General Comments (evaluating overall quality of the preprint)

The article is a well written analysis of regional trends in air quality, atmospheric deposition, and critical loads across the Canadian landscape. The authors clearly describe the environmental changes that have occurred as a result of emission reduction programs in Canada as well the United States. There continues to be a need for more measurements, including the number of monitoring sites as well as the pollutants measured. As the authors note, they may be missing up to 32% of the total nitrogen budget by not measuring NH3, HONO, PAN, organic nitrogen, and other unmeasured species. The paper provides an important reference for the continued need for ground-based monitoring to improve and constrain the atmospheric deposition models.

The authors only looked at acidification of aquatic and terrestrial (soil) ecosystems, while reduced nitrogen is becoming increasingly important in both Canada and the US. I would suggest that the authors, at a minimum, include language on the idea that eutrophication may also be contributing to regions experiencing algal blooms, loss of biodiversity, vegetation damage, etc. from increases in nitrogen deposition.

Overall, the paper as written provides a substantial contribution to the scientific literature by providing a complete summary of the latest trends from the CAPMON network. The data collected over the past 15 years have been validated and presented using accepted criteria, the results are clear with complementary figures and tables.

Specific Comments (addressing individual scientific questions/issues)

In the introduction include a short description on why NH₃ is difficult to include in dry deposition estimates but note that it is potentially a significant contributor to the overall nitrogen budget. Ammonia is not as important for looking at acidification, but it is worth mentioning as it’s an area of uncertainty in the estimates of dry deposition.

In the paragraph starting with line 128 it’s unclear how many detectors were used with the continuous analyzers. If multiple detectors were used, how were biases addressed? Some clarification is needed.

In the Results section it is important to be clear which time periods are being discussed. For example, it’s unclear on line 221 if the highest concentrations were measured over the 15-year time period (the whole period) or the beginning/end of the period. Make sure you consistently describe the time period for which results are being shared.

The values in Table 1 don’t match the results described on page 11. Please check the values in both places.

Around line 433, suggest including a statement about the potential increases in NH3 dry deposition also increasing resulting in the weaker dry N deposition trends. Also on line 433, were there shifts in the vehicle miles traveled which would have caused a trend in NOx emissions? What about changes in the energy sector?

Around line 471 – can you say anything about the role that satellites may play in helping to address the gaps in measurements to evaluate spatial and temporal trends in deposition.

In section 3.4, can you speculate why there are spatial differences causing difference sin the response of wet NO3 deposition due to NOx emission reductions?

In section 3.5 is was not clear what the threshold was for a level of protection/level at which a species would not be harmed. (was it set at a deposition level where it was expected that 50% of the lakes were unaffected?)

In the Discussion suggest including plans to assess eutrophication impacts due to nitrogen deposition in each of the regions. This is an area for future work and its not expected that results be presented but it should be mentioned as an important assessment for understanding ecological impacts around the monitoring locations.

Technical Corrections

Line 20: “included due to lack of monitoring data.”

Line 66: don’t include “and/or NH₃”. This should be a separate sentence since NH3 measurements were not included in this paper. The sentence should include why it is more difficult to include NH3 in dry deposition estimates (e.g., few ambient measurements, high deposition velocity, bi-directional flux).

Line 73: “both to agricultural and wildfire emissions of NH_3­. Emissions reductions in NOx (ref) have also been shown to contribute to increases in free NH₃ (Yu et al. 2018 Atmospheres https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2018JD028412).”

Line 83: suggest added Simkin et al 2016 from PNAS

Line 90: “total deposition (dry + wet) deposition as they relate to changes in anthropogenic emissions;”

Line 266: “Ongoing monitoring is required to assess the air quality and ecological impacts of these industrial activities on ambient N and S in the Northern Great Plains region (US) and Prairie (Canada).”

Line 324: “in agricultural and unmanaged areas, and its dry deposition estimates at EGB presented here should be treated as upper-end values due to emission of NH₃ being unaccounted for.”

Line 326: “including NH₃ concentrations and supplemental measurements that can be used to constrain bi-directional flux models (Walker et al. 2020)” https://www.sciencedirect.com/science/article/pii/S0048969719331717?via%3Dihub

Line 731: “the ecosystem critical loads are not exceeded, they do not indicate when ecosystems will fully recover from pre-2012 acidification.”

