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The thermal intemal boundary layer (TIBL) is associated with coastal pollution dispersion,which can result in high concentrations of air pollutants near the surface of the Earth.In this study,boundary layer height data which were obtained using a ceilometer were used to assess the effect of the TIBL on atmospheric pollutants in Qinhuangdao,a coastal city in North China.A TIBL formed on 33％ of summer days.When a TIBL formed,the sunshine duration was 2.4 hr longer,the wind speed was higher,the wind direction reflected a typical sea breeze,and the boundary layer height was lower from 9:00 LT to 20:00 LT compared to days without a TIBL.If no TIBL formed,the average concentrations of PM2.5 and PM10 decreased with increasing boundary layer height.However,when a TIBL was observed,the average concentrations of PM2.5 and PM10 increased with increasing boundary layer height.Because the air from the sea is dean,PM2.5 and PM10 concentrations reached minimums in the daytime at 16:00 LT.After 16:00 LT,the PM2.5 and PM10 concentrations increased rapidly on days when a TIBL formed,which indicated that the TIBL leads to the rapid accumulation of atmospheric pollutants in the evening.Therefore,the maximum concentrations of particulate matters were larger when a TIBL formed compared to when no TIBL was present during the night.These results indicate that it is suitable for outdoor activities in the daytime on days with a TIBL in coastal cities.