Development of an improved optical transmission technique for black carbon(BC) analysis

A new optical transmission technique far black carbon (BC) analysis was dev eloped to minimize interferences due to scattering effects in filter sample s. A standard thermal analysis method (VDI, 1999) is used to link light att enuation by the filter samples to elemental carbon (EC) concentration. Scat tering effects am minimized by immersion of the filters in oil of a similar refractive index, as is often done for microscopy purposes. Light attenuat ion was measured using both a white light sourer and a red LED of 650 nm. T he usual increase in overestimation of BC concentrations with decreasing BC amount in filter samples was found considerably reduced. Some effects of B C properties (e.g. fractal dimension, microstructure and size distribution) on the specific attenuation coefficient B-ATN however, are still present f or the treated samples. B-ATN was found close to 1 m(2)g(-1) for dry-disper sed industrial BC and 7 m(2)g(-1) for nebulized BC. Good agreement was foun d between the oil immersion, integrating sphere and a polar photometer tech nique and Mie calculations. The average specific attenuation coefficient of ambient samples in oil varied between 7 and 11 m(2)g(-1) fot while light a nd G and 9 m(2)g(-1) for red light (LED). B-ATN was found to have much less site variation for the treated than for the untreated samples. The oil imm ersion technique improved also the correlation with thermally analyzed EC. This new immersion technique therefore presents a considerable improvement over conventional optical transmission techniques and may therefore serve a s a simple, fast and cost-effective alternative to thermal methods. (C) 200 1 Elsevier Science Ltd. All rights reserved.