Line 739: suggesting adding a sentence about impacts from wildfires, extreme precipitation events, other climate related environmental stressors. May see new trends in deposition.

Line 740: include organic N in the list

Re:  Long-term Atmospheric Deposition of Nitrogen and Sulfur and Assessment of Critical Loads Exceedances at Canadian Rural Locations, Atmospheric Chemistry and Physics,

MS No.: acp-2022-400

Dear Editorial Support Team:

Thank you for the opportunity to review the subject manuscript.  In addition to this correspondence, I would like to send my suggested edits and technical comments in two .pdf files.  Please let me know if this would be acceptable as I’m not concerned about my anonymity.

The authors have prepared an important and valuable paper that documents the spatial and temporal variability of nitrogen (N) and sulfur (S) deposition across Canada for the 2000 – 2018 period.  They relate their results to estimated critical loads of acidity for aquatic and terrestrial ecosystems.  The paper is well written and has excellent graphics.  The supplementary materials are also very useful and informative.  In fact, the supplement is actually a true extension of the paper, no doubt due to limitations for publishing the many figures and tables in the article.  Therefore, preparation of the tables and figures deserves as much attention to detail as in the manuscript.

Most of my comments are editorial in nature.  I delve into technical issues only where the authors need to provide a clear or more detailed explanation of their findings.  One general correction that I suggest globally is to provide figure captions and table titles that are more descriptive such that the figures and table could stand on their own if copied and pasted into other media, especially emails.  Each caption and title should state that the data came from CAPMoN sites for the duration 2000 – 2018.

My technical comments are as follows.

Manuscript comments.

1. This is an important paper, and it deserves a catchy title. The paper is actually extremely heavy on spatial and temporal characterization of dry and wet deposition and very light on critical loads, but the title made me think that I was going to read a lot more about critical loads.  A title that better describes the content and makes the reader want to read the paper is needed.  I suggest something like:

"Long-term Declines in Atmospheric Nitrogen and Sulfur Deposition Supress Critical Loads Exceedances Across Canada, 2000 - 2018.”

