Observations of atmospheric chemical deposition to high Arctic snow

Macdonald, Katrina M.; Sharma, Sangeeta; Toom, Desiree; Chivulescu, Alina; Hanna, Sarah; Bertram, Allan K.; Platt, Andrew; Elsasser, Mike; Huang, Lin; Tarasick, David; Chellman, Nathan; McConnell, Joseph R.; Bozem, Heiko; Kunkel, Daniel; Lei, Ying Duan; Evans, Greg J.; Abbatt, Jonathan P. D.

doi:https://doi.org/10.5194/acp-17-5775-2017

Articles | Volume 17, issue 9

https://doi.org/10.5194/acp-17-5775-2017

© Author(s) 2017. This work is distributed under
the Creative Commons Attribution 3.0 License.

Special issue:

NETCARE (Network on Aerosols and Climate: Addressing Key Uncertainties...

https://doi.org/10.5194/acp-17-5775-2017

© Author(s) 2017. This work is distributed under
the Creative Commons Attribution 3.0 License.

Articles | Volume 17, issue 9

Research article

|

10 May 2017

Research article |

| 10 May 2017

Observations of atmospheric chemical deposition to high Arctic snow

Katrina M. Macdonald, Sangeeta Sharma, Desiree Toom, Alina Chivulescu, Sarah Hanna, Allan K. Bertram, Andrew Platt, Mike Elsasser, Lin Huang, David Tarasick, Nathan Chellman, Joseph R. McConnell, Heiko Bozem, Daniel Kunkel, Ying Duan Lei, Greg J. Evans, and Jonathan P. D. Abbatt

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Final revised paper (published on 10 May 2017)
Supplement to the final revised paper
Preprint (discussion started on 07 Nov 2016)
Supplement to the preprint

Interactive discussion

Status: closed

AC: Author comment | RC: Referee comment | SC: Short comment | EC: Editor comment

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- Supplement

RC1: 'Review for Macdonald et al.', Anonymous Referee #1, 06 Dec 2016
- AC1: 'Anonymous Referee #1 - Response', Katrina Macdonald, 27 Feb 2017
RC2: 'Review of Manuscript', Anonymous Referee #2, 06 Jan 2017
- AC2: 'Anonymous Referee #2 - Response', Katrina Macdonald, 27 Feb 2017

Peer-review completion

AR: Author's response | RR: Referee report | ED: Editor decision

AR by Anna Wenzel on behalf of the Authors (03 Mar 2017) Author's response

ED: Referee Nomination & Report Request started (03 Mar 2017) by Daniel J. Cziczo

RR by Anonymous Referee #1 (14 Mar 2017)

Suggestions for revision or reasons for rejection

Macdonald et al revised

This draft of the manuscript is significantly improved from the original submission. I am still far from convinced that the “effective deposition velocity” is a useful construct, but it is obvious that the authors feel that it is.

I also feel that the further attempt to explain the very high EVD for Ca^2+ is not convincing. It is troubling that the detection limit (reported in Supplemental Table 6) is so much higher (more than a factor of 7) than all other ions, suggesting very large variability in the blanks, or a serious analytical problem. (Note this is not true for the aerosol composition detection limits in Supplemental Table 8). The authors assert that qualitative agreement with ICPMS measurements of Ca gives confidence in both data sets, but only about half of all possible samples were above detection limits on both instruments. I find it hard to believe that so many samples were below ICPMS detection limits, raising concern about that data set as well. Given that the authors can not really explain the high EVD, they might be better served leaving Ca^2+ out of the story due to analytical issues causing large uncertainties.

One thing that the authors may want to expand on a bit more is the suggestion that the study period may have been unusual due to the active Icelandic volcano (mentioned in just one sentence near the end of discussion; pg 17 lines 2-5). Provision of the detailed aerosol data in Supplement Table 7 really makes it clear that the canonical “Arctic Haze” pattern for sulfate (and other pollution and dust derived constituents that usually covary with sulfate) with elevated concentrations in late winter into spring, generally peaking DJF, was not obvious. The really striking observation to me is that sulfate deposition to/with snow is so weak in the winter time , even in a year that may have had the usual haze peak in the atmosphere plus additional enhancement from the volcano. Cliff Davidson suggested very weak scavenging of sulfate by winter snow near Dye 3 way back (several papers before DGASP), based on snowpits that showed winter enhancements that were not proportional to increase in aerosol sulfate at Barrow, Alert, NyAesund during the haze season. DGASP showed that Haze really does not impact sites high on the Greenland ice sheet, so the inferred weak scavenging was not confirmed over Greenland. (Cliff actually claimed marked seasonality of sulfate scavenging, stronger in summer, since he did have aerosol measurements in summer that showed low sulfate compared to what he assumed it was in winter/spring.) Now it may be that you are confirming weak removal of sulfate during winter for Alert, and perhaps other parts of the Arctic basin, but this probably means untangling any impacts on this winter from the volcano.

