the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
07 Apr 2021
07 Apr 2021
Variability of Black Carbon mass concentration in surface snow at Svalbard
- 1Ca’ Foscari University of Venice, Dept. Environmental Sciences, Informatics and Statistics, via Torino, 155 - 30172 Venice-Mestre, Italy
- 2Università degli Studi di Perugia, Dipartimento di Chimica, Biologia e Biotecnologie, Perugia, Italy
- 3CNR-ISP, Institute of Polar Science – National Research Council – via Torino, 155 - 30172 Venice-Mestre, Italy
- 4Norwegian Polar Institute, Tromsø, Norway
- 5University of Warsaw, Institute of Geophysics, Warsaw, Poland
- 6Institute of Oceanology, Polish Academy of Sciences, Sopot, Poland
- 7CNR-ISP, Institute of Polar Science – National Research Council – Via Gobetti 101, Bologna
- 8Univ. Grenoble-Alpes, CNRS, IRD, Grenoble-INP, IGE, 38000 Grenoble, France
- anow at: Laboratory of Atmospheric Chemistry, Paul Scherrer Institute, 5232 Villigen PSI, Switzerland
- 1Ca’ Foscari University of Venice, Dept. Environmental Sciences, Informatics and Statistics, via Torino, 155 - 30172 Venice-Mestre, Italy
- 2Università degli Studi di Perugia, Dipartimento di Chimica, Biologia e Biotecnologie, Perugia, Italy
- 3CNR-ISP, Institute of Polar Science – National Research Council – via Torino, 155 - 30172 Venice-Mestre, Italy
- 4Norwegian Polar Institute, Tromsø, Norway
- 5University of Warsaw, Institute of Geophysics, Warsaw, Poland
- 6Institute of Oceanology, Polish Academy of Sciences, Sopot, Poland
- 7CNR-ISP, Institute of Polar Science – National Research Council – Via Gobetti 101, Bologna
- 8Univ. Grenoble-Alpes, CNRS, IRD, Grenoble-INP, IGE, 38000 Grenoble, France
- anow at: Laboratory of Atmospheric Chemistry, Paul Scherrer Institute, 5232 Villigen PSI, Switzerland
Abstract. Black Carbon (BC) is a significant forcing agent in the Arctic, but substantial uncertainty remains to quantify its climate effects due to the complexity of the different mechanisms involved, in particular related to processes in the snow-pack after deposition. In this study, we provide detailed and unique information on the evolution and variability of BC content in the upper surface snow layer during the spring period in Svalbard (Ny-Ålesund). Two different snow-sampling strategies were adopted during spring 2014 and 2015, providing the refractory BC (rBC) mass concentration variability on a seasonal/daily and daily/hourly time scales. The present work aims to identify which atmospheric variables could interact and modify the mass concentration of BC in the upper snowpack, the snow layer which BC particles affects the snow albedo. Despite the low BC mass concentrations, a relatively high daily variability was observed. Atmospheric, meteorological, and snow-related physico-chemical parameters were considered in a multiple statistical model to separate the factors determining observations. Precipitation events were the main drivers of the BC variability. Snow metamorphism and activation of local sources during the snow melting periods appeared to play a non-negligible role (wind resuspension in specific Arctic areas where coal mines were present). The BC content in the snow resulted in being statistically decoupled from the atmospheric BC load.
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Michele Bertò et al.
Status: final response (author comments only)
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RC1: 'Comment on acp-2021-39', Anonymous Referee #1, 08 Apr 2021
While this paper was submitted as a new paper, it is a revision of a paper previously submitted. I am reviewing it as I would a revised paper since I also reviewed the original version.
The paper is significantly improved from the earlier version. It still suffers from some of the difficulties of the original, but I will recommend publication with revision.