2. Lines 73: missing a space.
3. Line 75: continued to dominate? Over what period?   How about "...continues to exceed reduced N deposition or exhibit..."
4. Line 76: I did not take the time to read the cited articles for my review. Although these are well-respected studies, if you want to look at long-term trends, are not the more recent articles actually more important?  Do you really need to cite work back to 2005?  Are not the more recent studies sufficient?
5. Figure 1: Missing label for New Brunswick.
6. Line 122: I could not access the data on this site:  https://data-donnees.ec.gc.ca/data/air/monitor/monitoring-of-atmospheric-precipitation-chemistry/?lang=en .
7. Line 130: examination of
8. Line 190: Please specify whether this is base cation atmospheric flux or the surface-water loading into the catchment or both.  If this is done using surface-water concentrations, then I'm confused about where the data came from for these 31 lakes.
9. Line 194: This sounds like you have lake water quality data for these lakes as well as stream discharge. What is the source of those data?  Did I miss it?  The sources should be footnoted for figures and tables containing the data.
10. Line 227: See my comment on Figure S3 in the Supplement.  This ranking doesn't do much for me, but Figure S3 does, and it could convey more information if the circles were color-coded by region as in S4.
11. Line 241: Is ECCC, 2004 cited in your references list?
12. Line 280: Table 1.  I recommend that this information be displayed as a bar chart in the Supplement if you are limited to a certain number of figures.
13. Line 338: Reference to Fig. 2 is incorrect.  I think you mean Fig. S2 because Fig. 2 has no years, just sites.
14. Line 341: Fig. 2 does not show dates, nor does it differentiate between pSO4 and SO2. Is this a correct figure reference?
15. Line 375: For the novice dry deposition scientist, it is not obvious that when you say pSO4 in the text and refer to Fig. 3 where SO4 is shown in the graph that, since dry deposition is being plotted, the graph is actually SO2(g) and SO4(s).  Same goes for the N species where HNO3(g) and NO3(g or s) and NH3(g or s)?  I assume that this is correct since you talk about pNO3 and pNH4 in the text.  By the way, I hope that readers are not confused into thinking that pNO3 = log10[NO3].  Bottom line is that confusion about phases of species should be eliminated.
16. Line 380: Again, I would make captions more descriptive so that figures can stand alone when extracted from context of the article.  Specify that these are CAPMoN site IDs.  Might explain species represented in the legend with respect to phase as well. 
17. Line 401 and Fig. 4, Fig. 5: Those green labels are tiny!  Please make those larger.  Good caption that specifies “CAPMoN sites”!
18. Line 436: How can you "confirm future trends"? I don't think that is what you really mean.  How about “…required to enhance dry N deposition monitoring.”
19. Line 444: Table 4 title should read:  …) at CAPMoN sites, based on....
20. Line 481: Table 5 title should read:  …) at CAPMoN sites across Canada.
21. Line 486: Figure 6 caption:  …N at CAPMoN sites during 2000-2018.
22. Line 513: Figure 7 caption:  …sulfur at CAPMoN sites during 2000 - 2018.
23. Line 517: Awkward start of sentence.  "Mean cold and warm season fluxes of total N were 1.4 to 9.3 ...”
24. Line 526: I suggest revising these sentences:  “Seasonal differences in total S flux were large at only a few sites.  For example, the cold season flux at SAT ...”
25. Line 530: Same sites, right?  Suggest linking these 2 sentences:  “...(Fig. 8), whereas wet ...”
26. Line 534: Figure 8 caption:  “...seasons at CAPMoN sites during 2000-2018.”
27. Line 554: The pNH₄⁺ was not “relatively less important.”  I suggest:  contributed a small portion of total N deposition because pNH₄⁺.. (See line 556 as this is what you really mean).
28. Line 565: I suggest:  “...driven by a decline in oxidized N species.”
29. Line 569: Table 6.  I published Theil-Sen slopes with the same number of significant figures before as well.  However, I wonder if 2 sig. figs might be one too many.  If you round to 1 sig. fig, then you have a lot of these trends turn out to be equal for pre- 2010 and post-2010.  How much of a change in concentration or precipitation depth would constitute a change in trend on the order of 0.05 kg/ha-yr?  Could changes in the pre- and post-2010 slopes be attributed to shifts in chemical analysis methods or biases, age or calibration of precipitation gages, etc.?  I think that we (me included) can make conclusions about trends that might be due to changes in data quality that have nothing to do with the environmental signal.  I'm not saying that you're wrong, but you might want to be careful to note that subtle shifts like this could be due to changing data-collection methods, bias, etc. Perhaps a non-parametric test (Wilcoxon Signed-Ranks?) between pre- and post-2010 concentrations and precipitation depths would help confirm subtle differences in the trend slopes for these two periods.
30. Line 573: “...) at CAPMoN sites based...”
31. Line 611: Figure 9 caption: “...of nitrogen at CAPMoN sites, 2000 - 2018.”
32. Line 616: Figure 10 caption:  “... of sulfur at CAPMoN sites, 2000 - 2018.”
33. Line 636: You are not explaining that ratios >100 indicate removal of transboundary emissions in addition to domestic emissions, whereas ratios <100 indicate relative removal of only domestic emissions.  At least this is what I think you are implying here.  Seems to me like these ratios could also be influenced by the ways in which you delineate your source regions for the back trajectory analyses whereby ratios<100 could indicate that your source region is simply broader, whether the region is transboundary or not.
34. Line 656:  What do you mean by this?  This seems like an odd interpretation.  Do you mean that wet deposition removes pollutants from higher in the troposphere and higher up for some intense thunderstorms, thereby including contaminants from long-range transport, whereas dry deposition is more regionalized (i.e. "local")? This seems like what you're trying to say, which would make sense, but the brevity affects the clarity.
35. Line 659: “...(CL) for oxidant-produced acidity for lakes near the the five stations: ALG, ELA, LED, BAB, and KEJ, ranged ...”  Please be clear that your critical loads are for acidity produced by the oxidized N and S species, and that you are not evaluating critical loads of N, which could affect eutrophication or terrestrial plant communities.  However, addressing N critical loads would be a nice addition to the paper, but obviously more work.
36. Line 660: Specify that these are CLs for acidity.  The units should be eq H⁺ ha^-1 yr^-1, correct?
37. Line 661:  Specify CL for acidity.  I think you mean aquatic H⁺ CL or aquatic acidity CL, but not CL of N deposition to the surface water, correct?
38. Line 663: “...high CLs for these lakes (Table S9).”
39. Line 666: It is interesting that you show this shift in deposition around 2010, and right around this time, the lakes seemed to respond to reductions in acidic input.  However, you don't seem to draw that connection in the paper.  Why not?  Maybe the connection just needs to be more obvious?
40. Line 684: Be consistent and use the CL abbreviation.  “Terrestrial acidity CLs were estimated at 14 stations ...” Also, please see Table S10 comment in the supplement.  I recommend that you explain these “min” and “max” terms a little better and provide a bit of context here where you are using the terms as well.  Finally, “…(208 eq H⁺ ha^-1 yr^-1)”, right?
41. Line 687: I'm confused by this.  Are you evaluating S+N acidity or S for acidity and N for nutrient loading?  Why treat these independently if you are looking at total acidity?
42. Line 719: “Total S depositions for the respective periods were…”
43. Line 731: I’m not sure that this is really a conclusion of this paper.  It seems like there is a lot of discussion of items in this section that really are not conclusions of the study.  Are you able to use the subtitle:  Discussion and Conclusions, or maybe just Discussion?
44. Line 738: Well, Fig. 11 gives the impression that all is well and deposition is < CL for all areas now.  Am I missing something?

