Anthropogenic activities, by far the largest source of nox into the atmosphere, induce a weekly cycle of no 2 abundances in cities. Comprehensive analysis of the 2005-2017 OMI NO 2 dataset reveals significant weekly cycles in 115 of the 274 cities considered. These results are corroborated by a full year of highresolution tRopoMi no 2 observations. the oMi dataset permits us to identify trends in the weekly cycle resulting from nox emissions changes. the data show a clear weakening of the weekly cycle over european and U.S. cities, an evolution attributed to the decline in anthropogenic emissions and the resulting growing importance of background no 2 , whereas no 2 lifetime changes also play a minor role. in particular, the Sunday no 2 columns averaged over all U.S. cities are found to increase, relative to the weekly average, from 0.72 during 2005-2007 to 0.88 in 2015-2017. The opposite tendency is recorded in regions undergoing rapid emission growth. Multiyear simulations over the U.S. and the Middle east using the chemistry-transport model MAGRITTEv1.1 succeed in capturing the observed weekly cycles over the largest cities, as well as the observed long-term trends in the weekly cycle. Nitrogen oxides (NOx = NO 2 + NO) play a key role in atmospheric chemistry: they catalyse tropospheric ozone formation, they impact the self-cleaning capacity of the atmosphere, and they are precursors of secondary inorganic aerosol, with consequences for climate and human health 1. Fossil fuel combustion is the dominant source of NOx in the atmosphere, estimated at ~60% of the global total, whereas emissions from vegetation fires, lightning and soils make up the rest 2. Because of their relation to human activities, anthropogenic NOx emissions often display a weekly cycle, with reduced NOx levels in and around cities on rest days. Similar cycles have been also observed for other pollutants, e.g. aerosols 3,4 , and for meteorological parameters such as cloudiness 4,5. The NOx weekly cycle was previously investigated using ground-based, aircraft and satellite measurements 6-8 , whereas spatial patterns observed from satellites were used to study the urban photochemistry with the help of models 9. These studies, however, rely either on relatively short data records of at most several months 8,9 or on satellite data from coarse resolution sounders 6,7. Here we use NO 2 column data from two nadir-viewing satellite sensors, the Ozone Monitoring Instrument (OMI 10) launched in July 2004, and the high-resolution Tropospheric Monitoring Instrument (TROPOMI), single payload of the Sentinel-5 Precursor (S5P) launched in October 2017 11. Both sensors have an equatorial crossing time of ca. 13:40 (local time), and provide daily global coverage at resolutions of 13 × 24 km 2 and 7.2 × 3.5 km 2 (at nadir) for OMI and TROPOMI, respectively. The long data record compiled for this study (2005-2019) and the high resolution of both instruments allows to provide more robust information on the NO 2 weekly cycle over a larger number of cities around ...