Tracking the path to cleaner cities using global urban NO 2 monitoring from space

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Abstract Tracking air pollution is essential for evaluating the effectiveness of urban air quality and emission-control policies and their impact on public health. Unlike previous satellite-based urban NO₂ studies that typically rely on linear trends, aggregated data, and limited meteorological correction, we use daily high-resolution TROPOMI (TROPOspheric Monitoring Instrument) observations within a Generalized Additive Models-based framework (AirGAM) to estimate meteorology-adjusted, non-linear NO₂ TVCD (Tropospheric Vertical Column Density) trends across 5,435 cities with a population > 100,000 worldwide (2019–2024). ERA5-derived meteorological parameters are used to remove weather and seasonal effects so that trends primarily reflect changes in emissions. Of 5,435 cities, 1,400 cities had significant trends, with 79% showing declines, indicating an overall global drop in urban NO₂ TVCD. East Asia and particularly China (where 99% of cities with significant trends show declines), as well as Europe (95%) led the global reductions. Cities in the United States with significant trends ( n = 14) also experienced a decline. NO₂ TVCD levels increased most sharply in the cities of the Middle East, Central, and South Asia. The most populated examples are Tehran (3.1% yr −1 [95% CI: 0.7–5.5]) and Cairo (1.4% yr −1 [0.1–2.6]).

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