Abstract:A haze pollution event hit mid eastern China in January 2016. During the haze episode, concentrations of PM2.5 in Wuhan and Nanjing increased by three times in several hours. The meteorological observations and simulated data from an air quality model (CAMx) were used to analyze the transport of PM2.5 in this haze episode. The observations revealed that there were northerly winds in 1000-950 hPa and stable layer in the frontal zone above the two cities during the pollution period. Northerly winds in 1000-950 hPa transported pollutants to the cities, and stable layer in the frontal zone suppressed vertical diffusion of pollutants on the surface. Thus, the transport and the stable layer jointly caused the rapid increase of PM2.5 concentrations in the two cities. The pollution was cleared after the main cold air arrived, especially after the weak wind zone in the frontal zone moved. The simulation data showed that local contribution to the two cities in the severe pollution progress was less than the average value from 15 to 19 January. The transport contribution to PM2.5 in Wuhan and Nanjing was 51% and 58%, respectively. The transport contribution rate was not very certain, because the model underestimated the transport of PM2.5.