Fig. 1. Flow chart of ground-level NO₂ concentration estimation

Fig. 2. Influence of ground station distribution on estimation results. (a) Geographic position of ground stations; (b) estimation of model with latitude and longitude coordinate features; (c) estimation of model with spherical coordinate features; (d) estimation of model without sample geographic information features

Fig. 3. Analysis of influence of predictive variables on model. (a) Variance inflation factor (VIF) from each feature of estimation model; (b) absolute mean about SHAP value of each feature from OMI dataset; (c) Beeswarm image from SHAP value of each feature in OMI dataset

Fig. 4. Spatio-temporal coverage of satellite data. (a) Spatial distribution of tropospheric NO₂ data coverage by OMI; (b) spatial distribution of tropospheric NO₂ data coverage by TROPOMI; (c) daily coverage of data about TROPOMI and OMI from 2018 to 2021

Fig. 5. Scatter plot of cross-validation from ground-level NO₂ estimation by TROPOMI and OMI datasets from 2018 to 2021. (a) Scatter plot of estimation from TROPOMI; (b) scatter plot of estimation from OMI

Fig. 6. Comparison and analysis of spatial resolution of TROPOMI and OMI data. (a) Satellite image of Southeast China from Google Earth; (b) L2 orbit data of TROPOMI; (c) L2 orbit data of OMI; (d) ground-based data; (e) estimation of TROPOMI; (f) estimation of OMI; (g) satellite image of central China from Google Earth; (h) L2 orbit data of TROPOMI; (i) L2 orbit data of OMI; (j) ground-based data; (k) estimation of TROPOMI; (l) estimation of OMI

Fig. 7. Comparison of scatter plots between previous and optimized models from 2018 to 2021. (a) Estimation of optimized model; (b) estimation of previous model

Fig. 8. Comparison of optimization results of high-value models. June 5, 2019: (a) estimation of previous model around Yangtze River Delta; (b) estimation of optimized models; (c) ground-based data; August 8, 2019: (d) estimation results of original model in area around Pearl River Delta; (e) estimation of optimized models; (f) ground-based data