Nitrogen dioxide (NO₂) is a harmful air pollutant that originates from both anthropogenic and natural emissions sources, including fossil fuel combustion, biomass burning, lightning, and soils (Dix et al., 2020; Jin et al., 2021; Schumann and Huntrieser, 2007; Huber et al., 2024), with fossil fuel combustion accounting for ∼45 % of total global nitrogen oxide emissions (Song et al., 2021). Only a small amount of NO₂ is emitted from these sources directly, with nitric oxide (NO) being the primary emissions product that quickly cycles to NO₂ in the presence of oxidants such as ozone (O₃) or peroxy radicals (HO₂ or RO₂). The summed concentrations of NO and NO₂ are referred to as nitrogen oxides (NOx= NO + NO₂), as the concentrations of NO and NO₂ are inherently linked. NO₂ is more commonly targeted by regulatory measures than NO, as it constitutes the majority of atmospheric NO_x concentrations and is linked to increased morbidity and mortality from long-term exposure, particularly within urban environments (Chen et al., 2024). While NO_x is commonly associated with health risks, the direct association between NO_x exposure and adverse health outcomes remains uncertain (Anenberg et al., 2022). Despite this, NO_x contributes to known harmful secondary pollutants, including O₃ and fine particulate matter.

NO₂ concentrations are measured using: (1) in-situ monitoring, e.g. chemiluminescence analyzers at the surface, or (2) remote sensing instrumentation leveraging the unique spectral properties of NO₂, that absorbs light most efficiently in the visible portions (405–465 nm) of the electromagnetic spectrum (Lamsal et al., 2015). The latter method relies upon spectrometers detecting in the UV-Visible spectral range to infer NO₂ vertical column densities (VCDs), defined as the summed concentration of NO₂ in a column from the surface to an upper limit of the atmosphere, with the tropopause often used as the upper limit. Spectrometers have been used to measure NO₂ VCDs from ground-level directed upward, from aircraft directed downward, or from space-based satellites directed downward, including from the TROPOspheric Monitoring Instrument (TROPOMI) onboard the Sentinel-5P satellite (Herman et al., 2009; Fishman et al., 2012; Veefkind et al., 2012). NO₂ can also be remotely sensed from ground-based instruments capable of inferring vertical profiles of NO₂, such as using multi-axis differential optical absorption spectroscopy (MAX-DOAS; Vlemmix et al., 2010).

The earliest space-based spectrometers detecting NO₂ were flown on low-earth polar orbiting satellites and were launched within the mid-1990s to mid-2000s. These include the Global Ozone Monitoring Experiment (GOME; Burrows et al., 1999) and GOME-2 satellites, the SCanning Imaging Absorption spectroMeter for Atmospheric CHartographY (SCIAMACHY; Bovensmann et al., 1999) and the Ozone Monitoring Instrument (OMI; Levelt et al., 2006). The data collected using these instruments provided unique insight into atmospheric chemistry and composition across the globe, including in mostly unmonitored regions. OMI, launched in 2004, provided NO₂ VCDs at a spatial resolution of 13×24 km² at nadir and has remained operable for more than two decades at the time this was written, providing a valuable long-term record of NO₂ globally. OMI remained the highest resolution space-based NO₂ product until TROPOMI launched in 2017, which ultimately provided NO₂ VCDs at a spatial resolution of 3.5×5.5 km² at nadir. Observations at this resolution facilitated the evaluation of satellite NO₂ at previously unprecedented spatial scales, including at the intra-urban level (Goldberg et al., 2021a, 2024).