Supplementary Information comments

Globally, I suggest that each figure in this Supplement should have a caption that is more descriptive of data sources and locations so that the figures will stand on their own if extracted from the document and displayed somewhere else.  For example, in Figure S4, the graphs could be excerpted from the supplement, and they would not stand on their own because people generally do not know that EGB and LON are CAPMoN sites, nor do they know where they are located.

1. Section S1.2: Citation for Zhang et al., (2008) is not in the references list.
2. Globally include specifications that data were obtained at CAPMoN sites during 2000 – 2018. Also, for Tables S6, S7, and S8 captions should read “...deposition at CAPMoN sites to % change in NOx emissions in back-trajectory determined source regions.”
3. Tables S9 and S10 – What are the data sources for the tables? Please include footnotes to reference data sources.
4. Table S11: “...) near CAPMoN sites after including...”  Also, what are the source(s) of the lake and soil chemistry data?  Please footnote those.
5. Figure S2: One cannot read those tiny green labels.
6. Figure S3: Perhaps include in the captions that the sites are ordered longitudinally from west to east?  Wouldn't this also convey more information if the circles were colored by the same regional colors in S4 along with the same legend as in S4?
7. Figure S4: Caption:  “Annual trends in atmospheric S and N species concentrations (mg/m³) at CAPMoN sites grouped by region.” 
8. Figure S5a: This caption is great!  This is what you need on every figure!
9. Figure S6: Caption:  “…(kg N or S/ha/yr) measured at CAPMoN sites during 2000-2018…”
10. Figure S7: Caption:  “…for the cold (Nov-Apr) and warm (May-Oct) seasons during 2000 - 2018 at CAPMoN monitoring sites denoted by site IDs plotted longitudinally from left (west) to right (east).”
11. Figure S8: Change caption in the same way as described above for S7.
12. Supplementary Information References: Add Zhang, 2008 as mentioned above.

End of Comments

General

This paper is well written and generally clear, with strong support for the key takeaways. It provides a comprehensive overview of almost two decades of changes in observed or observationally derived deposition estimates in Canada, and uses this data to assess the impact on sensitive ecosystems with a critical load comparison. It’s likely that this paper will serve as a key reference for future deposition research, and I believe that there are some places where the manuscript would benefit from the authors’ experienced perspective on their vision for future directions in deposition research. Including this is optional, so that I have marked specific locations for this in the minor technical comments section. Although I list below several suggested revisions, most are optional/very minor and relate to re-formatting the figures or sentence re-wording for clarity. The manuscript is overall in very good shape and in my opinion would be ready for publication following a few regarding the language around the reduced and dry deposition analyses.

Major, conceptual comments

In general, I take issue with the representation of “total N deposition” and in particular “total reduced N deposition,” when the authors estimate that neglecting dry ammonia deposition results in a 32% underestimate in N deposition overall. With respect that this analysis is well-founded on the best available measurements, I find the comparisons of wet and dry (section 3.3.2) and reduced versus oxidized deposition unconvincing because the reduced component only includes a minority contributor, pNH₄⁺. At present the reduced versus oxidized N discussion undermines one of the paper’s key conclusions, that more observations of NH₃ and organic N deposition are needed. While the authors include careful discussion on the limitations of the approach given the available measurements, it would improve the clarity of this manuscript to refer to reduced N deposition more specifically. For example, the comparison of oxidized N with reduced N is really a comparison between the majority component of oxidized N and pNH₄⁺.

