From the journal Environmental Science: Atmospheres Peer review history

11-May-2022

Dear Dr Cowan:

Manuscript ID: EA-CRV-02-2022-000012
TITLE: Review of methods for assessing deposition of reactive nitrogen pollutants across complex terrain with focus on the UK

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Associate Editor, Environmental Science: Atmospheres

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Reviewer 1

This is a nice authoritative review on nitrogen deposition assessments across complex terrain. It is comprehensive, well-cited, and written clearly. I have no doubt this review will be of great interest to the deposition community, and be recognized as an important summary on the topic. Especially later in the manuscript (Sections 3.1.2 to 3.1.4), the authors draw important conclusions, and point to many valuable potential directions for future investigation.

I have some minor comments and suggestions that could be considered, mainly technical in nature and to improve clarity in some areas.

General comment about the abstract:

Is there a strict word count for the abstract? I wondered if some additional "key points" from the text could be highlighted, to help guide a reader to the critical content that has been compiled here. At present, the main message according to the abstract is that there is no coherent effort in constraining Nr-deposition uncertainties over complex terrain, but the text is rich with other important take home points that might deserve attention in the abstract. In my opinion, the brevity of the abstract also results in an overly large emphasis on the speculative calculations that are only introduced for the first time at the very conclusion of the entire manuscript.

Line 53: The expression "semi-natural" is used throughout the introduction, and elsewhere in the text, but is not defined clearly. What do the authors specifically mean by "semi-natural", and is the emphasis on this specific type of land throughout the introduction intentional?

Lines 65-67: Perhaps the authors feel this statement is widely accepted, but I think this could use some citations that generally support the point.

Line 70-71: The wording about some fraction of the deposited Nr being "transformed from a solid into a gaseous chemical species" strikes me as awkward. Is it exclusively solid-phase nitrogen species that are converted during denitrification?

Line 81: Isn't the strict definition of critical load the prolonged exposure <i>below</i> which harmful effects <i>cannot</i> be detected? The distinction seems trivial, but I think serves a technical purpose.

Lines 91-94: this list of several processes at work in complex terrain that can increase deposition (relative to flat surfaces) could use some citations. I also wondered if the "seeder-feeder" effect could use a brief explanation.

Line 101: the "CBED" acronym is not defined until much later in the manuscript (line 548) and should be introduced here

Figure 1: I know this figure is mainly for demonstrative purposes, but I was wondering whether the EMEP4UK and UKIAM model outputs were based on identical emissions inventories (and if so, would further accentuate the point the authors are making)

Figure 2: Why do the dry and wet CBED bars end in 2013? I would gather from Figure 1 that there is additional CBED output available, through at least 2016.

Line 186-187: Is this comment about emissions specific to NH3? This could use some careful clarification. I would also encourage inclusion of a citation mentioning the improved inventories from, for example, Paulot et al. (e.g., doi:10.1002/2013JD021130).

Lines 303-311: It is not immediately apparent that the focus of this paragraph is referring to dry deposition. This could use clarification.

Lines 331-332: This sentence about wet deposition seems out of place here, when this process is treated in a separate section.

Lines 335-338: Given the "large amount of literature" on the topic, the specific (and lone) citation to Chazette et al. (2005) seemed a bit awkward. Is this the only citation specific to the topic in complex topography? If so, the comment about a "large body of literature" could be qualified by the caveat that much less work has been done on the topic at hand.

General comment about Section 2.2:

The authors neglect to mention the use of passive sampling for aerosol/particulate matter, which is becoming more frequently used and can be accompanied by compositional analysis, e.g. https://doi.org/10.1016/j.atmosenv.2019.116905, https://doi.org/10.1080/02786826.2010.509747, https://doi.org/10.1080/02786826.2021.1895429

Lines 444-455: The citations to relevant throughfall analysis work is extremely slight. For a whole paragraph on this approach, only Draajers et al. 1994 is cited. Again, this may be related specifically to the lack of applications in complex terrain? But a few more general references on the approach could be included in this paragraph (e.g. the statement "for many pollutants like Nr this requires a correction..." could use some example references from the literature; I would review some of the other statements for potential citations that could be inserted).