NO₂ trends have been characterized in urban and broader environments using space-based instruments. Earlier satellite studies used the GOME and SCIAMACHY satellites to identify increasing NO₂ VCD trends in China from the mid-1990s to the mid-2000s (Richter et al., 2005; Stavrakou et al., 2008; van der A et al., 2008), driven primarily by economic growth and industrialization. Later studies, incorporating OMI observations, highlighted further increases in China through the early 2010s, with VCDs and satellite-inferred surface concentrations steadily declining since (Miyazaki et al., 2017; Wang et al., 2019; Jiang et al., 2022). Europe has exhibited steady NO₂ VCD declines since the start of the satellite NO₂ record (Richter et al., 2005; Krotkov et al., 2016; Duncan et al., 2016), driven largely by the implementation of various emissions control technologies. In the United States, NO₂ concentrations generally exhibited a decreasing trend from 2005 through the mid-2010s (Lamsal et al., 2015), with VCD decreases more gradual since, in part due to an increased influence from regional background NO₂ levels (Jiang et al., 2018; Goldberg et al., 2021b; Dang et al., 2023). In contrast, urban regions of India have shown NO₂ increases over the past few decades, linked to urbanization and energy demand growth (Hilboll et al., 2013; Ghude et al., 2020). Over Africa and South America, NO₂ VCD trends through the mid-2010s have been less pronounced, reflecting limited industrialization and more dominant contributions from biomass burning and natural sources (Geddes et al., 2016; Castellanos et al., 2014). Additionally, numerous studies have highlighted the influence that the COVID-19 pandemic had on NO₂ globally, with most regions globally exhibiting broad NO₂ decreases in 2020 during numerous lockdowns and subsequent, regionally-distinct rebounds in emissions (Lonsdale and Sun, 2023; Fisher et al., 2024).

Satellite studies have been used to characterize trends within the urban environment specifically, using different methods to characterize the urban extent. Geddes et al. (2016) used GOME, SCIAMACHY and GOME-2 oversampled to a 0.1° × 0.1° grid to highlight NO₂ VCD trends globally, as well as in select urban areas, with the urban region defined as the surrounding ∼ 200 km × 200 km. Fioletov et al. (2022) and Fioletov et al. (2025) used urban density from the Gridded Population of the World (SEDAC, 2017) as a proxy for the extent of the urban environment to identify changes in urban NO_x emissions. Anenberg et al. (2022) used urban boundaries provided from the 2019 version of the Global Human Settlement Layer-Settlement model (GHS-SMOD) to evaluate NO₂ trends from 2000–2019 using surface NO₂ estimates derived from OMI NO₂ and a land-use regression model.

Here, we use TROPOMI tropospheric NO₂ VCDs to quantify general NO₂ changes globally from 2019 to 2024, with a particular focus on urban areas. The urban boundaries are defined by the 2023 version of GHS-SMOD, which provides urban cluster boundaries for all urban regions globally. We evaluate changes in annual mean urban NO₂ VCDs against NO_x emissions inventories and characterize the influence of different seasons on annual variations. We additionally note changes in select oil, gas, and other mining regions, which exhibit the largest changes globally outside of urban areas. This study represents the first detailed global-scale analysis of urban TROPOMI NO₂ from 2019 to 2024. Our findings illustrate how NO₂ responded to specific societal events during this timeframe, such as the impact of clean air policies, population migration away from urban areas due to war, the increased demand for fossil fuels and rare-Earth minerals, and the emergence and waning of a global pandemic. Furthermore, by directly linking observed NO₂ urban enhancements with NO_x emission inventory data from the updated EDGARv8.1, our work provides valuable insights into regions where emissions inventories align closely with observations, as well as areas exhibiting potential inventory discrepancies. This work underscores the critical value of satellite-derived NO₂ as a tool for urban air quality assessment and emissions management.

Using the method outlined in Sect. 2.2.1, the GHS-SMOD urban cluster boundaries are used to determine mean TROPOMI NO₂ concentrations for all urban clusters globally. Of all 11 534 GHS-SMOD urban clusters, 58.1 % are in Asia and Oceania, 18.5 % are in Africa, 10.9 % are in Europe, 6.2 % are in North America and 6.3 % are in South America. Looking at VCD changes from 2019 to 2024 in the 50 cities representing the ten most populous urban clusters on each continent, with Asia and Oceania considered jointly, East Asian cities represent four and European cities represent five of the ten largest VCD decreases (Fig. 2a). Seoul experienced the greatest absolute reduction in annual mean NO₂ VCD of any of these 50 cities (Fig. 2b), representing a significant decrease of -9.4 ± 1.0 % yr⁻¹ (p < 0.001; Fig. 2c). London, England produced the greatest NO₂ VCD decrease of the ten most populous European cities (-5.4 ± 1.3 % yr⁻¹; p < 0.001), occurring alongside the introduction of the city's ultra-low emission zone introduced in 2019 and expanded in 2023, which has contributed to decreased local NO₂ concentrations (Hajmohammadi and Heydecker, 2022).

