (Page 9) listing the emission inventories.

 

8. Section 3.2: It is better to demonstrate by figures how much results become different by different ‘gas-particle partitioning’ scheme.

This is indeed a useful exercise, but we feel it is beyond the scope of this review, as it will require detailed inter-comparison of model runs with varied schemes. In Section 3.2, we have provided a general overview of the differences expected between equilibrium assumptions and explicit mass transfer schemes. Table S2 shows the main (species and temperature-dependent parameters) differences in these schemes with references for further details, and there is the in-text (Lines 333–335) exhortation to the reader interested in more detailed inter-comparisons to the works of Kim et al. (1993), Pilinis (1999), Zhang et al. (2000), and Amundson et al. (2006).

 

9. Section 5: it might be better to add comparison or reviewing figures for diurnal and seasonal variation from different studies, regions and different methods.

We had thought of incorporating such a figure in our initial draft of this review. Considering the complexities of the varied techniques and locations (source type, topography, meteorology, etc.), which would result in a muddled comparison, we refrained from doing so. The supplement provides an extensive list of studies that the reader may peruse for information beyond the overview presented.

Author Response File: Author Response.pdf