Black carbon concentrations and modeled smoke deposition fluxes to the bare ice dark zone of the Greenland Ice Sheet
Abstract. Ice-albedo feedbacks in the ablation region of the Greenland Ice Sheet (GrIS) are difficult to constrain and model due in part to our limited understanding of the seasonal evolution of the bare-ice region. To help fill observational gaps, 13 surface samples were collected on the GrIS across the 2014 summer melt season from patches of snow that were visibly light, medium, and dark colored. These samples were analyzed for their refractory black carbon (rBC) concentrations and size distributions with a Single Particle Soot Photometer coupled to a characterized nebulizer. We present a size distribution of rBC in fresh snow on the GrIS, as well as from surface hoar in the bare ice dark zone of the GrIS. The size distributions from the surface hoar samples appear unimodal, and were overall smaller than the fresh snow sample, with a peak around 0.3 µm. The fresh snow sample contained very large rBC particles that had a pronounced bimodality in peak size distributions, with peaks around 0.2 µm and 2 µm. rBC concentrations ranged from a minimum of 3 µg-rBC/L-H2O in light-colored patches at the beginning and end of the melt season, to a maximum of 32 µg-rBC/L-H2O in a dark patch in early August. On average, rBC concentrations were higher (20 µg-rBC/L-H2O ± 10 µg-rBC/L-H2O) in patches that were visibly dark compared to medium patches (7 µg-rBC/L-H2O ± 2 µg-rBC/L-H2O) and light patches (4 µg-rBC/L-H2O ± 1 µg-rBC/L-H2O), suggesting BC aggregation contributed to snow aging on the GrIS, and vice versa. Additionally, concentrations peaked in light and dark patches in early August, which is likely due to smoke transport from wildfires in Northern Canada and Alaska as supported by the Navy Aerosol Analysis and Prediction System (NAAPS) reanalysis model. According to model output, 26 mg/m3 of biomass burning derived smoke was deposited between April 1st and August 30th, of which 85 % came from wet deposition and 67 % was deposited during our sample collection timeframe. The increase in rBC concentration and size distributions immediately after modelled smoke deposition fluxes suggest biomass burning smoke is a source of BC to the dark zone of the GRIS. Thus, role of BC in the seasonal evolution of the ice-albedo feedback should continue to be investigated in the bare-ice zone of the GrIS.
Alia Lauren Khan et al.
Status: open (until 19 Apr 2023)
RC1: 'Comment on tc-2022-258', Anonymous Referee #1, 10 Mar 2023
AC1: 'Reply on RC1', Alia Khan, 12 Mar 2023
- RC2: 'Reply on AC1', Anonymous Referee #1, 13 Mar 2023 reply
- AC1: 'Reply on RC1', Alia Khan, 12 Mar 2023 reply
- RC3: 'Comment on tc-2022-258', Anonymous Referee #2, 27 Mar 2023 reply
Alia Lauren Khan et al.
Alia Lauren Khan et al.
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Comment on tc-2022-258 - Black carbon concentrations and modeled smoke deposition fluxes to the bare ice dark zone of the Greenland Ice Sheet
This manuscript analyses 13 surface snow samples collected in the Greenland Ice Sheet for refractory black carbon (rBC), measuring their concentrations and size distributions on a Single Particle Soot Photometer (SP2) + a customized but extensively tested nebulizer. Concentrations of rBC in the samples were compared with the Navy Aerosol Analysis and Prediction System (NAAPS) reanalysis model to establish rBC dry and wet deposition fluxes, and were linked to biomass burning smoke from wildfires in Northern Canada and Alaska. The sampling occurred in the 2014 summer melt season, from three different types of snow patches, visually classified as light, medium and dark colored, which presented increasing rBC concentrations, respectively. The aim of this work was to provide observational data to models that analyze ice-albedo feedbacks in the ablation region of the Greenland Ice Sheet. The authors’ comparison of rBC measurements in-situ with NAAPS model showed that global aerosol models may be overestimating BC deposition, and recommend further investigation of this issue.
This manuscript presents relevant research for the scope of this journal, addressing the role of black carbon (here as rBC) in the surface snow of a region of Greenland with restricted data. Considering that BC effects in ice and snow are still highly uncertain, field measurements are important and necessary to provide data for models involving this particle.
The title clearly reflect the contents of the paper, and the abstract provide a concise and complete summary of the research carried out. The methods are well detailed and their usage is backed by previous works cited in the manuscript. The language is adequate and clear, as well as the manuscript structure. The authors do not extrapolate their conclusions more than their results go, and acknowledge their dataset is not big enough to observe patterns in the rBC size distribution on their samples. There are interesting comparisons of the ground observations and the NAAPS model, showing the model to be off for one specific snow sample – and thus justifying the importance of evaluating the model with a larger sample size of rBC ground observations across the Arctic – but also validating the model results with rBC measurements in dark patches of snow, as those show an increase in concentrations just after specific smoke events. This work also presents more evidence on post-deposition aggregation of rBC particles (seen in the increase of the modal peak of rBC particle size in light patches over the duration of the season).
My understanding is that this research has merit: it presents new data, is scientifically sound and, based on results, raises an important question about models overestimating BC deposition in Greenland snow. It is the first work presenting rBC size distributions in snow and surface hoar of the bare ice zone of the Greenland Ice Sheet. Thus, I recommend publication after minor revision.
I have only one specific comment and very few minor corrections. I hope my review was useful to this journal and to the authors, and would be glad to review this manuscript a second time should the editors decide for it. Also hope to see the questions raised in this manuscript to be further investigated in future works.
Ln 108-122: Is it possible to add the volume of samples in this section, somewhere?
Data availability: would the rBC data be made available with the manuscript?
Line 63: “… and was determined…” - shouldn’t it be “were”?
Figure 1: The source of the images is mentioned in duplicity (both in the bottom of the image and in the caption). In my opinion this is unnecessary, and the authors could remove one of them.
Lines 317-318: “… especially during episodically…” - seems odd to me, please check and if wrong, rephrase.