Thermal/Optical Methods for Elemental Carbon Quantification in Soils and Urban Dusts: Equivalence of Different Analysis Protocols

Abstract

Quantifying elemental carbon (EC) content in geological samples is challenging due to interferences of crustal, salt, and organic material. Thermal/optical analysis, combined with acid pretreatment, represents a feasible approach. However, the consistency of various thermal/optical analysis protocols for this type of samples has never been examined. In this study, urban street dust and soil samples from Baoji, China were pretreated with acids and analyzed with four thermal/optical protocols to investigate how analytical conditions and optical correction affect EC measurement. The EC values measured with reflectance correction (ECR) were found always higher and less sensitive to temperature program than the EC values measured with transmittance correction (ECT). A high-temperature method with extended heating times (STN120) showed the highest ECT/ECR ratio (0.86) while a low-temperature protocol (IMPROVE-550), with heating time adjusted for sample loading, showed the lowest (0.53). STN ECT was higher than IMPROVE ECT, in contrast to results from aerosol samples. A higher peak inert-mode temperature and extended heating times can elevate ECT/ECR ratios for pretreated geological samples by promoting pyrolyzed organic carbon (PyOC) removal over EC under trace levels of oxygen. Considering that PyOC within filter increases ECR while decreases ECT from the actual EC levels, simultaneous ECR and ECT measurements would constrain the range of EC loading and provide information on method performance. Further testing with standard reference materials of common environmental matrices supports the findings. Char and soot fractions of EC can be further separated using the IMPROVE protocol. The char/soot ratio was lower in street dusts (2.2 on average) than in soils (5.2 on average), most likely reflecting motor vehicle emissions. The soot concentrations agreed with EC from CTO-375, a pure thermal method.

Multi-wavelength optical measurement to enhance thermal/optical analysis for carbonaceous aerosol

“…By category average, ECR and ECT account for 10-50 and 5-46 % of TC, respectively. In general, ECR > ECT, as reported in previous studies (Khan et al, 2012;Han et al, 2013 Schmid et al, 2001), consistent with organic vapors pyrolyzed within the filter leaving the sample after native EC and POC in the surface deposit have evolved (Chen et al, 2004). POC was least apparent for the diesel exhaust samples where optical adjustments were negligible.…”

Multi-wavelength optical measurement to enhance thermal/optical analysis for carbonaceous aerosol

“…By category average, ECR and ECT account for 10-50 and 5-46 % of TC, respectively. In general, ECR > ECT, as reported in previous studies (Khan et al, 2012;Han et al, 2013; Chow transmittance measurements (LR λ and LT λ , respectively, in millivolts (mV)) from the retrofitted seven-wavelength carbon analyzer at room temperature against absolute filter reflectance and transmittance (FR λ and FT λ , respectively) quantified by the Lambda integrating-sphere spectrometer, using eight Fresno ambient samples (5/6, 6/6, 6/19, 7/3, 9/29, 11/4, 11/13, and 12/28 of 2003) of various loadings as transfer standards. The 633 nm data are from a conventional carbon analyzer.…”

“…Thermal/optical analysis that combines thermal separation and optical monitoring is potentially a powerful tool for analyzing carbonaceous aerosol on filters. Spatiotemporal variations and long-term trends in aerosol loading, chemical composition, sources, and effects have been inferred from OC and EC measurements (e.g., Chen et al, 2012;Hand et al, 2012;Malm et al, 1994;Murphy et al, 2011;Park et al, 2006). As many archived samples may be retrieved for reanalysis and ∼ 40 000 new samples are collected per year in the US long-term networks alone, an enhanced multiwavelength thermal/optical analyzer would benefit the scientific community that uses the data.…”

Black carbon concentrations and sources in the marine boundary layer of the tropical Atlantic Ocean using four methodologies

“…Few BC concentrations have been reported for the Caribbean Sea, leading to poor resolution in model predictions in the marine boundary layer. Mean summertime BC observation and model predictions in this region display concentrations ranging from 0.01-0.1 µg m −3 (Hansen et al, 1990;Koch et al 2007;Wang et al, 2013). All measurements of EC and BC in this study exceed the previous model simulations and field investigations.…”