Assessing 3-D variability of ultrafine particle using a Geo-AI modelling approach: A case study in Zhunan-Miaoli, Taiwan.

Previous air pollution modeling studies have predominantly emphasized horizontal distributions, overlooking the critical vertical variability of pollutant concentrations in urban environments. Therefore, the three-dimensional (3-D) behavior of air pollutants, and of ultrafine particulate matter (PM0.1) in particular, is insufficiently characterized. This study examined the 3-D distribution of PM0.1 in the Zhunan and Toufen regions in Miaoli, Taiwan. Using a hexacopter drone, PM0.1 concentrations were measured at 12 locations, at altitudes of 40, 60, and 100 m. A geospatial-artificial intelligence (Geo-AI) model was developed to estimate 3-D PM0.1 concentrations, incorporating databases such as land use, meteorology, and 3-D building data as predictor variables. SHapley Additive exPlanations (SHAP) analysis for variable selection showed that key predictors were building height, temperature, carbon dioxide, nitric oxide, forest coverage and 3-D spatial distance from buildings. Five machine learning algorithms were used for modeling. Extreme Gradient Boosting Regressor (XGBR) achieved the best performance with a training R2 of 0.98. The model's robustness was further examined through 10-fold cross-validation and stratified validation, which yielded R2 values exceeding 0.85, indicating a strong ability to capture the spatial variation of PM0.1 across different environmental conditions. These findings underscored the crucial role of vertical pollutant dispersion in urban environments and the need to incorporate detailed 3-D measurements into urban planning and public health policies.