Soot evolution in an ethylene lab-scale Rich-Quench-Lean (RQL) combustor is investigated using a sectional model coupled with the Conditional Moment Closure turbulent combustion model with detailed chemistry and Large Eddy Simulation. The focus is on developing capability for capturing particle size distributions (PSD) locally and as a function of the burner operating conditions. Two such conditions are analysed by varying the airflow provided in the burner primary and dilution regions, which has a drastic effect on soot emission as shown by previous experiments in this burner. Results show a good agreement with experiments for the gaseous phase, while trends in soot formation tendency and location are also fairly captured. The analysis shows the sensitivity of soot evolution on mixing and indicates the effect of dilution air in modifying the soot PSD at the burner exit. Notably, the presence of dilution air and the associated changes in the mixing field lead to a broadly sustained uni-modal mean soot PSD shape in the primary region and a decrease in mean particle size compared to the corresponding dilution-free condition, consistent with experimental trends. The results demonstrate a promising improvement in capturing soot PSD in realistic combustion devices.