Comment on tc-2022-258

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.

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