The Influence of Dust on Quantitative Measurements of Black Carbon in Ice and Snow when Using a Thermal Optical Method

Abstract

Accurate measurements of black carbon concentrations in snow and ice are essential to quantify its impact on glacial melting and sequential climate forcing via snow albedo. However, snow and ice contain dust that may severely bias the precision of the elemental carbon (EC) and organic carbon (OC) measurements of filters with a thermal/optical method. To evaluate the effects of dust on black carbon analysis and to optimize filtration methods, meltwater from ice core and surface snow samples with variable dust content were filtered with different methods, including filtration of the entire material (including settling) and supernatant liquid, mechanical stirring and sonication, as well as utilization of single and double quartz filters. In this research, it is shown that dust can induce an extra decrease in optical reflectance during the 250°C heating stage in the thermal/optical method and an improper OC and EC split. To address this problem, a correction procedure was suggested and used to revise the OC and EC results. The OC, EC, and TC concentration variations from different filtration methods along the ice core depth and along surface snow elevation were illustrated. These results indicate that black carbon and dust generally mix as agglomerates. The agglomerate structure will contribute to the underestimation of EC and OC in the measurement. However, carbonaceous matter can be efficiently detached from dust particles by ultrasonic agitation of the meltwater samples, which significantly improves carbon volatilization during the thermal/optical analysis.