Extreme haze episodes have frequently occurred in Seoul since mid-2010s by the combined contributions of transboundary transported aerosols as well as locally emitted pollutants. In this study, we developed a novel method to estimate the contribution of long-range transport (LRT, aerosols are transported from any regions except local area near Seoul) and local pollution (LP, aerosols are originated from local area near Seoul) cases to the PM10 concentration in Seoul, Korea, using the PM10 concentration ratio between surface (PM10S) and mountaintop (PM10M) sites and the lidar-derived mixing layer height. The overall contributions of LRT and LP events to nighttime high-PM10 episodes (PM10 > 50 μg m−3) during the period of May 2008–April 2019 were estimated to be approximately 32% and 47%, respectively. The monthly contribution of LRT events to the PM10 concentration varied from approximately 18% (July) to 43% (January), whereas the contribution of LP events was estimated between 39% (March) and 69% (July); this pattern was associated with seasonal synoptic circulations. The similar PM10S values between the LRT (71 ± 22 μg m−3) and LP (73 ± 26 μg m−3) cases during the nighttime suggest that a reduction in local PM10 emissions is crucial to decrease the PM10 concentration during high-PM10 events. The high PM10S for daytime LRT cases can be explained by the combined effects of increased local emissions and LRT aerosols. Display omitted •Develop a novel method to distinguish between long-range transport (LRT) and local pollution (LP) episodes•Overall contributions of the LRT and LP cases to the high-PM10 episodes were estimated as 32% and 47%, respectively.•Monthly contribution of LRT events to the PM10 concentration varied from approximately 18% (July) to 43% (January).•Contribution of LP events ranged from approximately 39% (March) to 69% (July).•A reduction in the local PM10 emissions is crucial to decrease the PM10 concentration during high-PM10 events.