Unexpected Delay in the Real‐Field Outdoor Aging of Biodegradable <scp>PBAT</scp> Mulch Films in Rainy Season: Revisiting the Role of Solar Irradiation

ABSTRACT Accurately predicting the service life of biodegradable mulch films (BPMs) is critical for their replacement of conventional polyethylene in sustainable agriculture. Here, we systematically investigated the aging behavior of poly(butylene adipate‐co‐terephthalate) (PBAT)‐based films under contrasting seasonal field conditions and controlled indoor accelerated weathering. It is unexpected to find that the aging rate of BPMs in dry season is almost double that in rainy season. By combining mechanical, thermal, molecular, and morphological analyses, we demonstrate that direct normal irradiance (DNI)—rather than humidity or total solar radiation—governs degradation and determines film service life. Notably, high humidity in rainy season indirectly extended film longevity by lowering DNI exposure. We further show that acceleration factors derived from DNI establish a robust correlation between indoor accelerated aging and real‐field performance, enabling reliable lifespan prediction across climates. Additives such as photo‐stabilizers, carbon black, and montmorillonite exerted environment‐dependent effects, highlighting the importance of climate‐adapted formulations. This study identifies DNI as a mechanistic predictor of biodegradable film degradation and provides a practical framework for region‐specific design, evaluation, and deployment of sustainable agricultural plastics.