Refractory Black Carbon Concentrations and Mass Size Distribution in a West Antarctic Shallow Core

Abstract:

Black carbon (BC) is a carbonaceous particle formed during the incomplete combustion of fossil fuels and biomass, and plays an important role in the climatic system due to its effect in the planetary albedo. Although there are several records of Southern Hemisphere paleo-biomass burning, there are only a few publications on BC variability in ice cores from Antarctica. We present refractory black carbon (rBC) concentrations from a 20 meter long snow and firn core collected from Pine Island Glacier, West Antarctica, in the 2014/2015 austral summer. The rBC record was used to investigate BC emissions in the Southern Hemisphere. The core was analyzed at Central Washington University, WA, USA, using a Single Particle Soot Photometer (SP2, Droplet Measurement Technologies) coupled to a CETAC Marin-5 nebulizer. The core was dated to 47 years (1968 - 2015), using rBC, sodium (Na), sulfur (S), strontium (Sr) and the ratio of non-sea-salt sulfur over sodium (nssS/Na) as dating parameters. We observed very low rBC concentrations in the core, with a well-defined seasonality and peak concentrations in the austral dry season (July to December, geometric mean concentrations = 0.057 µg L^-1) due to increased biomass burning comparing to the austral wet season (January to June, geometric mean concentrations = 0.015 µg L^-1). Annual rBC geometric mean concentration was 0.03 µg L^-1, the lowest rBC annual average observed for Antarctica, and the rBC fluxes are the lowest fluxes measured in West Antarctica (6.25 µg m^-2 a^-1). We did not observe long-term trends in rBC concentrations along the record. HYSPLIT trajectory modelling from 1968 to 2015 suggest Australia and New Zealand as the most probable rBC source areas, followed by South America. Mass size distributions of rBC particles indicate a significant contribution of rare, large particles of mass-equivalent diameter (DBC) > 500 nm to the total rBC mass (36%), while small particles (DBC < 100 nm) are abundant but contribute < 8% to total rBC mass. We observed a primary mass median diameter of 163 ± 68 nm, smaller than reported for snow compared to other regions of the globe, but similar to East Antarctica rBC size distributions. In addition, we observed a secondary mode at 1880 nm, larger than already reported for Antarctic snow.