Major, technical comments

Line 201: Including the limitations of this approach related to soil N saturation and the role of N deposition in acidity is helpful. However, this signal is offset by an under-estimate in total N deposition used in this calculation, and I would suggest mentioning that.

Table 1: As this table is described as for the period 2000-2018, should the Atlantic sites include only KEJ? I believe that the others could bring down the average significantly because they were only operational in the latter part of the period.

Lines 452 – 453: I’m not convinced that the N fluxes in Canada were lower than in the USA because of the underestimate in total N used in this study, which could bring the Canada-based fluxes to a closer level to those in the USA (9.5 * 130% = 12.4). Indeed, the following paragraph cites Zhang et al. 2009 to express that the site-based fluxes can be as high as 11.6 kg/ha-y when including NH₃, NO₂ and organic N. Consider restricting this clause to only S or clarify the estimate for Canada-based N fluxes when accounting for the species included in the US flux estimates.

Lines 478-479: Is the clause “however, oxidized N deposition continued to be greater than reduced N deposition in the west coast, prairie, and Atlantic regions” correct? It seems to contradict lines 457-458.

Figures 6, 7: Include in the legend or elsewhere that oxidized N includes only HNO₃ and pNO₃^-, while reduced includes only pNH₄⁺.

Line 548-549: Clarify this is specific to pNH₄⁺.

Line 566: Again, please clarify that “reduced N” is specific to pNH₄⁺.

Table 6: I do appreciate the note in the caption of this table, but I think it would be clearer to clarify that “total reduced N” is pNH₄⁺ (and possibly also that oxidized N is HNO₃ and pNO₃^-) in the table heading.

Minor, technical comments

Line 99: Consider rephrasing “meaning that the influence of local pollution sources is minimal” to "local pollution sources do not have an outsized influence"

Line 128: The way that this is written, “total reactive N” NO_y seems to exclude NO_x because NO_x is listed separately, but the below text indicates the NO_y measurement includes NO_x. Reactive N generally also includes reduced forms. Consider re-naming NO_y as “total oxidized N.”

Following paragraph lines 255-267: This is one place where I believe that offering some insight or context into anticipated directions or implications for your research would be helpful. For example, if trends continue (NO_x/SO₂ decrease while NH₃ increases out west), do you think that western ecosystems will be threatened? Would it be worth a more tailored study of NH₃ impacts in this region?

Line 295: It took me a while to understand that this paragraph is comparing the gas versus aerosol phase components of dry deposition, so it may be worth mentioning that specifically in the opening sentence, potentially: “the relative role of gas versus aerosol phase dry deposition of routinely-measured species…”

Lines 299-300: Based on Figure 2, I think that it is probably true that the Atlantic sites had lower fluxes through the whole period, but maybe it would be worth clarifying whether this is still true if you include only KEJ.

Figure 2: Consider clarifying which species are in gas or aerosol phase in the legend to improve agreement with the text.

Figures 4, 5, 9, 10: Consider including only the sites that you discuss in-text and moving the remaining figures to supplement. It is difficult to read the text in these figures in this format.

Line 441: Do you have any thoughts on why the rate of decline in annual dry S deposition would have accelerated at SAT and ELA after 2010?

Table 4: Nice, very clear table.

Figure 6: Is there a reason the sites are sorted in this way? Would it be possible to change to either alphabetical or sorted so that the % shifts in a consistent direction? I am concerned that passive observers may currently view this as a temporal trend.

Lines 530-531: Consider expanding on the impact of the seasonal patterns observed here—for example, does this have implications for terrestrial acidification?

Line 541: Consider noting that BON, MIN and BAB were not operational early in the period of study, which might influence the lack of significance in their annual trends.

Line 557: I’m not sure I understand what “is possible” – that dry deposition increased? Please consider clarifying. An alternative interpretation is that this references the ambient NH₃ concentration increases, which the cited studies demonstrate did increase (in other words, “possible” is not an appropriate descriptor).