None of the ten largest South American cities experienced statistically significant changes in NO₂ VCD, with relative changes typically less than ±0.6 × 10¹⁵ molecules cm⁻² (Fig. 2b). The most notable exception is Santiago, Chile, which experienced an annual mean VCD difference of nearly -2.2×1015 molecules cm⁻² between 2019 and 2024. Of the largest North American cities, significant decreases occurred in Los Angeles (-2.8 ± 0.6 % yr⁻¹; p = 0.004), and the San Francisco Bay Area (-2.8 ± 0.6 % yr⁻¹; p = 0.023), while significant increases occurred in the Mexican cities of Guadalajara (+1.9 ± 0.8 % yr⁻¹; p = 0.019) and Mexico City (+2.7 ± 0.8 % yr⁻¹; p = 0.010).

Most of the largest African cities experienced increased NO₂ VCDs from 2019 to 2024, with Abidjan, Ivory Coast experiencing the largest urban increase (+6.6 ± 1.2 % yr⁻¹; p < 0.001), with additional increases occurring in Cairo, Egypt (+2.3 ± 0.8 % yr⁻¹; p= 0.006); Addis Ababa, Ethiopia (+2.4 ± 1.1 % yr⁻¹; p= 0.012); and Kinshasa, DR Congo (+3.8 ± 0.6 % yr⁻¹; p < 0.001). In the Sudanese capital of Khartoum, NO₂ VCDs started decreasing in 2023, coinciding with the onset of conflict within Sudan (Guo et al., 2023; Fig. S10). This resulted in the largest absolute NO₂ VCD decrease of any African city from 2019 to 2024 (Fig. 2b), and a decrease of -13.1 ± 1.4 % yr⁻¹ (p < 0.001).

Of the cities presented in Fig. 2, the three largest absolute decreases between 2019 and 2024 were in the East Asian cities of Seoul, South Korea (Fig. 3a); Shanghai, China (Fig. 3b); and Guangzhou, China (Fig. 3c). Decreases in Seoul coincide with known policies implemented by the South Korean government since the early 2000s to reduce local emissions, as well as changes in emissions that began following the COVID-19 pandemic (Ho et al., 2021; Seo et al., 2021). Moscow experienced the largest NO₂ VCD increase of any large GHS-SMOD city through 2024, with a VCD increase of +9.97 % yr⁻¹ (p= 0.001). This increase was accompanied by anomalously high monthly mean concentrations in early 2022 (Fig. S11), following the onset of the Russia-Ukraine war in Ukraine, when monthly mean NO₂ VCDs for March reached 59×1015 molecules cm⁻² (see Sect. 3.3).

Aggregating the NO₂ VCD changes to the country level by considering the population of each urban cluster (Eq. 1), we identify population-weighted VCD changes in countries globally (Fig. 4). The majority of urban NO₂ VCD increases were observed in much of Central America including Mexico, in Africa, in the Middle East and in Central Asia. Russia experienced the largest population-weighted VCD increase of 6.2 ± 3.6 % yr⁻¹ (p= 0.046). Broad urban VCD decreases were observed in numerous countries across Western and Central Europe, as well as Eastern Asian countries. The largest urban population-weighted decrease occurred in South Korea (-8.74 ± 0.9 % yr⁻¹; p < 0.001).

