Identification of NO₂ and SO₂ over China: Characterization of polluted and hotspots Provinces

Increasing emissions of aerosol and trace gases (e.g. nitrogen dioxide: NO₂ and sulfur dioxide: SO₂) have resulted in severe air pollution in China due to its rapid industrialization, economic growth, and urbanization. This resulted in numerous environmental and health problems, and poor air quality mainly in industrial areas and major cities. This study identifies long-term (2005‒2020) Ozone Monitoring Instrument (OMI) based NO₂ and SO₂ pollution hotspots across China by analyzing spatiotemporal distributions and variations, with characterization of polluted provinces, SO₂/NO₂ ratio, trend, and assessing how effective China’s Air Pollution Control Policy (APCP) is on NO₂ and SO₂. Results show that NO₂ and SO₂ pollution hotspots were seen in China's central (Hubei), eastern (Anhui, Jiangsu, Shandong, Zhejiang), northern (Beijing, Hebei, Henan, Shanxi, Tianjin), northeast (Liaoning, Jilin), northwestern (Urumqi), southern (Guangdong, Hong Kong), and southwest (Chongqing, Sichuan). However, the pollution level was higher in winter, followed by autumn, spring, and summer. China’s eight provinces (Tianjin, Shanghai, Shandong, Jiangsu, Beijing, Hebei, Hong Kong, and Henan) were identified as extremely polluted with high NO₂ levels ranging from 16.86 − 9.75 (10¹⁵ molecules/cm²), whereas Shandong, Tianjin, Hebei, Beijing, Henan, Shanxi, Jiangsu, Shanghai, Anhui, and Liaoning were deemed to extremely polluted provinces with high SO₂ levels ranging from 20.62 − 14.30 (10¹⁵ molecules/cm²). Moreover, the SO₂/NO₂ ratio for 27 Chinese provinces fluctuates between 1.02 to 4.98, indicating industries emit more SO₂ than NO₂. Finally, China’s air pollution control policies (APCP) led to the largest annual reductions in NO₂ during the 12th five-year plan (FYP) (6%‒94%) and SO₂ during the 11th FYP (6%‒74%). The present study concludes, however, that China’s APCP improved air quality by easing NO₂ and SO₂ emissions. This study recommends that the Chinese government may adopt a comprehensive strategy to reduce air pollution, including investing in clean energy, promoting electric vehicles, enforcing strict emission standards for industries, implementing green building practices, and raising public awareness about pollution reduction.