Global NO 2 changes between 2019 and 2024 as observed by TROPOMI in urban areas and emerging hotspots

We present a global assessment of space-based urban nitrogen dioxide (NO 2 ) observations from 2019 to 2024 using annual and monthly mean tropospheric vertical column densities (VCDs) from the TROPOspheric Monitoring Instrument (TROPOMI). Across 11 500 cities defined by the Global Human Settlement Layer-Settlement Model (GHS-SMOD), we find population-weighted annual mean urban NO 2 VCDs were lower in 2024 than 2019 in Europe (−13 %) and Asia and Oceania (−17 %), with seasonal decomposition indicating that annual changes are largely driven by concentration decreases during November–March. Aggregated urban VCD changes in North America, South America and Africa were statistically insignificant, though numerous individual cities exhibited significant changes. Of larger cities, Tehran had the largest annual mean NO 2 VCD ( > 30 × 10 15 molecules cm −2 ) and Seoul experienced the largest reduction (−9.4 ± 1.0 % yr −1 ; p < 0.001). We then calculate NO 2 VCD urban enhancements (VCD ENH ) by removing background concentrations from urban signatures and compare VCD ENH to changes in nitrogen oxide (NO x ) emissions from two emissions inventories, highlighting regions with potential inventory discrepancies. We find VCD ENH changes exceed changes in inventory NO x emissions in Europe, North America and Asia and Oceania, with worse agreement in the Global South. We further identify changes in NO 2 near fossil fuel operations and note conflict-related changes in NO 2 , highlighting the responsiveness of satellite NO 2 to certain societal disruptions. This work demonstrates the value in space-based remote sensing being an accountability agent for air pollution emissions on a global scale and to identify changes in NO 2 in otherwise unmonitored regions.