Evaluating Spatial and Temporal Patterns of Airshed Classification in Thailand Using Object-Based Classification (OBC)

Abstract. Fine particulate matter (PM_2.5) remains a major public-health concern in Thailand, with seasonal peaks amplified by terrain-induced stagnation. We delineate nationwide airshed regimes by integrating topography with multi-variable meteorology in an object-based workflow coupled to K-means clustering. Monthly composites for 2003–2024 were assembled for digital elevation (DEM), planetary boundary-layer pressure (PBL; proxy for PBL height), 10-m wind speed (WS10), surface pressure (SUR), surface shortwave radiation (SRAD), TMAX/TMIN, vapor pressure (VAP), and precipitation (PPT), harmonized to a common grid and aggregated to objects. Candidate K=2–10 was evaluated; the Elbow criterion supported K=4.
Four physically interpretable clusters emerge—lowland, two transition belts, and highland—monotonically ordered by elevation and accompanied by coherent atmospheric gradients. From lowland to highland, SUR, TMAX, TMIN, VAP, and PPT decline, winds weaken in transition/highland zones, and PBL pressure decreases (consistent with higher PBL tops); SRAD is modestly lower aloft. Seasonality is systematic: November–February features expansion of transition/highland classes into basins under cooler, more stable conditions, whereas during the Southwest monsoon (May–October) transition belts retract as ventilation and wet scavenging strengthen. Despite boundary shifts, cluster identity at fixed locations is highly repeatable.
The zoning provides an operational scaffold for airshed-aware management: targeting monitoring and advisories in transition belts and basin interiors, supporting cross-provincial coordination, and informing regime-conditioned forecasting. Limitations include a dispersion-focused design and reliance on gridded products. Planned extensions will incorporate ventilation capacity (VC=WS10×PBLH), stagnation frequency, and expanded validation (Silhouette/Gap, bootstrap stability, external/temporal holdouts with independent PM_2.5).