Long-term spatiotemporal dynamics, hotspot persistence, and extreme AOD days over Bursa, Türkiye

Satellite-based aerosol optical depth (AOD) enables long-term monitoring of atmospheric aerosol loading in regions with sparse ground-based observation networks. In this study, the spatial and temporal variability of 550 nm AOD over Bursa during 2003–2025 was investigated using Moderate Resolution Imaging Spectroradiometer (MODIS) Multi-Angle Implementation of Atmospheric Correction (MAIAC) MCD19A2 Collection 6.1 data at 1 km spatial resolution. Annual and seasonal composites were used to evaluate long-term trends, seasonality, the persistence of high-AOD areas, and extreme aerosol days. The common analysis domain covered 83.20% of the provincial area. The area-weighted annual AOD series showed a small negative trend in the full-period analysis (Theil-Sen slope = -0.510 × 10 − 3 AOD year − 1 ; p = 0.035). The negative slope direction remained consistent across the regional trend sensitivity analyses, whereas statistical significance at p < 0.05 was not retained under several annual-coverage and leave-one-year-out scenarios. After correction for multiple testing, 1,550 pixels exhibited significant trends, of which 1,539 showed decreases. AOD was highest in summer and lowest in winter, while none of the seasonal trends was significant. Persistent high-AOD areas were concentrated along the northern plain and urban corridor, with 102 pixels remaining in the annual upper-decile class throughout all 23 years. No robust trend was detected in the frequency of extreme AOD days. Overall, the different dimensions of the aerosol record did not evolve in parallel: mean AOD showed a modest negative long-term tendency, the spatial structure of recurrent high-AOD areas remained persistent, and extreme-day frequency showed no detectable trend.