Line 599: Is this finding robust when considering the impact of dry NH₃ deposition? Similarly, for lines 607-608, I think that it would be worth mentioning that this is also a conservative estimate of the contribution of dry deposition, in addition to reduced, for the same reason.

Figures 9, 10: Consider adopting the same color scheme, or some variant, as in Figure 2. It’s a little confusing that some of these colors were used to denote the warm season in the more recent figures.

Sentence on lines 623-625: Is there a citation for the back trajectory analyses?

Paragraph lines 641 – 648: Would it be possible to speculate what the cited analyses imply for the reduced N deposition response in Canada? Do you have thoughts on what are the process-based drivers of the distinction in response between the USA and the UK (maybe based on precipitation, or aerosol formation chemistry, given differences in NO_x trends in either region)?

Also: Do your findings on the relatively high efficiency of dry versus wet N or S deposition suggest that an increase in dry N associated with NH₃ would cause a >100% increase in deposition per unit?

Line 702: The discussion on uncertainty in the calculation of CL in this section is very nice. Consider including also a discussion on the uncertainty associated with the atmospheric measurements as well, possibly related to this paper:

Walker, et al. (2019), Aspects of uncertainty in total reactive nitrogen deposition estimates for North American critical load applications. Science of The Total Environment, 690: 1005-1018, ISSN 0048-9697, https://doi.org/10.1016/j.scitotenv.2019.06.337.

Line 728: This doesn’t change the overall point of this sentence, but from my read, Figure 11 seems to suggest that SBR had an exceedance in 2014.

Sentence starting line 738: could you give examples of where those sites should be based on your research?

Sentence starting line 741: consider expanding to include organic N as well as emphasizing the need for dry deposition constraints specifically. Also, please consider including more citations as this has been discussed recently, possibly:

Sutton et al. (2013). Towards a climate-dependent paradigm of ammonia emission and deposition. Phil. Trans. R. Soc. B3682013016620130166. http://doi.org/10.1098/rstb.2013.0166.

Walker et a. (2020). A review of measurements of air-surface exchange of reactive nitrogen in natural ecosystems across North America, Science of The Total Environment, 698(133975), ISSN 0048-9697, https://doi.org/10.1016/j.scitotenv.2019.133975.

Minor, syntactical comments

Sentence on lines 141 – 142: Not sure why this statement about NO_y follows a description of the NH₃ instrument. Consider moving it to start ~ line 134 (following the description of the NO_y instrument).

Line 225: “their pSO₄^2- and pNH₄⁺ concentrations” – clarify whether “their” refers to west coast and prairie or southeastern.

Table 3: Move to precede last paragraph, so that it follows first mention of this table.

Figure 6: Move up to where it is first introduced.

This study presents an analysis of sulfur and nitrogen deposition monitored at 15 rural CAPMoN sites for the 2000-2018 time-period to characterize the spatiotemporal trends in atmospheric deposition across Canada and assess likely changes in ecosystem damage through inferences in changes in critical load exceedance during the period analyzed. The changes in atmospheric S and N deposition and the relative proportions of wet and dry deposition amounts reported by this analysis are along expected lines given the changes in NO_x/SO_x/NH₃ emissions and those reported in several similar analyses (both measurement and model based) for atmospheric deposition across the Continental U.S. Nevertheless, the documentation of these changes across Canada and from the CAPMoN measurements is a useful contribution. Before the manuscript can be considered suitable for publication, I feel there are aspects of the analysis that would benefit from additional elaboration as well as several conclusions that need more substantiation. The following comments are offered, addressing which may help improve the usefulness of this manuscript.

