[Estimation of Near-surface O₃ Concentration in Chengdu-Chongqing Urban Agglomeration Based on CatBoost-LSTM Model]

Due to spatio-temporal differences in ozone sources and influencing factors， ozone （O₃）， an important air pollutant in the atmosphere， often exhibits spatial heterogeneity and temporal correlations. To enhance the spatial resolution and estimation accuracy of ozone， a high-precision ozone estimation model （CatBoost-LSTM） was constructed by combining CatBoost and LSTM. The model utilized ground-based ozone observations as response variables， Sentinel-5P TROPOMI offline data， ERA5 reanalysis data， and elevation as explanatory variables. The experimental results of Chengdu-Chongqing urban agglomeration showed that： ① In the overall model， CatBoost-LSTM exhibited the highest estimation accuracy compared to that of other models in the study. The coefficient of determination （R²）， root mean square error （RMSE）， and mean absolute error （MAE） on the verification set were 0.965， 5.81 μg·m^-3， and 4.42 μg·m^-3， respectively. ② Considering the seasonal differences in ozone concentration and its influencing factors， the CatBoost-LSTM seasonal model demonstrated varying levels of improvement in accuracy on the verification set， with the winter model showing the most significant enhancement. ③ The average monthly near-surface ozone concentration in the study area exhibited an inverted "V" trend， with a slight decrease observed in May， followed by reaching its peak concentration in August （89.08 μg·m^-3）， and then declining to its lowest level in December （29.3 μg·m^-3）. ④ There were obvious seasonal differences in the near-surface ozone concentration， with higher levels observed in summer （84.59 μg·m^-3）， followed by those in spring （72.62 μg·m^-3）， autumn （53.59 μg·m^-3）， and winter （35.23 μg·m^-3）. ⑤ In terms of spatial distribution， the areas with higher near-surface ozone concentration were mainly distributed in the western regions characterized by higher elevations， dense mountains， frequent industrial activities， high traffic density， dense population， and numerous pollution sources. Due to the limited intensity of industrial activities and traffic， as well as the relatively minor emissions from pollution sources and the flat topography， the overall ozone concentration in the eastern region at lower altitudes was comparatively lower.