## 6 Conclusions  
The micro-pore structure of coal plays a crucial role in the storage and adsorption of coalbed methane. This study systematically reveals the regulation patterns of volatiles on the nano-pore structure, surface morphology, and fractal characteristics of coal through multi-scale characterization using AFM and SEM and establishes quantitative relationships between key parameters and volatiles. The specific conclusions obtained are as follows:  
(1) Combining AFM surface morphology images with SEM technology, the study observed that volatiles have a decisive effect on the surface morphology and pore structure of coal. There is a significant positive correlation between the surface relief of coal samples and V\({}_{\rm{daf}}\) (volatile matter on an air-dried, ash-free basis). High-volatile coal samples exhibit significant gully undulations on their surfaces, with pores dominated by irregular macropores and a loose bedding structure. SEM images show a loose structure with coarse pores. In contrast, low-volatile coal samples tend to have smooth surfaces, with pores mainly consisting of dense and regular small pores, and a compact bedding structure. SEM images show a dense structure with numerous regular small pores on the surface. This indicates that an increase in volatiles leads to a loosening of the coal structure and coarsening of pores, while a decrease in volatiles promotes structural densification and pore refinement. (2) The surface roughness parameters R\({}_{\rm{a}}\) and R\({}_{\rm{q}}\) range from 1.54-4.55 nm to 1.98-5.83 nm, respectively. As the volatile content of coal samples increases, R\({}_{\rm{a}}\) and R\({}_{\rm{q}}\) exhibit the same linear upward trend. Additionally, the number of pores, area porosity, and shape factor of coal are negatively correlated with volatiles, attributed to gas impact during volatile exhalation damaging pore walls, leading to the merging of small pores and the closure of unstable micropores. In contrast, the average pore diameter shows a positive correlation, mainly due to the merging of small pores to form macropores and the preferential closure of less stable small pores, increasing the relative proportion of macropores. The differences in coal volatiles result in changes in its pore characteristics, which collectively affect the coal's surface morphology, thereby increasing its roughness. (3) Using the Gwyddion software, the study calculated the fractal dimension of each coal sample through three different fractal dimension calculation methods and found a negative correlation between the fractal dimension of coal samples and their volatiles. High-volatile coals have lower fractal dimensions due to a homogenization of pore sizes, while low-volatile coals exhibit higher structural complexity due to the coexistence of multi-scale pores. (4) As a new indicator of pore evolution, the total number of nanorings shows a negative correlation trend with coal sample volatiles. Meanwhile, low-volatile coals have significantly more nanorings with diameters of 10-200 nm than high-volatile coal samples, suggesting that low-volatile coals are more likely to form regular nanorings due to their compact structure. The nanoring roughness shows a positive correlation trend with increasing volatiles, as gas escape and surface spallation during thermal evolution increase the roughness of high-volatile coals. (5) Using advanced nanoscale characterization techniques and multi-parameter quantitative analysis, this study systematically elucidates the linear regulation mechanism of volatiles, as a core indicator of coal rank, on the nano-pore structure, morphology, and complexity of coal. It reveals changes in the pore structure of coal at different volatile contents, providing critical experimental evidence for refining theoretical models of coal pore evolution. Additionally, the study measures the fractal dimension of coal using multiple methods through Gwyddion software, providing reliable methodological conventions for studying the complexity of coal pore structures.