Status of Traceability in BC Measurements: Need of an Agreed Method

Abstract

The accurate measurement of Black Carbon (BC) holds significant importance for regulatory compliance and for effectively identifying sources to mitigate associated health and climate effects. Presently, BC measurement techniques can be broadly classified into three categories: (i) Optical Absorbance, (ii) Evolved Carbon, and (iii) Laser-Induced Incandescence. Among these, photometers based on optical absorbance principle are widely used for continuous, long-term measurements due to their operational simplicity and ability to provide near real-time data. However, there is a lack of consensus on the traceable calibration procedures for commercially available optical absorbance-based photometers (e.g., aethalometer, COSMOS (continuous soot monitoring system), MAAP (multi-angle absorption photometer), and PSAP (particle soot absorption photometer)). Moreover, there is inconsistency in critical measurement parameters, despite all photometers being working on the same Beer-Lambert principle.   Optical photometers are generally calibrated by comparing them against a reference instrument of higher accuracy. However, there is no agreement upon instrument or method which should serve as a reference standard. Researchers utilize different methods, including laser-induced incandescence (Malik et al., 2022), the evolved carbon (Li et al., 2023), and photoacoustic spectrometry (Malik and Aggarwal, 2021), each grounded in distinct measurement principles, resulting in inconsistent calibrating procedures. Furthermore, there is no consensus on reference BC particles, as various material like Aqua Dag, fullerene soot, glassy carbon spheres, as well as BC core mass from the diesel engines are being used for calibrating these reference instruments (Malik and Aggarwal, 2021). These ambiguities and variability in calibration protocol poses a challenge to traceability of BC measurements by optical photometers.   Apart from traceability issue, there exists an inconsistency in different parameters involved in absorption coefficient (which is ultimately converted into BC maas) measurement by optical photometers. Firstly, there is ambiguity regarding the optimal wavelength for measuring the absorption coefficient to derive BC mass concentrations. For example, the absorption coefficient derived at 880 nm wavelength channel of Aethalometer is used to derive the BC mass concentration, whereas in other photometers the measurements are done at 565 nm (COSMOS, PSAP), and 630 nm (MAAP). Additionally, there is no uniformity in defining the particulate matter (PM) size cutoff for BC measurement, nor in the selection of filter media (quartz, glass etc.) for PM mass collection.   Addressing these ambiguities highlights the need of standardization of measurement method for continuous long-term measurement of BC. Additionally, establishing a traceable calibration method is essential to ensure uniformity in BC measurements.