As noted last time, the authors really need to look at Dibb et al., 2007 (Seasonal variations in the soluble ion content of snow at Summit, Greenland: Constraints from three years of daily surface snow samples, Atmos. Environ., 41, 5007-5019). As suggested by the title, this paper reports on 3 years, through the winters, of daily snow sampling at Summit, including ion concentrations and fluxes, and specifically separates freshly fallen from surface snow aged days to weeks. It probably needs to be considered in relation to the summary of previous work at top of page 2, and contradicts the statement made in lines 4-5 on page 3. Also, a main point of Dibb et al., 2007 was to compare seasonality of ions in snow falling at Summit to the seasonality inferred from snowpit studies, so it would seem an additional valid source for the comparisons made at the end of section 3.1 in this manuscript (page 9, lines 5-15).

Editorial comments/suggestions

P3/L30 is unavoidable and natural within natural snowpack systems. ---→is unavoidable within natural snowpacks and still expected to smaller extent on the snow table.

P3/L30-32 It is actually not necessary to know the depth of a sampled surface stratigraphic layer to calculate fluxes, in fact the depth can, and often does, vary horizontally. All that is needed is the area sampled and the mass of snow in that area (e.g., g snow/cm^2). Given concentrations in mass or mole per g of melted snow, conc x g snow/cm^2 give the amount of chemical in the layer per cm^2. The harder challenge is to only sample from single stratigraphic layer, but that can be done with care.

P4/L3-7 Possible underestimate of snowfall up to a factor of 10 seems too large to just pass over. Here in the text it is stated that large discrepancies between operator reports and a met station separated by 6 km justify ignoring the station date, referring to section 4.2 in the supplement for details. However, there are no details on the discrepancies in the supplement, just a rephrased version of the same statement.

P4/L30 Be more specific what you mean by underestimate of BC mass due to size cut-off. First point is that 0.02-50 fg is not directly a size (need to assume shape factor and density). Also not clear whether you are worried about truncation on the small end or the large end of the distribution (both could be possible). It also seems that the cited mass range is based on the SP2 detection principle, probably also need to think about any issues related to transmission efficiency of the nebulizer.

P7/L3-6 and Table 1 It is a little confusing that the list in the text does not overlap very well with the elements in the table (e.g. Ba, Ti, K, Cd, Ca, Cr, and Na are in text but not table while V, Se, Sb, and Tl are in the table but not listed in the text). Could either make the list in text agree with this table, or expand the list to include all of the elements in supplemental tables S3 and S4 and make it clear you are referring to those tables and not Table 1.

Footnotes to Table 1. MSA, ACE, PRP, and FOR are abbreviations, so OK to define, but C2O4^- and NH4+ are not. If you need to define these ion formulae, you probably need to define all of them. Better choice would seem to stop after the first 4.

Related comment, MSA usually signifies methylsulfonic acid, not the ion methylsulfonate that you correctly indicate was measured by IC. I think that MS^- would be better shorthand to indicate methylsulfonate (here and everywhere else in the MS).

Sections 3.2.2, 3.2.3 and section 4 Need to be consistent and show the valences for all of the ions everytime they appear in the text. Right now only a few are shown. This is especially important since you also discuss results of elemental analyses for Na, K, Mg and Ca. Throughout these 3 sections the references are all to the ions, not elements.

P12/L6 Not so clear that NH4^+ is always dominant compared to NH3. Maybe in the Arctic, and during cold season, but this reads like a global declaration.

P12/L8-9 As noted earlier, Ca^2+ is a striking exception to this statement, for no plausible reason. Do you really think this is real?

P13/L8 seems “in-cloud scavenging” should be “below-cloud”

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RR by Anonymous Referee #2 (22 Mar 2017)

ED: Reconsider after minor revisions (Editor review) (23 Mar 2017) by Daniel J. Cziczo

AR by Katrina Macdonald on behalf of the Authors (03 Apr 2017) Author's response Manuscript

ED: Publish as is (03 Apr 2017) by Daniel J. Cziczo

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Short summary

Rapid climate changes within the Arctic have highlighted existing uncertainties in the transport of contaminants to Arctic snow. Fresh snow samples collected frequently through the winter season were analyzed for major constituents creating a unique record of Arctic snow. Comparison with simultaneous atmospheric measurements provides insight into the driving processes in the transfer of contaminants from air to snow. The relative importance of deposition mechanisms over the season is proposed.