• From a total N deposition perspective, one of the shortcomings of this analysis relates to the limitations in the estimates of dry deposition amounts: (1) ambient measurements of select N species are combined with a deposition model to estimate their dry-deposition fluxes; (2) air sampling at majority of the locations only report HNO3, pNO3, and pNH4. The lack of measurements of NO2 (possibly) and NH3 (likely) represent a potentially large proportion of the dry deposited N and total atmospheric deposition budget. This largely renders the comparisons of wet vs dry N deposition amounts somewhat qualitative and confounds any quantitative interpretation of the relative contribution of oxidized vs reduced N to the total atmospheric N deposition amounts and their influence on subsequent CL exceedance and changes. To the authors credit, they do present the possible impact of these missing pieces through analyses of a more complete measurement set at the EGB site, which suggests non-negligible impacts in terms of overall budget. It is not readily apparent whether the relative contributions of the missing components inferred from the EGB site could be extrapolated to the other locations, but perhaps a more explicit recognition of the missing N-deposition components in the discussion of results at other locations may help caveat the results more appropriately.  As examples:
  1. L286: would dry deposition of N decrease by 50% if the estimates also included changes induced by NH3 emissions?
  2. L303-304: conceivably these contributions of HNO3, pNH4 and pNH3 to dry N deposition would be different if NH3 dry deposition was included. The particulate bound N dry deposition contributions could in fact be much smaller?
  3. L306: It is possible that pNO3 decreased in response to HNO3 decreases because there was sufficient NH3 available. The relative amounts of oxidized N to total N deposition could in fact be different if the atmospheric NHx budget increased and that became a larger fraction of the N dry deposition. Should probably caveat this discussion by this important missing component.
  4. L305: how representative is the fraction of non-routinely monitored N-species to the dry deposition budget (70%) at EGB of other locations?
  5. L476-479: would the oxidized N deposition be greater than that of reduced N if NH3 dry deposition estimates were also available? Perhaps this conclusion should be caveated since a potentially large contributor to the reduced N deposition is not accounted for. It is possible that some of the sites in Table 5 are not in proximity to NH3 sources so that NH3 dry deposition may not influence the conclusion – if that is the case it would be helpful to state that more explicitly.
  6. L551: could the lack of trend in total deposition of reduced N be due to missing dry deposition of NH3?

• L70: the large variations in the contributions of dry deposition to total N-deposition across the U.S. convey significant spatial variability that likely arises from a combination of effects associated with spatial heterogeneity in emission source distribution, land-use and precipitation – it may be useful to state that explicitly.

• L99: Is there a formal way to assess whether a site is “regionally representative” or is it just based on the rural settings of the site?

• Equations 1-5: while a reference to the work of Zhang et al. (2008) is provided for the basis of these equations, for the sake of completeness it may be useful to briefly state (if possible) the assumptions on aerosol mixing state (if any) used in this formulation. It would also be useful to state what size fraction pSO4, pNO3, and pNH4 represent – I assume it is PM10?

• L255: Do the west coast sites also see influence of intercontinental transport of airborne S and N species and their subsequent contributions to deposition? Could such contributions also lead to the noted weaker trends despite reduction in local emissions?

• L255-265: Are trends in seasonal values similar to those shown for the annual estimates?

• L272-274: I was not sure what the physical significance of providing mean values (across all sites) of the deposition fluxes is?

• Figure S3: please clarify in figure caption (here and elsewhere) whether the units are for ug-N/m3 and ug-S/m3.

• L34-342: It would be helpful if the authors could explain why the relative proportion of pSO4 dry deposition increased?

• L415: what is the significance of comparing the annual rate of decrease in S and N dry deposition? Should also probably qualify this statement to indicate that the rate of decrease is expressed as percent.

• L470: I agree that quantifying the contribution of NO2 and NH3 dry deposition is important and that the current suite of measurements do not allow for their quantification. What other methods may be available to fill these critical data gaps? Can modeled deposition estimates of different species be used to fill in the gaps in the interim – are such estimates available from models such as GEM-MACH?

• L474: What does comparing the deposition fluxes of N and S tell us? Is there an ecological exposure significance of the relative mass fluxes of S and N?

• L638-640: Please elaborate why emission reductions were more effective in decreasing dry N deposition than wet N deposition? Some discussion of possible reasons leading to the noted "super-linear" (>100%) response in dry N deposition to emission reductions in the southeastern US should also be provided. Do contributions from transboundary transport play a role here? If so, which species would cause such behavior since HNO3 deposits readily and would not be expected to undergo long-range transport?

• L659-660: It is not clear to me how the CL for the lakes were estimated - brief description or references to other work would be useful. Please also provide some discussion of what the max and min values of CL in Table S10 represent and how to interpret these in context of the discussion in L684-697.

• L731: The sentence “the critical load analysis presented in this paper is a case study” is vague? Should a case study not be representative of conditions? Reasons for continued acidic state of lakes in eastern Canada should be mentioned briefly.