Modified Thermal-Optical Analysis Using Spectral Absorption Selectivity To Distinguish Black Carbon from Pyrolized Organic Carbon

Abstract

Black carbon (BC), a main component of combustion-generated soot, is a strong absorber of sunlight and contributes to climate change. Measurement methods for BC are uncertain, however. This study presents a method for analyzing the BC mass loading on a quartz fiber filter using a modified thermal-optical analysis method, wherein light transmitted through the sample is measured over a spectral region instead of at a single wavelength as the sample is heated. Evolution of the spectral light transmission signal depends on the relative amounts of light-absorbing BC and char, the latter of which forms when organic carbon in the sample pyrolyzes during heating. Absorption selectivities of BC and char are found to be distinct and are used to apportion the amount of light attenuated by each component in the sample. Light attenuation is converted to mass concentration based on derived mass attenuation efficiencies (MAE) of BC and char. The fraction of attenuation due to each component are scaled by their individual MAE values and added together as the total mass of light absorbing carbon (LAC). An iterative algorithm is used to find the MAE values for both BC and char that provide the best fit to the carbon mass remaining on the filter (derived from direct measurements of thermally evolved CO 2 ) at temperatures higher than 480°C. This method was applied to measure the BC concentration in precipitation samples collected from coastal and mountain sites in Northern California. The uncertainty in measured BC concentration of samples that contained a high concentration of organics susceptible to char ranged from 12 to 100 percent, depending on the mass loading of BC on the filter. The lower detection limit for this method was approximately 0.35 µg BC and uncertainty approached 20 percent for BC mass loading greater than 1.0 µg BC.

Measurement and modeling of the multiwavelength optical properties of uncoated flame-generated soot

“…The AAEs for the coateddenuded particles decrease strongly with d p,VED , with the mean AAE ∼ 1.6 for the smallest particles (x = 0.5) and an AAE mean of ∼ 0.5 for the largest particles (x = 0.9). Values below 1 contrast with the methane data but have been observed in a few previous studies (Clarke et al, 2007;Hadley et al, 2008;Lack et al, 2008). While such a decrease with size is consistent with Mie theory predictions, given the MAC results, it seems more likely that the size dependence of the AAE is related to changes in the soot maturity with particle size.…”

Evaluation of the carbon content of aerosols from the burning of biomass in the Brazilian Amazon using thermal, optical and thermal-optical analysis methods

“…6a shows an unrealistically high value of 0.51. This ratio is much higher than what has been previously reported for biomass burning samples in the same location (Mayol-Bracero et al, 2002a), and suggests that EC a was strongly overestimated due to the presence of a large amount of refractory water-soluble material or OC char (Mayol-Bracero et al, 2002a;Hadley et al, 2008). Our results in Fig.…”

Effects of Black Carbon on Climate: Advances in Measurement and Modeling

“…Three principal methods are currently used for quantifying the mass concentration of BC in liquid water samples. The first method utilizes the thermal optical transmittance (TOT) technique (Ogren et al, 1983;Hadley et al, 2008;Wang et al, 2011). In this method, the liquid water sample is filtered, and the BC particles retained on the filter are thermally converted into CO 2 .…”