General comment about Section 3.1.1:

While totally inadequate at the present time for applications in complex terrain, I wondered whether the review of Liu et al., 2020 (https://doi.org/10.5194/acp-20-8641-2020, or related work) on satellite-derived indirect estimates of N deposition deserves being mentioned. The present authors could draw attention to the fact that such satellite estimates are particularly impractical in the environments of interest to this manuscript: the scale of the satellite observations are too coarse, and satellite retrievals can be extremely uncertain in complex terrains.

General comments about Section 3.1.2, 3.1.3, and 3.1.4:

I would like to say that these sections are particularly valuable and well-conceived. The authors have done excellent work critically reviewing these applications in complex terrain environments. Here, the authors draw valuable conclusions and point to suggestions for future research directions that should be of great interest to the community.

Reviewer 2

General comments:
This manuscript provides an excellent review on methods, including monitoring and modeling methods, for assessing (wet, dry and fog) deposition of reactive nitrogen (Nr) pollutants across complex terrain with special focus on the UK. The authors summarized more than 300 references together with contribution from a panel of international experts. They highlight no coherent global research effort to constrain the uncertainties in Nr deposition quantification over complex terrain. An important finding is that enhancement of Nr deposition over complex terrain (which was in the range of a factor of 1.4 to 2.5) is identified, using simplifed estimates for Nr deposition. If this is true, it means more ecosystems in the UK would be at a greater direct risk of degradation and eutrophication of water bodies. This review also highlights the importance of future studies on this area. The manuscript fits well the scope of this journal and merits to be accepted for publication after minor revision.
Specific comments:
L170: Use “North America” rather than “N America” to avoid misunderstanding;
L1154-1156: Update the publishing year to be 2010 (rather than 2009) and provide vol. and pages “44, 243-249.”
L1270-1273: Insert “378, 20190314.” between “Phil. Trans. R. Soc. A” and “http://dx.doi.org/10.1098/rsta.2019.0314”.

Dear editor and reviewers,

We thank you for taking the time to review our manuscript and for your comments and insights which will help us improve our submission. Please find below a list of amendments made to our manuscript for re-submission, which we hope is now suitable for publication.

Sincerely,
Dr Nicholas Cowan & authors


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REVIEWER REPORT(S):
Referee: 1

Is there a strict word count for the abstract? I wondered if some additional "key points" from the text could be highlighted, to help guide a reader to the critical content that has been compiled here. At present, the main message according to the abstract is that there is no coherent effort in constraining Nr-deposition uncertainties over complex terrain, but the text is rich with other important take home points that might deserve attention in the abstract. In my opinion, the brevity of the abstract also results in an overly large emphasis on the speculative calculations that are only introduced for the first time at the very conclusion of the entire manuscript.

Our manuscript already exceeds the “guideline” abstract length.

Line 53: The expression "semi-natural" is used throughout the introduction, and elsewhere in the text, but is not defined clearly. What do the authors specifically mean by "semi-natural", and is the emphasis on this specific type of land throughout the introduction intentional?

Description added: (i.e. an ecosystem with most of its processes and biodiversity intact, though altered by human activity)

Lines 65-67: Perhaps the authors feel this statement is widely accepted, but I think this could use some citations that generally support the point.

Reference added:
Aas, W., Mortier, A., Bowersox, V., Cherian, R., Faluvegi, G., Fagerli, H., Hand, J., Klimont, Z., Galy-Lacaux, C., Lehmann, C.M.B., Myhre, C.L., Myhre, G., Olivié, D., Sato, K., Quaas, J., Rao, P.S.P., Schulz, M., Shindell, D., Skeie, R.B., Stein, A., Takemura, T., Tsyro, S., Vet, R., Xu, X., 2019. Global and regional trends of atmospheric sulfur. Sci Rep. https://doi.org/10.1038/s41598-018-37304-0

Line 70-71: The wording about some fraction of the deposited Nr being "transformed from a solid into a gaseous chemical species" strikes me as awkward. Is it exclusively solid-phase nitrogen species that are converted during denitrification?

