Transboundary air pollution is one of the critical environmental problems in Southeast Asia; nevertheless the interaction between transboundary haze and local planetary boundary layer (PBL) remains unclear due to a lack of sufficient observations and sophisticated simulations. This study applied LiDAR observations and model simulations to comprehensively evaluate the interaction between transboundary haze and local PBL during a recent transboundary haze episode in Singapore in October 2023. Results show that upper-level southeasterly wind, especially the nocturnal low-level jet, created favorable conditions for aerosol transport when fire hotspot numbers increased in Indonesia. The rapid diurnal increase in PBL height facilitated the entrainment of upper-level haze into the PBL, interacting with ground-level aerosols through turbulent mixing. Model simulations further show the significant contributions of fire emissions to the transboundary haze episode in maritime Southeast Asia. The impacts extended toward the southern region of Peninsular Malaysia, driven by the persistent prevailing southeasterly wind. Plain Language Summary In Southeast Asia, transboundary air pollution that crosses borders is a significant environmental issue. However, we don't fully understand how the remote haze interacts with local upper-level atmosphere. Our study used LiDAR observations and model simulations to thoroughly examine this problem during a recent episode in Singapore in October 2023. Our findings indicate that upper-level southeasterly wind, particularly a low-level jet during the night, played a crucial role in transporting aerosols when there was an increase in fire hotspot numbers in Indonesia. In the daytime, the rise of planetary boundary layer (PBL) height helped drive the upper-level haze into the PBL, where it mixed with aerosols near the ground through the vertical movement of air parcels. Additionally, our simulations revealed that emissions from fires significantly contributed to the transboundary haze episode in maritime Southeast Asia. The impact of this haze extended toward the southern region of Peninsular Malaysia, primarily due to the persistent southeasterly wind prevailing in the area.
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