Regional haze episodes have been frequently reported in east China since 2000.In the present study,two regional haze episodes over east China in the spring of 2011 were examined by observations and simulations conducted by a three-dimensional regional chemical transport model (NAQPMS) with an on-line tracer-tagged module.The model reproduced accurately the observed PM2.5 with correlation coefficient ranging from 0.52 to 0.76 and root mean square error (RMSE) of 20-50μg/m3 in four city clusters (Yangtze River Delta,Shandong Peninsula,Huabei Plain and Central Liaoning) over east China.Our results indicate that a northward cross-border transport from the Yangtze River Delta to Central Liaoning below 2km above ground played an important role in the formation of these regional high PM2.5 episodes.Contributions of regional transport from outside city clusters presented an increasing trend from south to north.In the northernmost cluster (Central Liaoning),the contribution from other city clusters reached 40–50% during the two episodes.In contrast,it was below 10% in the Yangtze River Delta (southernmost cluster).Mixing accumulation of pollutants from various city clusters during transport was responsible for this trend.Furthermore,a preliminary estimate shows that cross-border transport of PM2.5 might increase 0.5-3% daily mortality during the high PM2.5 episodes.