As noted in my review of the original paper, the difficulty is that the sampling and analysis wasn’t really designed to answer the questions being posed by this analysis. (Most specifically the snow sampling was done to a fixed depth in both experiments, rather than being designed to isolate, e.g., the influence of newly fallen snow, melt layers, hoar frost, etc.) As noted by the other reviewer of the original paper, correlative analysis is also not really the right tool to be using here for processes-level insight (e.g. see comment 1 below), and that is still the approach taken. There isn’t anything that can be done at this point about the sampling, and the more focused analysis presented in this revised paper is an improvement. The data collected a useful contribution to the quantification of snow BC concentrations in the Arctic so with correction of the issues below I recommend publication.
1) There is still some difficulty with the explanations for the selected variables. See my comments below about SZA as a driving variable for snow rBC concentrations. The reasoning behind expecting surface snow temperature to correlate with daily variations in surface snow rBC concentrations is unclear. Once melting commences, any time the temperature goes above freezing surface rBC will continue to increase. Based on physical understanding of process driving surface snow concentration changes you don’t expect there to be a correlation between air temperature and surface snow concentration because the process of surface snow BC concentrations increasing with melt is cumulative; when temperatures go back down, there’s no physical reason why rBC would then decrease. Therefore applying correlation at the hourly or daily timescale doesn’t make sense.
2) The one major issue with the paper that still needs to be addressed is that it argues there is a diurnal cycle in rBC observed during the 3-days experiment (lines 420-421 and 465, referencing Figure 3), then hypothesizes that the formation and evaporation of hoar frost is driving this cycle. However:
i.) Most problematic of all is that I don’t see the purported diurnal cycle in rBC that is referenced as existing in Figure 3. As noted in my review of the original paper, the dark blue line in the bottom panel simply looks like smoothed random variations, superimposed on an overall decreasing trend. If the authors are going to argue there was a diurnal cycle and then explain why this diurnal cycle exists, they first need to show that there actually IS a diurnal cycle with some sort of statistical analysis. Simply asserting it’s there is not sufficient.
ii) No observations are presented to support that hoar frost formed as hypothesized. Was hoar frost observed during the 3-days experiment measurements? Were the weather conditions (e.g. clear skies and low winds) conducive to hoar frost formation?
Smaller comments:
3) Line 149: Solar zenith angle is cited here as a likely primary driver of snow BC concentrations. It’s really solar insolation that is relevant, and then really only as it drives temperature and therefore snow metamorphism changes (e.g. melting and hoar frost formation). Indeed, “Solar radiation” (W/m2) is what’s shown in Figures 2 and 3, along with temperature. This text should be edited to reflect this.
4) lines 253-255 then 256-258: I don’t understand this explanation given that RH is an observational variable of interest since “high RH might favour the deposition of BC suspended by the formation of water droplets through the cloud condensation nuclei.” It’s then stated that precipitation amount is monitored, so that would be a more direct measure of wet deposition (if that’s indeed the type of deposition monitoring RH was supposed to reveal).
5) Lines 301-302 vs lines 320-322: The same result is expressed two different ways in two different places. Why? Lines 301-302 express this as “variability”, then notes it declined through the campaign; lines 320-322 simply expresses it as a trend. The partitioning of information into Sections 3.1.1 (atmospheric eBC) and Section 3.1.2 (atmospheric conditions) is a bit odd.
6) “coarse mode particles” (used in text) and “dust particles” (Figures 2 and 3) are used interchangeably. What’s really shown in the figure is coarse mode particles. In much literature “dust” has a fairly specific meaning (mineral dust). Here, the coarse mode particle composition isn’t determined so it would be best to stick with terminology that reflects what is actually measured. In the text you could then note that the coarse mode particles are likely a mix of soil, mineral dust and (as noted) possibly coal dust.
7) Lines 308-311: The number concentration of coarse mode particles in snow, as noted, is lower during the first half of the 85-days campaign, then increases during the second half of the campaign (Figure 2). On line 355 an ‘average concentration’ of 4914+/-4109 per ml is given but it’s not clear whether this is across the full duration of the campaign (in which case it’s not a very meaningful number, given the clear difference between the first and second half of the campaign) or it’s only over the second half of the campaign (in which case the time period used for this statistic should be given).
