Fig. 1. Study area

Fig. 2. Linear fitting between ground measured NO₂ and OMI NO₂ in major cities of Shaanxi-Gansu-Ningxia region from 2014 to 2019. (a) Xi'an; (b) Lanzhou; (c) Yinchuan

Fig. 3. Monthly average value of NO₂ column density in Shaanxi-Gansu-Ningxia region from 2005 to 2019

Fig. 4. Seasonal variation of NO₂ column density in Shaanxi-Gansu-Ningxia region from 2005 to 2019

Fig. 5. Seasonal average NO₂ column density of Shaanxi-Gansu-Ningxia region during 2005 to 2019. (a) Spring; (b) summer; (c) autumn; (d) winter

Fig. 6. Annual variation trend of NO₂ column density in Shaanxi-Gansu-Ningxia region from 2005 to 2019

Fig. 7. Spatial distribution of annual mean concentration of NO₂ column density in Shaanxi-Gansu-Ningxia region from 2005 to 2019

Fig. 8. Seasonal wind direction and frequency maps of Shaanxi (a), Gansu (b) and Ningxia (c)

Fig. 9. Relationship between monthly average temperature and NO₂ column density in Shaanxi-Gansu-Ningxia region from 2005 to 2019. (a) Scatter diagram; (b) trend graph

Fig. 10. Relationship between monthly average precipitation and NO₂ column density in Shaanxi-Gansu-Ningxia region from 2005 to 2019. (a) Scatter diagram; (b) trend graph

Fig. 11. Variation tendency of economic indicators in Shaanxi-Gansu-Ningxia region from 2005 to 2018