Enhancing Global Estimation of Fine Particulate Matter Concentrations by Including Geophysical a Priori Information in Deep Learning.

Global fine particulate matter (PM 2.5 ) assessment is impeded by a paucity of monitors. We improve estimation of the global distribution of PM 2.5 concentrations by developing, optimizing, and applying a convolutional neural network with information from satellite-, simulation-, and monitor-based sources to predict the local bias in monthly geophysical a priori PM 2.5 concentrations over 1998-2019. We develop a loss function that incorporates geophysical a priori estimates and apply it in model training to address the unrealistic results produced by mean-square-error loss functions in regions with few monitors. We introduce novel spatial cross-validation for air quality to examine the importance of considering spatial properties. We address the sharp decline in deep learning model performance in regions distant from monitors by incorporating the geophysical a priori PM 2.5 . The resultant monthly PM 2.5 estimates are highly consistent with spatial cross-validation PM 2.5 concentrations from monitors globally and regionally. We withheld 10% to 99% of monitors for testing to evaluate the sensitivity and robustness of model performance to the density of ground-based monitors. The model incorporating the geophysical a priori PM 2.5 concentrations remains highly consistent with observations globally even under extreme conditions (e.g., 1% for training, R 2 = 0.73), while the model without exhibits weaker performance (1% for training, R 2 = 0.51).