Black carbon concentrations and modeled smoke deposition fluxes to the bare-ice dark zone of the Greenland Ice Sheet

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 and ice that were visibly light, medium, and dark colored. These samples were analyzed for their refractory black carbon (rBC) concentrations and size distributions with a singleparticle soot photometer coupled to a characterized nebulizer. We present a size distribution of rBC in fresh snow on the GrIS and from the weathering crust in the bare-ice dark zone of the GrIS. The size distributions from the weathering crust samples appear unimodal and were overall smaller than the fresh snow sample, with a peak around 0.3 mu m. The fresh snow sample contained very large rBC particles that had a pronounced bimodality in the peak size distributions, with peaks around 0.2 and 2 mu m. rBC concentrations ranged from a minimum of 3 mu g-rBC=L-H2O in lightcolored patches at the beginning and end of the melt season to a maximum of 32 mu g-rBC=L-H2O in a dark patch in early August. On average, the rBC concentrations were higher (20 +/- 10 mu g-rBC+L-H2O) in patches that were visibly dark, compared to medium patches (7 +/- 2 mu g-rBC=L-H2O) and light patches (4 +/- 1 mu g-rBC=L-H2O), suggesting that 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. A ccording to the model output, 26 mg m(-3) of biomass-burning-derived smoke was deposited between 1 April and 30 August, of which 85% came from wet deposition, and 67% was deposited during our sample collection time frame. The increase in the rBC concentration and size distributions immediately after the modeled smoke deposition fluxes suggest that biomass burning smoke is a source of BC to the dark zone of the GrIS. Thus, the role of BC in the seasonal evolution of the ice-albedo feedbacks should continue to be investigated in the weathering crust of the bare-ice zone of the GrIS.