After a severe accident, a substantial amount of radioactive material will be released into the containment vessel in the form of aerosols. Under the influence of coagulation and deposition effects, the aerosol concentration will decrease over time. In aerosol concentration decay experiments, different measurement equipment can yield results in terms of either the decay rate of number concentration or the decay rate of mass concentration. To investigate the implications of these two types of concentration decay rates, a custom experimental setup was constructed, and extensive aerosol deposition data were collected under pure air conditions. Based on coagulation and deposition models and the experimental data, a comprehensive analysis revealed that when the total number concentration of aerosols is below 0.8 × 104 particles/cm3, deposition effects dominate the concentration decay process, and the reduction in number concentration is primarily attributed to deposition. As the aerosol number concentration increases, the accelerating effect of coagulation on concentration decay becomes more significant during the deposition process. Given that deposition effects have a greater impact on mass concentration, while coagulation effects have a greater impact on number concentration, the decay rate of mass concentration can be used to measure the strength of removal mechanisms, and the decay rate of number concentration can characterize the intensity of particle coagulation.