8) lines 337 and 341: BC should be rBC
9) lines 373-375: “Dry deposition is the main depositional process for the coarse mode particles. Recently it has been suggested to have a significant contribution to the BC surface content (up to 50-60%; Liu et al., 2011; Jacobi et al., 2019).”
This needs to be explained or written differently: Atmospheric BC is not a coarse-mode particle so dry deposition of coarse-mode particles as a significant source of BC to surface snow is a confusing statement.
10) Lines 379-382: “Our data support the hypothesis related to local sources' activation in enhancing the dry deposition impacts in an old mining town as Ny-Alesund. Especially during poor snow cover conditions, as during the snow-melting season, dust particles as residuals of carbon extraction mining activities are available for wind lift\suspension.” It’s argued that this is a significant source of BC to snow. Yet on lines 536-537 it’s stated that: “We believe our results to be representative at least of the Arctic coastal areas, characterized by similar processes and seasonality.” Svalbard is fairly unusual for the Arctic in the degree to which past coal mining is likely influencing the addition of rBC to the snowpack from local surface sources (lines 379-358), so how can you assert that you think the results here are generalizable to the Arctic coastal areas overall (lines 536-537)?
11) lines 393-395: The impact of surface snow melt on surface snow concentrations of particulates is presented too much as a hypothesized process; in fact there is significant support for this in the literatures. In addition to the modeling that simulates this and in addition to Aamaas et al 2011 there are at least three other studies showing this in observational data:
Xu, B., T. Yao, X. Liu, and N. Wang, Elemental and organic carbon measurements with a two-step heatinggas chromatography system in snow samples from the Tibetan Plateau, Ann. Glaciol., 43, 257–262, doi: 10.3189/172756406781812122, 2006.
Doherty, S. J., T. C. Grenfell, S. Forsström, D. L. Hegg, S. G. Warren and R. Brandt, Observed vertical redistribution of black carbon and other light-absorbing particles in melting snow, J. Geophys. Res., 118(11), 5553-5569, doi:10.1002/jgrd.50235, 2013.
Doherty, S. J., D. A. Hegg, P. K. Quinn, J. E. Johnson, J. P. Schwarz, C. Dang and S. G. Warren, Causes of variability in light absorption by particles in snow at sites in Idaho and Utah, J. Geophys. Res. - Atmos., 121, doi:10.1002/2015JD024375, 2016.
12) lines 424-425: It’s noted that the concentration of rBC in snow is 6x higher in the 3-days experiment than in the 85-days experiment. Earlier it’s pointed out that the concentrations during the 85-days experiment were consistent with those found in previous studies. Any thoughts on why the very large increase in snow rBC?
13) line 427: Bond et al. 2013 didn’t give snow rBC sized so this isn’t an appropriate citation. The work of Schwartz et al. could be cited instead.
14) line 435: “All the measured snow impurities time series show two common features…: Which variables are your referring to here? The description that follows matches that for rBC but not for e.g. dust/coarse mode particle concentration. What do you mean by “all the measured snow impurities time series”?
15) line 520: Remaining “unaffected” is different from “not being a primary driver of variations in surface snow rBC over XX timescales”. What you’ve shown supports the latter statement but not the former, and only sort of, since the observational period did not include any highly elevated atmospheric eBC periods.
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RC2: 'Comment on acp-2021-39', Anonymous Referee #2, 06 May 2021
The manuscript ‘Variability of Black Carbon mass concentration in surface snow at Svalbard’ by Michele Bertò et al. reports the cumulative processes (such as atmospheric, snowpack and meteorological conditions) in governing the refractory Black Carbon (rBC) mass concentrations in the upper snow layers at Svalbard. The database (85 days in 2014, 1 Apr-24 Jun and 3-days in 2015, 28 Apr- 1 May) in this study is useful to the characterization of aerosol-cryosphere interaction over the Arctic. However, there are many weak elements and lack of clarity in several aspects. I recommend publication of this manuscript in the Atmospheric Chemistry and Physics after my comments have been addressed.
