How Do Planetary Boundary Layer Schemes Perform in PM2.5-O3 Composite Pollution in the Beijing-Tianjin-Hebei (BTH)? An In-Depth Mechanism Analysis From the Underlying Code

Parameterization schemes undergo continuous updates, with most refinements documented in code implementations, necessitating in-depth code analysis for evaluation. Focusing on PM_2.5-O₃ composite pollution in the Beijing-Tianjin-Hebei region, 10 planetary boundary layer (PBL) schemes were evaluated for their simulation performance. The 2-m temperature (T₂) demonstrates daytime dependence on temperature gradient and transport coefficient, shifting to surface temperature dominance at night. Saturated mixing ratio predominantly governs 2-m relative humidity (RH₂) variations, while 10-m wind speed (WS₁₀) derives from the first-level wind speed. Vertical profile comparisons reveal degraded simulation accuracy at lower levels, attributed to human activities’ influences within the PBL. The total energy-mass flux scheme exhibited distinct deviations due to failure to recognize the existence of shallow cumulus, consequently suppressing vertical mixing. Turbulent diffusion coefficient (K) discrepancies in nonlocal closure schemes primarily stem from PBL height (PBLH) variations, whereas local closure schemes show mixing length dominance (∼63%). Dissipation rate considerations exert stronger impacts at lower levels. For hybrid schemes, K calculated by Monin-Obukhov similarity theory shows enhanced magnitudes during daytime. Statistical analysis indicates the Mellor-Yamada-Janjić scheme's minor T₂ deficiencies become insignificant when weighed against its superior vertical profiles of T, RH, and WS₁₀. These insights offer critical references for PBL scheme optimization.