Much of the Middle East exhibited substantial increases in urban population-weighted NO₂ VCDs from 2019 to 2024, including in Saudi Arabia (+2.0 ± 0.9 % yr⁻¹; p= 0.009), Iraq (+3.7 ± 0.8 % yr⁻¹; p < 0.001), and Iran (+2.1 ± 0.9 % yr⁻¹; p= 0.013), with broad increases that extend beyond the urban environment. One of the most salient VCD decreases in the Middle East occurred in Lebanon (-8.5 ± 1.0 % yr⁻¹; p < 0.001), coinciding with the country's severe economic and financial crisis that began in late 2019 (Harake et al., 2021). VCD decreases through 2024 were particularly stark in the Lebanese capital Beirut (-7.9 ± 1.1 % yr⁻¹; p < 0.001). Additional Middle Eastern countries that exhibited decreased urban NO₂ VCDs through 2024 include much of Israel (-4.5 ± 0.9 % yr⁻¹; p< 0.001), Qatar (-3.4 ± 1.2 % yr⁻¹; p= 0.004), and Afghanistan (-3.8 ± 0.8 % yr⁻¹; p= 0.003). Notable urban NO₂ VCD changes in less populated countries of Asia and Oceania include decreases in Cambodia (-5.0 ± 0.9 % yr⁻¹; p< 0.001), Sri Lanka (-5.4 ± 0.9 % yr⁻¹; p< 0.001) and Australia (-3.0 ± 0.9 % yr⁻¹; p= 0.008). Urban increases were observed in much of Central Asia, including Uzbekistan (+4.4 ± 0.9 % yr⁻¹; p< 0.001) and Turkmenistan (+4.5 ± 0.5 % yr⁻¹; p< 0.001).

NO₂ VCD decreases for more populous countries with an urban population of at least nine million were largest in East Asia, including China (-6.0 ± 1.0 % yr⁻¹; p< 0.001) and Japan (-4.1 ± 0.6 % yr⁻¹; p< 0.001) (Fig. 5). Urban population-weighted VCD decreases in South Korea were particularly pronounced, with a population-weighted concentration difference of -5.6×1015 molecules cm⁻² between 2019 and 2024. In South Asia, the neighboring countries of Afghanistan (-3.8 ± 0.8 % yr⁻¹; p= 0.003) and Pakistan (-3.0 ± 0.8 % yr⁻¹; p= 0.012) exhibited some of the only significant country-level VCD decreases for the region. Significant decreases also occurred in numerous countries of Western and Central Europe, with Germany experiencing the largest VCD decrease in Europe through 2024 (-6.7 ± 0.9 % yr⁻¹; p< 0.001). Of the most-populous European countries, Russia was the only country to experience increased population-weighted NO₂ VCDs through 2024.

A majority of larger African countries exhibited insignificant urban VCD changes, with 2024 population-weighted VCDs changing by less than 0.25×1015 molecules cm⁻² relative to 2019 levels (Fig. 5b). Exceptions include larger changes in Sudan (-6.1 ± 0.8 % yr⁻¹; p < 0.001) and Ivory Coast (+4.9 ± 1.0 % yr⁻¹; p < 0.001). Middle Eastern and Central Asian countries experienced some of the largest urban VCD increases, with Iraq experiencing the largest difference between 2019 and 2024 levels of any larger country (+1.2×1015 molecules cm⁻²). Chile saw the largest difference in annual mean urban NO₂ VCD between 2019 and 2024 of any South American country, due in large part to lower 2024 annual mean NO₂ VCDs in the capital city of Santiago (Fig. 5b).

The following subsections describe NO₂ VCDs in five global subregions: Asia and Oceania, Africa, Europe, North America and South America

NO₂ can be readily observed over oil, gas, and other mining regions due to emissions from drilling and extraction equipment, processing plants, compressors, truck traffic, and routine or episodic flaring. In these settings, increases or decreases in NO₂ can signify shifts in production levels or changes in mining activity. Because NO₂ responds quickly to changes in combustion-related activity, satellite retrievals serve as an effective proxy for monitoring relative operational intensity in major extraction regions (Dix et al., 2022).

