## 1 Introduction  
Bitumen ageing refers to a series of physical and chemical changes that occur under the natural climatic conditions of light, oxygen, and water during the production, construction, and use [1; 2; 3]. Thermo-oxidative ageing accounts for more than 80% of the total life cycle ageing of bitumen [4; 5; 6]. Rotating thin film oven test (RTFOT) or thin film oven test (TFOT) is usually used to simulate the short-term ageing process of bitumen [7]. The technical indexes of bitumen after ageing have changed to a certain extent. Current studies suggest that the change in bitumen components is the cause of ageing [8; 9; 10; 11]. Some of the light components, such as saturate and aromatic, are volatilized, and some of them are converted into resins, while some of the resins are converted into asphaltenes with largermolecular weight and greater hardness [12; 13; 14]. [PERSON] et al. [15] used the method of prolonging ageing time to obtain the ageing degree and ageing rate of bitumen in different periods. [PERSON] et al. [5] reported that the short-term ageing temperature had a significant effect on long-term chemical and rheological properties. [PERSON] et al. [4] reported that the rheological properties of bitumen depended on its age state as related to thermal history. [PERSON] and [PERSON] [16] considered that absorption spectroscopy was useful to determine the extent of ageing resembling multiple periods of service life for bitumen. With the gradual development of micro-testing technology, the research on bitumen ageing has changed from macro-performance to micro-parameters [17]. In recent years, atomic force microscope (AFM) technology has been widely used in the field of bitumen testing [18; 19]. Compared with scanning electron microscope (SEM), AFM test conditions do not require vacuum, avoiding volatilization, or sputtering of light components in bitumen [20]. More importantly, the AFM images have nanometer-scale high resolution, which can clearly observe the nano-morphological characteristics of bitumen [21; 22]. In the AFM image, the bitumen has a structure similar to that of the bee body, which [PERSON] [23] named the bee-like structure. At present, there are still controversies about the formation mechanism of the bee-like structure [24]. There are two main viewpoints. Viewpoint 1 is that wax is the cause of the formation of bee-like structure, viewpoint 2 is that asphaltene is the root cause of the formation of bee-like structure [25; 26; 27]. Viewpoint 2 has been accepted by more and more scholars. The relationship between the nano-morphology characteristics of bitumen represented by bee-like structure and the macro-technical indexes of bitumen has become a hot topic. [PERSON] et al. [28] observed the changes of bee-like structure in the process of loading by using AFM. [PERSON] et al. [29] found that the number of bee-like structures decreased during ageing. [PERSON] et al. [7] reported that the main reason for the nanomechanical changes during ageing or regeneration was the change of chemical composition of bitumen. [PERSON] et al. [17] considered that the ageing increased the surface stiffness, so the ageing affected the bitumen nano-morphology significantly. In summary, there are still some problems in the study of the relationship between the AFM nano-morphology and ageing of bitumen [28; 30; 31]. (1) Lack of parameters and methods for quantitative analysis of nano-morphological parameters of bitumen;  
(2) The effect of ageing on the nano-morphology of bitumen is not clear enough;  
(3) The quantitative relationship between bitumen nano-morphology and technical indexes after ageing is not clear. The purpose of this paper is to find out the effect of thermal oxygen ageing on the nano-morphology of bitumen. The research contents include the nano-morphological parameters of bitumen for quantitative analysis, the effects of ageing on the nano-morphology parameters of bitumen, and the quantitative relationship between bitumen nano-morphology and technical indices after ageing.