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.
Highlights
Elemental carbon (EC, often referred to as black carbon, BC, in soil and sediment research) is produced from incomplete combustion of biomass or fossil fuel [1,2,3]
Among the four temperature protocols investigated in this study, IMPROVE-675 is most similar to EUSAAR, but yet it is the STN120 produces the closest EC values measured with reflectance correction (ECR) and EC values measured with transmittance correction (ECT) results and smallest bounds for actual elemental carbon (EC) concentration
The acid pretreatment of geological samples removes most of the interfering materials, and this makes the samples suitable for EC analysis with thermal/ optical methods
Summary
Elemental carbon (EC, often referred to as black carbon, BC, in soil and sediment research) is produced from incomplete combustion of biomass or fossil fuel [1,2,3]. Thermal/optical methods were introduced for measuring EC in sediments and soils in conjunction with acid pretreatment to minimize interferences from carbonate, metal oxides, salts, and water soluble organic compounds (WSOC) [18,27] This allows better comparability between EC contents measured in geological material and aerosols [27]. Surface soil samples with relatively low expected EC contents and street dust samples with relatively high expected EC contents (from motor vehicle exhausts and coal combustion) were collected in the central China These samples as well as three standard reference materials (SRMs) were used to compare EC concentrations determined using a pretreatment procedure coupled with IMPROVE, STN, and CTO-375 protocols. These protocols were further modified to evaluate the influence of analytical parameters as well as EC measurement uncertainty
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