The planetary boundary layer top as a valve: Unraveling bidirectional aerosol transport

Cross-boundary transport at the planetary boundary layer (PBL) top regulates aerosol vertical structure. Using unmanned aerial vehicle (UAV) profiles along the Jiangsu coast (March-April 2025), we quantify bidirectional aerosol exchange across the PBL top (PBLt). The results suggest that diurnal PBL variation controls transport direction under the observed coastal conditions. During PBL ascent, aerosols are transported from the base of the free troposphere (FTb) into PBLt, while during PBL descent, the flow reverses from PBLt to FTb. During PBL growth, FTb-to-PBLt transport dominates under all three conditions, though driven by distinct mechanisms. On cleaning days, significant PBL growth (reaching 1500-1750 m) and a strong upward aerosol number concentration (Na) gradient (peaking at 12.76 cm^-3/m) by 16:00LT indicated approaching transport reversal. During haze, suppressed PBL height (550-600m) corresponded to weakened FTb-to-PBLt transport activity. For external aerosol intrusion, direct contact between the external aerosol layer and the PBLt, combined with downdrafts and PBL uplift, enhanced FTb to PBLt input from 13:00-16:00LT. During the descent phase, PBLt-to-FTb export occurred in all cases, with duration modulated by stability and surface wind fields. On cleaning days, minimal nocturnal PBL height (100-150m) initially favored PBLt-to-FTb transport until a low-level jet (LLJ) reversed the direction. During haze, an earlier PBL collapse (to 100m by 19:00LT) and premature LLJ onset shortened the export phase. Under external aerosol intrusion, extreme nocturnal stability (PBL: 50m), surface inversion, and absent wind shear nearly halted cross-layer transport after 22:00LT. The Na gradient and aerosol transport showed significant variations based on weather type and diurnal variations. During PBL growth (07:00-16:00LT), the upward gradient strengthened toward 16:00LT and peaked at 12.76 cm^-3/m (clean), 4.44 cm^-3/m (haze), and 1.57 cm^-3/m (intrusion). During PBL decay, the Na gradient weakens, with a downward gradient at the PBL minimum (22:00LT; 4.10 cm^-3/m) only on clean days, while haze and intrusion remain near-neutral (0.60 and 0.18 cm^-3/m). Aerosol cross-layer transport exhibited marked size dependence: dominated by 0.14-0.16 μm particles on clean days; broadened to 0.14-0.20 μm during haze; and dependent on PBL diurnal phase during external aerosol intrusion, primarily 0.16 μm and 0.20 μm during PBL uplift, and predominantly 0.20 μm during PBL decline.