A systematic review of the variability of freshly-emitted and aged black carbon based on various measurement...

Black carbon (BC) poses acute negative health and environmental impacts. Accurate BC quantification is important for assessing its impacts and developing effective control strategies. However, the unclear optical properties and numerous methods of BC restrict its accurate quantification. Elemental carbon (EC), refractory black carbon (rBC), and equivalent black carbon (eBC) are terminology used for BC quantification by various techniques, with an essential parameter of mass absorption cross-section (MAC) characterizing its optical properties. This review summarizes the measurement principles, along with the advantages, limitations, and uncertainties of mainstream instruments used for BC quantification, based on findings from numerous studies. Then, we categorically compared the MAC of BC for different emission sources and regions, and those are not consistent. The MAC values of freshly-emitted and aged BC are both influenced by the combustion sources, while the transport process present significant impact on aged BC. Average MAC values of BC for urban, rural, and other areas are 10.1 ± 3.7 m²/g, 13.4 ± 5.7 m²/g, and 13.7 ± 8.2 m²/g at 550 nm, respectively. Furthermore, we systematically explored the relationship between eBC, EC, and rBC of freshly-emitted and aged BC to establish unambiguous links between these terms. Averaged mass ratios of eBC/EC, eBC/rBC and EC/rBC are 0.81, 0.94, and 1.32 for freshly-emitted BC, 1.17, 2.14, and 1.60 for aged BC, respectively. Overall, it is urgent to establish standard methods for accurate BC quantification in the future to reduce the uncertainty of quantitative results by different techniques.