Text changed to:
A fraction of the deposited Nr is transformed into a gaseous chemical species (NO, N2O or N2) via denitrification (Davidson et al., 2000).

 
Line 81: Isn't the strict definition of critical load the prolonged exposure <i>below</i> which harmful effects <i>cannot</i> be detected? The distinction seems trivial, but I think serves a technical purpose.

Agree: Corrected

Lines 91-94: this list of several processes at work in complex terrain that can increase deposition (relative to flat surfaces) could use some citations. I also wondered if the "seeder-feeder" effect could use a brief explanation.

Description added to text:
“(precipitation from an upper-level precipitating cloud falling through a lower-level orographic stratus cloud that caps elevated terrain)”

Line 101: the "CBED" acronym is not defined until much later in the manuscript (line 548) and should be introduced here

Definition moved

Figure 1: I know this figure is mainly for demonstrative purposes, but I was wondering whether the EMEP4UK and UKIAM model outputs were based on identical emissions inventories (and if so, would further accentuate the point the authors are making)

Both models are based on the national atmospheric emission inventory (NAEI) for the UK and EMEP for wider Europe; but there might be slight differences in implementation (i.e. shipping). Emission inputs are very similar, but not sufficiently controlled to be identical enough to make a statement in the manuscript.


Figure 2: Why do the dry and wet CBED bars end in 2013? I would gather from Figure 1 that there is additional CBED output available, through at least 2016.

The figure has now been updated to a later version that has become available during the review process now covering the timeframe 2002-2019 for both deposition estimates.


Line 186-187: Is this comment about emissions specific to NH3? This could use some careful clarification. I would also encourage inclusion of a citation mentioning the improved inventories from, for example, Paulot et al. (e.g., doi:10.1002/2013JD021130).

Different models superimpose different seasonal and diurnal temporal profiles to emissions for a range of compounds. Whilst there is uncertainty for all of them, the uncertainty is particularly large for agricultural and soil emissions. We have clarified this in the text and added a sentence to highlight the fact that new information on intra-year variability and seasonality in emissions is coming from inversion studies such as the one of Paulot et al. (2014) and Marais et al. (2022) for the UK.

Lines 303-311: It is not immediately apparent that the focus of this paragraph is referring to dry deposition. This could use clarification.

Clarified that it is dry deposition.

Lines 331-332: This sentence about wet deposition seems out of place here, when this process is treated in a separate section.

Text moved to final paragraph where passive samplers are further discussed.


Lines 335-338: Given the "large amount of literature" on the topic, the specific (and lone) citation to Chazette et al. (2005) seemed a bit awkward. Is this the only citation specific to the topic in complex topography? If so, the comment about a "large body of literature" could be qualified by the caveat that much less work has been done on the topic at hand.

Three additional references are now included.

Comerón, A., Muñoz-Porcar, C., Rocadenbosch, F., Rodríguez-Gómez, A., Sicard, M., 2017. Current Research in Lidar Technology Used for the Remote Sensing of Atmospheric Aerosols. Sensors 17(6), p.1450. https://doi.org/10.3390/s17061450
Dubovik, O., Li, Z., Mishchenko, M.I., Tanré, D., Karol, Y., Bojkov, B., Cairns, B., Diner, D.J., Espinosa, W.R., Goloub, P., Gu, X., Hasekamp, O., Hong, J., Hou, W., Knobelspiesse, K.D., Landgraf, J., Li, L., Litvinov, P., Liu, Y., Lopatin, A., Marbach, T., Maring, H., Martins, V., Meijer, Y., Milinevsky, G., Mukai, S., Parol, F., Qiao, Y., Remer, L., Rietjens, J., Sano, I., Stammes, P., Stamnes, S., Sun, X., Tabary, P., Travis, L.D., Waquet, F., Xu, F., Yan, C., Yin, D., 2019. Polarimetric remote sensing of atmospheric aerosols: Instruments, methodologies, results, and perspectives. Journal of Quantitative Spectroscopy and Radiative Transfer 224, pp.474-511. https://doi.org/10.1016/j.jqsrt.2018.11.024
Xiao-Min, T.I.A.N., Dong, L., Ji-Wei, X.U., Zhen-Zhu, W.A.N.G., Bang-Xin, W.A.N.G., De-Cheng, W.U., Zhi-Qing, Z.H.O.N.G., Chen-Bo, X.I.E. and Ying-Jian, W.A.N.G., 2018. Review of lidar technology for atmosphere monitoring. Journal of atmospheric and Environmental Optics, 13(5), p.321.

