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Evaluation of a Multiwavelength Characterization of Brown and Black Carbon from Filter Samples
AuthorJohnson, Megan M.
AdvisorWatson, John G
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An ultraviolet – visible (UV-VIS) spectrometer coupled with an integrating sphere was used to measure diffuse reflectance and transmittance of particulate matter (PM) samples collected on quartz-fiber (QF) and Teflon-membrane (TM) filter media over the wavelength range 250 – 1000 nm at 1 nm resolution. These measurements were used to calculate PM sample attenuation, absorption, and Ångstrom absorption exponents (AAE). Samples included laboratory generated source samples (e.g. biomass burning emissions, diesel engine exhaust, and resuspended dusts) and ambient samples. PM sample attenuation and absorption were compared to other PM light absorption measurement methods including densitometer, dual-wavelength (370 & 880 nm) transmissometer, and 3-λ (405, 532, 781 nm) photoacoustic spectrometer (PAS). Large differences were found between filter-based methods and QF and TM absorption was found to be higher than PAS by average factors of 5.1 and 3.6, respectively. AAE values calculated for all samples compared well with values previously reported in literature. Comparison of the filter media showed that attenuation and absorption values from TM samples are, on average, ~1/2 of the values obtained using QF samples. Filter media comparison also revealed evidence of shadowing effects on TM filter media with high sample loading. Comparison of absorption approximation methods using various AAEs and a power law extrapolation exhibited large differences in radiative forcing estimates, indicating that PM absorption is not always well represented by the power law assumption.