A PM2.5 pollution episode over Hangzhou, China during 8 to 16 December 2011 was simulated using the Models-3 Community Multiscale Air Quality (CMAQ). Relative contributions from local and regional emission sources to the pollution event were also investigated through numerical sensitivity tests. Comparisons between simulations and measurements at six meteorological sites over the Yangtze River Delta Region (YRDR) and four air monitoring stations at Hangzhou were satisfactory. The temporal mean of the PM2.5 mass concentration in Hangzhou was lower than those at most areas of Jiangsu province and Shanghai during the episode. Process analysis of the four air monitoring stations at Hangzhou shows that emissions and aerosol processes contributed to the primary and secondary PM2.5 concentrations, with the mean accumulated rates of 1.2–25.5 µg/m3/hr and 0.5–1.2 µg/m3/hr, respectively. The process of advection also increased the PM2.5 mass concentration (1.2–3.4 µg/m3/hr). Diffusion was the dominant removal process at most air monitoring stations, with the removal rates of 4.1–20.7 µg/m3/hr. The dry (–3.0 to –3.6 µg/m3/hr) and wet deposition and heterogeneous processes (–0.4 to –1.8 µg/m3/hr) contributed to the loss of PM2.5. Process analysis also indicates that the maximum concentrations of PM2.5 that occurred during 13–14 December were mainly due to ineffective removal through diffusion. Results of the sensitivity tests suggest that non-Hangzhou pollutants made significant contributions to the PM2.5 pollution in Hangzhou, reaching up to 70% during the focal episode. Under certain meteorological conditions, pollutants transported from outside of Hangzhou not only increased the PM2.5 concentration, but also extended the pollution episode period in Hangzhou by one day. Nevertheless Hangzhou’s local emissions were not negligible, because they had important impacts on PM2.5 peak values.