Abstract: Significance　Black carbon (BC) aerosols, as short-lived atmospheric pollutants with strong light absorption, have significant impacts on global climate and have become a research hotspot in the field of atmospheric environment. At present, there are technical problems in the measurement of BC aerosols based on optical methods, such as significant measurement differences between the instruments based on different principles, multiple influencing factors on measurement results, and a lack of metrological standards. Therefore, establishing a unified BC metrological standardization system can help reduce the uncertainty of BC measurement and lower its radiative forcing assessment error. In order to promote the development of BC metrological research in China, this article reviews the reference materials and their generation and aging techniques, as well as the traceable in-situ methods currently used for BC aerosol measurement internationally, and looks forward to the future development directions of BC metrology in China. Progress　Due to the complexity of carbonaceous aerosols in the atmosphere, researchers have established various working groups to explore the applicability of existing reference materials and summarize relevant suggestions, but a unified standard and specification have not yet been formed. At present, the reference materials used for BC aerosol calibration are mainly divided into two categories. One is the simulated particle state standard substances, which are mostly dispersed in water or other solvents to form suspensions. When used for calibration, aerosols are generated by atomizing these suspensions, which is easy to operate and can be used to transfer standards. The other is to generate persistent soot particles based on the generators with different principles. Through comparison, evaluation, and optimization of the stability of these generators, someone is selected as a standard generator for calibration of BC aerosol related instruments.The aging of BC aerosols in the atmosphere can affect their particle size, optical properties, and absorption, thereby affecting the measurement sensitivity of different instruments. Using two types of aerosols with different particle sizes and optical properties to simulate fresh BC and aged BC respectively as reference materials can better calibrate indirect optical instruments and reduce random uncertainty. Common devices for simulating aging process include smog chambers, oxidation flow reactors, and heated reservoirs. However, the limitations for these devices lie in the difficulty of controlling parameters such as reaction light intensity, and ensuring the reproducibility of the generation of aged BC. When simulating the generation of standard aged BC based on instrument calibration, the stability and repeatability of aging devices are the most important. In addition, factors such as reaction time, construction cost, and portability also need to be considered. In existing studies on BC aerosols, direct instruments are commonly used to calibrate indirect instruments. Therefore, determining a unified and traceable calibration method for direct instruments is one of the foundations of BC optical metrology research. In addition, it is also necessary to explore whether some direct instruments can be traced back to the International System of Units (SI). At present, the national metrology institutes in many Eruopean countries have conducted applicability studies on extinction minus scattering (EMS) and photothermal interferometry (PTI) methods to establish benchmark devices for BC absorption calibration and subsequent measurement specifications. Conclusions and Prospects　At present, the measurement of BC aerosols based on optical methods has been widely applied. However, there are significant differences in the measurement results of different instruments, and it is urgent to establish a metrological system of BC light absorption based on a unified standard substance. This article provides a review of the commonly used reference materials and their generation and aging methods, as well as the traceable reference methods for BC calibration. Reference materials with stable and controllable optical properties and microstructures as close as possible to real atmospheric BC particles are the basis for BC instrument calibration. Currently, the reference materials used in research are diverse, making it difficult to ensure the accuracy, repeatability, and comparability of calibration results. Further research is needed on the stability of reference materials and the sensitivity of instrument. The government also needs to establish a unified and applicable standard generation method or reference materials for optical instruments based on different principle, and develop corresponding metrological specifications. In addition, most of the existing reference materials characterize fresh BC, while further consideration is needed on the impact of BC aging process on instrument measurement. A standard aging simulation device based on photochemical smog chamber should be established to improve its repeatability, reduce calibration uncertainty, and construct calibration schemes for indirect instruments under different single scattering albedo (SSA). The stability and uncertainty of commonly used direct measurement methods such as extinction scattering, photoacoustic spectroscopy, and PTI, should be compared and evaluated to establish a metrology system of BC absorption in China.

Key words: black carbon aerosol, optical method, reference material, ageing device, standard method