Known coal-dominated mining regions showed pronounced NO₂ VCD increases from 2019 to 2024 (Fig. 12). The sparsely-populated Santanghu Basin (Fig. 12a), a region in eastern Xinjiang Province with a relatively nascent coal mining industry (Zhang et al., 2018; Liu et al., 2018), represented the most substantial increase in VCD over China through 2024 (23.9 ± 1.6 % yr⁻¹; p< 0.001). The recent expansion of mining operations is evident in visible satellite imagery (Fig. S20). The largest regional increase in VCD anywhere in India from 2019 to 2024 (+2.1×1015 molecules cm⁻²) was observed in the Ib Valley in northwestern Odisha state (Fig. 12b). The region contains multiple surface coal mines and coal-fired power plants (Varma et al., 2015), with VCDs increasing at a rate of 8.2 ± 1.3 % yr⁻¹ (p< 0.001). NO₂ VCDs near numerous other coal mines and power plants throughout India exhibited changes, but NO₂ VCD increases were more prevalent than decreases. In the Kuzbass Region of Siberia, one of Russia's largest coal mining regions, 2024 annual mean VCDs were 2.4×1015 molecules cm⁻² higher than in 2019, though annual changes were not statistically significant (Fig. 12c). A previous study identified a correlation between space-based NO₂ observations and regional coal production in the Kuzbass region (Labzovskii et al., 2022), providing relevant context for the observed VCD changes. Increased VCDs were also observed over rare earth metal mines. In a mining region known as the Copperbelt in the south of the Democratic Republic of the Congo (DRC), broad NO₂ VCD increases were observed, including at a large surface copper and cobalt mine near the city of Kolwezi (Fig. 12e). VCDs at the Kolwezi mine increased at a rate of 10.1 ± 0.9 % yr⁻¹ (p< 0.001) from 2019 to 2024. Numerous surface mines exist in the region, with most observing increases in NO_x emissions from mining operations in recent years (Martínez-Alonso et al., 2023).

Not all coal regions experienced increased VCDs. Northwest of Johannesburg, South Africa in Limpopo Province, NO₂ VCDs near the Grootegeluk surface coal mine, together with two adjacent power plants (Faure et al., 1996; Shikwambana et al., 2020) decreased at a rate of -5.2 ± 1.2 % yr⁻¹ (p< 0.001) from 2019 to 2024 (Fig. 12d). The region represented one of the largest NO₂ signatures in Africa in 2024, despite the significant decrease for this period (Fig. 8a).

Oil and gas extraction areas in North America experienced diverse patterns. Annual mean NO₂ VCDs at the Athabasca oil sands in Alberta, Canada were slightly lower in 2024 than in 2019, although the decrease for the period was insignificant (p> 0.05; Fig. 12f). The Bakken region in North Dakota, US experienced a similarly insignificant decrease in VCDs (Fig. 12i). Notable increases occurred in the Permian (Fig. 12g) and Uintah (Fig. 12h) Basins in the southwestern US experiencing significant increases of 5.8 ± 0.7 % yr⁻¹ (p< 0.001) and 7.6 ± 1.1 % yr⁻¹ (p< 0.001), respectively.

We present a global analysis of urban TROPOMI tropospheric NO₂ VCD from 2019 to 2024 using GHS-SMOD-defined urban boundaries, encompassing more than 11 500 cities. Our results reveal statistically lower urban population-weighted NO₂ VCDs in 2024 than in 2019 in Asia and Oceania (-17 %) and Europe (-13 %) with particularly strong reductions in cities including Seoul (-9.4 ± 1.0 % yr⁻¹; p< 0.001), Guangzhou (-5.6 ± 1.3 % yr⁻¹; p< 0.001), and London, England (-5.4 ± 1.3 % yr⁻¹; p< 0.001). These decreases generally reflect a combination of long-term emissions control policies and economic incentives, indicating policies to tackle NO₂ pollution have broadly worked. COVID-19 induced reductions in activity often caused a temporary NO₂ reduction but is unlikely to have caused much of the long-term changes between 2019 and 2024. Conversely, urban NO₂ in numerous African cities have increased over the same period, with Abidjan (+6.6 ± 1.2 % yr⁻¹; p< 0.001), Cairo (+2.3 ± 0.8 % yr⁻¹; p= 0.006) and Addis Ababa (+2.4 ± 1.1 % yr⁻¹; p= 0.012) representing larger cities that are leading the continent's upward tendency. Though numerous populous North American cities exhibited significant VCD decreases, population-weighted urban levels for the continent as a whole did not show a significant change. Similarly, South American cities exhibited an insignificant VCD change from 2019 to 2024, apart from May-September in 2020. Population-weighted NO₂ VCDs increases were most notable in countries in the Middle East and Africa, highlighting a potential degradation in air quality in regions of the world that lack extensive ground-level monitoring.

