Monitoring long-term changes in NO2 pollution from global to city scale: A journey guided by environmental laws and policies

Air pollution presents a formidable global challenge, exerting profound impacts on climate, human well-being and the world economy. Notably, atmospheric NO₂, predominantly of anthropogenic origin, arises from diverse sources, including vehicular and industrial emissions. This study investigates the changes in global atmospheric NO₂ through analysis of satellite and ground-based data, focusing on the period from 2002 to 2019. Elevated NO₂ levels (> 8 × 10¹⁵ molec./cm²) are identified in regions such as the USA, Europe, India, China, the Middle East (MDE), South Africa (SA), Central Africa (CA) and selected regions in Brazil. Seasonal variability is evident, with peak concentrations in winter and troughs in summer, largely influenced by meteorological conditions and biomass burning. While NO_x emissions from road transport dominate the USA and Europe, various industrial activities drive elevated NO₂ levels in East China (EC), the Indo-Gangetic Plain (IGP) and SA. Noteworthy is the substantial decline (approximately -0.1 × 10¹⁵ molec./cm²/year) in NO₂ observed in the USA and Europe during the study period. In contrast, significant positive trends (approximately 0.06–0.1 × 10¹⁵ molec./cm²2/year) are noted in the MDE, EC, SA, CA and IGP. An additional analysis of NO₂ pollution in 3000 global cities reveals a declining trend in most cities in the USA and Western Europe (WE) at -0.1 × 10¹⁵ molec./cm²/year. Conversely, cities in India, China, Africa, Southeast Asia, MDE, and South America exhibit positive trends in NO₂, ranging from 0.04 to 0.1 × 10¹⁵ molec./cm²/year. The decreasing NO₂ trends in developed nations of North America and Europe are attributed to the enforcement of stringent vehicular norms, resulting in a significant reduction in road transport emissions. This study offers a comprehensive overview of recent NO₂ pollution trends across nations and cities, highlighting the contrasting trajectories between developed and developing regions. It also suggests potential strategies for developing nations to mitigate air pollution.