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Pollution challenges in India intensify during the post-monsoon period, particularly across the Indo-Gangetic Plain, where multiple emission sources intersect with adverse meteorological conditions to elevate particulate matter concentrations. This study investigates PM2.5 variations in Delhi-National Capital Region (NCR) during October-November of 2024 and 2025, with specific focus on the influence of crop-residue burning in Punjab and Haryana, local firecracker emissions, and temperature-driven atmospheric processes.In 2024, a total of 779 stubble-burning incidents were detected between 1st to 12th October in Punjab and Haryana, coinciding with days that recorded high PM2.5 levels in Delhi-NCR. Satellite-based Fire Radiative Power (FRP) signals and air-mass back-trajectory analyses further validated the long-range transport of smoke plumes into the NCR. In contrast, severe flooding in 2025 in large parts of Punjab and pockets of Haryana resulted in a 77.5% decline in fire incidents during this period. This created a unique natural experiment to assess Delhi’s pollution baseline under minimal agricultural burning influence. Correspondingly, Delhi’s seasonal average PM2.5, reduced by 15.5%, highlighting the substantial contribution of transported biomass burning aerosols to regional air quality degradation.However, despite significantly fewer regional stubble burning fire events, Delhi-NCR continued to experience notable PM2.5 spikes. Analysis indicates these increases in PM concentration were primarily driven by local emissions such as vehicular exhaust, road dust resuspension, refuse burning, episodic firecracker bursts around festive periods, construction and demolition activities. Additionally, secondary aerosol formation, particularly the conversion of NOx to nitrate under high humidity and low temperatures, contributed to elevated pollution loads.Meteorological conditions further intensified pollution build-up in this region. Significant night-time temperature dips, shallow planetary boundary layers heights, and low wind speeds limited vertical mixing and hindered pollutant dispersion, causing pollutants to remain trapped near the surface.The findings demonstrate that early-winter PM2.5 levels in Delhi-NCR are shaped by a complex interplay of transboundary smoke transport, persistent local emissions, and temperature-driven atmospheric processes that favour pollutant accumulation.