The major concern is the main outcome of the study. Authors state that ‘precipitation events were the main drivers of the BC variability (line-33)”. However, the snow precipitation amount is negatively associated with the rBC mass concentration during 3-days experiment, as against the positive association during 85-days experiment. How do authors explain these contrasting effects of precipitation on BC concentration in snow? In line 353-367, authors try to connect atmospheric eBC concentration with the wet scavenging processes, which requires better investigation. Do authors want to highlight (quantify?) how effective the wash out processes in compared to various other factors considered in this study?
The interpretation of the standardized estimated coefficients derived from multiple linear regression models amongst rBC (in snow) and water conductivity, coarse mode particles number concentration, equivalent black carbon atmospheric concentration, snow precipitation episodes, solar radiation and the snow temperature during 85- and 3-days experiments lacks coherent interpretation as well as proper explanation of the physical processes involved. The positive and negative associations seen in the case of eBC, fresh snow and SWR during 85 days and 3-days experiments must be clearly described. Information about both dry and wet deposition processes are randomly put in different context, this could be avoided.
The BC in the upper snowpack affects the snow Albedo. There are multi-layer approaches to understand the effect of vertical distribution of BC in the snow pack (e.g., Dang et al., 2017). In this study, what is the criterion of selecting “upper snowpack” as 10 cm in 85-days and 3-cm in 3-days experiments?
If SZA is the primary driver of the diurnal variation of BC in snow, what about the cloud cover? SZA is important while estimating snow albedo change, but does it really influence (under the given circumstances) the BC concentration on a diurnal scale in 3 cm snow? How magnificent is snow metamorphism when the diurnal temperature remains below -6 C?
Line 35-36: “The statistical analysis suggests that the BC content in the snow is decoupled from the atmospheric BC load.” This is not clear.
Line 82-85: The citation about the dry deposition parameters is not suitable in the context of present study
Line 98: seasonal > intra-seasonal variability
Line 121/ 99: Sampling period is contradicting; is it May end or till June 24?
Section 2.3.1: In general, filter loading effect is negligible at Arctic due to very low BC concentrations. Why MAC at 530 nm is used? Do aethalometer derived absorption coefficients agree well with PSAP measurements?
Why MAC = 7.25 m2g-1 is considered for PSAP estimates of eBC? Virkkula (2010) is more appropriate for PSAP data correction.
Section 2.4.2: This section is very important, which explains the estimates of rBC in snow. However, the methodology using SP2 (please expand) for rBC (in snow) estimation is not clear. The following statement needs to be refined:
“The nebulization efficiency was evaluated daily by injecting Aquadag® solutions with different mass concentrations, ranging from 0.1 to 100 ng g-1, obtaining an average value of 61%, that was used to correct all the BC mass concentrations reported in this manuscript.”
Line 345-347: Please include the values of R2?
Line 381: “…. as residuals of carbon extraction mining activities”, add reference to this statement
Line 432: Why average coarse mode number concentrations are significantly higher in 3-days experiment (~ 26642 ± 9261 ml-1) than that in 85 days experiment (~ 4914 ± 4109 ml-1)?
Fig-3: Why rBC concentration is relatively higher in 3 days experiment (> 10 ppb) as compared to the values in the corresponding period of 85 days experiment (< 6 ppb)?
Fig-4: 80 or 85 days? In the regression model, how the values of SWIR are considered? Diurnal average or normal incident condition?
Lines 465-469: “… low mass concentrations when the solar radiation is high and vice versa. The BC particles are known to be non-volatile and not photo chemically active, therefore the decrease in their concentration observed when the solar radiation is higher could not be explained as a re-emission process from the snowpack into the atmosphere as observed for other aerosol species”. The sentence is not clear. Please rewrite.
Michele Bertò et al.
Michele Bertò et al.
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