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This study proposes an improved approach for monitoring the spatial concentrations of hourly particulate matter less than 2.5 μm in diameter (PM_2.5) via a deep neural network (DNN) using geostationary ocean color imager (GOCI) images and unified model (UM) reanalysis data over the Korean Peninsula. The DNN performance was optimized to determine the appropriate training model structures, incorporating hyperparameter tuning, regularization, early stopping, and input and output variable normalization to prevent training dataset overfitting. Near-surface atmospheric information from the UM was also used as an input variable to spatially generalize the DNN model. The retrieved PM_2.5 from the DNN was compared with estimates from random forest, multiple linear regression, and the Community Multiscale Air Quality model. The DNN demonstrated the highest accuracy compared to that of the conventional methods for the hold-out validation (root mean square error (RMSE) = 7.042 μg/m³, mean bias error (MBE) = −0.340 μg/m³, and coefficient of determination (R²) = 0.698) and the cross-validation (RMSE = 9.166 μg/m³, MBE = 0.293 μg/m³, and R² = 0.49). Although the R² was low due to underestimated high PM_2.5 concentration patterns, the RMSE and MBE demonstrated reliable accuracy values (<10 μg/m³ and 1 μg/m³, respectively) for the hold-out validation and cross-validation.