Optical determination of black carbon mass concentrations in snow samples: A new analytical method.

Black carbon (BC) aerosols in the atmosphere strongly affect direct radiative forcing and climate, not only while suspended in the atmosphere but also after deposition onto high albedo surfaces. Snow surfaces are especially sensitive to BC deposition, because of their high surface albedo and additional positive feedbacks further enhance faster snowpack melting caused by BC deposition, resulting in modifications in water resources and recession of glaciers. For the analysis of BC deposition on snow, a precise quantification of BC mass is needed. Instead, optical methods have the potential of quantifying only BC, based on its characteristic spectral absorption. Commercial optical transmissometers commonly use quartz filters to filter BC and measure its optical attenuation. They are calibrated for the determination of BC mass concentrations in air, but not adapted or calibrated for their determination in water or snowmelt samples. Additionally, they are generally calibrated using BC-simulating materials that are not representative of ambient BC particles. Here, a new analytical method is demonstrated for the quantitative determination of BC mass concentration in snow samples that considers filtering of melted snow with polycarbonate filters in a new device, and optical filter attenuation BC mass concentration measurement (880 nm). The attenuation can be obtained with any optical equipment that can measure the 880-nm attenuation of filters impacted with BC/snow impurities. This method has been calibrated using real diesel vehicle exhaust soot with well-known optical properties as reference material, yielding a multipoint calibration curve for common BC concentration levels in snow. The limits of detection (0.011 mg of BC), quantification (0.036 mg of BC) and reproducibility (96.39%) of this new analytical method have been determined. Real surface snow samples collected at different locations in "Los Andes" mountains of Chile were measured with this method given a BC concentrations ranged from 151 to 5987 μg kg.