General comment about Section 2.2:

The authors neglect to mention the use of passive sampling for aerosol/particulate matter, which is becoming more frequently used and can be accompanied by compositional analysis, e.g. https://doi.org/10.1016/j.atmosenv.2019.116905, https://doi.org/10.1080/02786826.2010.509747, https://doi.org/10.1080/02786826.2021.1895429

We have added a paragraph on passive samplers for aerosol for completeness, but do not think that they are of much use to assess variability in complex terrain:

“Passive samplers for aerosol measurements are also becoming more commonplace (e.g. Castillo et al., 2019; Sparks and Wagner, 2021), however, the capture particles via deposition and require an estimate of the deposition velocity to infer air concentrations. The deposition rate to such artificial surfaces is not the same as to real vegetation (cf. Section 2.5 below), but is nevertheless likely to be impacted by the heterogeneous turbulence associated with complex terrain. This introduces additional uncertainty in the relationship between the captured amount and the concentration in air at complex sites.”


Lines 444-455: The citations to relevant throughfall analysis work is extremely slight. For a whole paragraph on this approach, only Draajers et al. 1994 is cited. Again, this may be related specifically to the lack of applications in complex terrain? But a few more general references on the approach could be included in this paragraph (e.g. the statement "for many pollutants like Nr this requires a correction..." could use some example references from the literature; I would review some of the other statements for potential citations that could be inserted).

The reviewer is correct in that small number of references relate to the papers that use throughfall specifically to assess the impact of terrain or heterogeneity. We only found two relevant references, the one of Draajers et al. (1994) and also Kirchner et al (2014) which was discussed in a slightly difference context later in the manuscript, but which we have now reiterated here. In addition, we have added several key references to the statements made about throughfall in general throughout the paragraph.

General comment about Section 3.1.1:

While totally inadequate at the present time for applications in complex terrain, I wondered whether the review of Liu et al., 2020 (https://doi.org/10.5194/acp-20-8641-2020, or related work) on satellite-derived indirect estimates of N deposition deserves being mentioned. The present authors could draw attention to the fact that such satellite estimates are particularly impractical in the environments of interest to this manuscript: the scale of the satellite observations are too coarse, and satellite retrievals can be extremely uncertain in complex terrains.

These approaches really just represent a satellite constraint to the atmospheric chemistry and transport modelling and rely on the latter to simulate the vertical distribution (to infer from the satellite the surface concentration rather than the column loading) and the deposition. In terms of deposition, they are subject to the same limitations as the ACTMs are. In addition, the spatial resolution is too coarse to infer the impact of complex terrain.

Referee: 2

L170: Use “North America” rather than “N America” to avoid misunderstanding;

Corrected


L1154-1156: Update the publishing year to be 2010 (rather than 2009) and provide vol. and pages “44, 243-249.”

Corrected


L1270-1273: Insert “378, 20190314.” between “Phil. Trans. R. Soc. A” and “http://dx.doi.org/10.1098/rsta.2019.0314”.

Corrected

02-Jul-2022

Dear Dr Cowan:

Manuscript ID: EA-CRV-02-2022-000012.R1
TITLE: Review of methods for assessing deposition of reactive nitrogen pollutants across complex terrain with focus on the UK

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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Reviewer 1

The authors have sufficiently addressed all reviewer concerns.

Reviewer 2

After revision, I believe the manuscript can be accepted for publication in current form.