In summary, the inception mechanism of PAH radicals can be divided into three groups: chemical, "physical+chemical", and physical, as shown in Fig. . When the radical molecular mass is lower than 200 Da, the physical interaction is too weak at flame temperature, and chemical nucleation is the dominant mechanism. With increasing radical mass, the physical interaction becomes stronger, and a stack cluster structure can be formed with the combined effect of physical and chemical nucleation. For large radicals (>400), the physical interaction becomes the dominant nucleation mechanism.