Evaluating annual changes in TROPOMI NO₂ urban enhancements (VCD_ENH) – the difference between mean urban and background VCDs – against changes in EDGAR and CEDS NO_x emissions inventories, we highlight potential discrepancies in inventory estimates in urban regions. In African, Asian and European cities, changes in VCD_ENH tend to exceed changes in both EDGAR and CEDS emissions, pointing to potential inventory overestimates in NO_x emissions. In North America, EDGAR agrees well with VCD_ENH (mean difference of 0.3 % relative to 2019 values), while CEDS NO_x emissions are 6.1 % lower than VCD_ENH, relative to their respective 2019 values. These mismatches may stem from rapidly evolving emission sources or limitations in the EDGAR and CEDS bottom-up inventory methods. Similar discrepancies in emissions inventories in the Global South have been reported in previous studies (Ahn et al., 2023), suggesting larger emissions uncertainties in regions where unmonitored emissions activity may be significant.

In most regions, VCD changes from 2019 to 2024 were driven by changes during the colder months (November–March). This was most pronounced in Asian cities, where mean cold season VCDs decreased by -1.2×1015 molecules cm⁻² (-18 %) from 2019 to 2024, compared with warm season VCD decreases of -0.5×1015 molecules cm⁻² (-13 %). Large changes in NO₂ were not confined to urban regions alone. We identified localized increases near fossil fuel and other mining operations, including in the Santanghu Basin in China (+23.9 ± 1.6 % yr⁻¹; p< 0.001), the Permian (+5.8 ± 0.7 % yr⁻¹; p< 0.001) and Uintah (+7.6 ± 1.1 % yr⁻¹; p< 0.001) Basins in the US, and the Copperbelt region of the DRC (10.1 ± 0.9 % yr⁻¹; p< 0.001), signaling expanding industrial activity. In Khartoum and Kyiv, conflict and displacement drove sharp reductions in NO₂, demonstrating the utility of satellite data in detecting societal disruptions.

Several limitations of this work should be noted. First, satellite NO₂ column densities may not always reflect surface-level NO₂ concentrations, particularly in regions with vertically elevated sources. In urban areas dominated by surface-based transportation emissions, NO₂ VCDs are likely more representative of surface exposure. However, in areas with tall-stack sources, such as power plants, NO₂ columns may be decoupled from near-surface levels (Brett et al., 2025). Second, we assume static city boundaries defined by the 2023 version of GHS-SMOD, with population estimates from 2020. This is likely a reasonable approximation for urbanized regions in Europe and North America, where built-up area changes are slow, but may introduce uncertainty in rapidly urbanizing regions of Africa and Asia over a six-year period. Future analyses could incorporate time-varying urban boundaries to address this. Additionally, while many of the changes presented here reflect variability in anthropogenic NO_x emissions, it is important to recognize that atmospheric chemistry also influences the observed NO₂ variability. Seasonal differences in photochemical lifetimes (i.e., longest in winter), boundary layer mixing (i.e., more vertical mixing in summer), chemical partitioning between NO and NO₂, meteorological variability, and contributions from additional emissions sources including soil NO_x and fire emissions, can all modulate the magnitude and timing of observed NO₂ concentrations. These processes likely contribute to some of the regional and seasonal differences highlighted in this study.

Taken together, these results demonstrate the utility of high-resolution satellite instruments for characterizing both broad regional NO₂ signals and localized changes, and linking with anthropogenically induced factors such as urban growth, industrial expansion, policy interventions, and conflict. This highlights potential in using TROPOMI observations as an accountability agent to determine how local changes in human activities affect local and global air pollution. As the TROPOMI record lengthens and newer, geostationary satellites come online and begin to detect changes in atmospheric composition, continued space-based monitoring will be essential for improving our understanding of atmospheric composition and chemistry around the globe.