Satellite evidence for changes in the NO₂ weekly cycle over large cities

Anthropogenic activities, by far the largest source of NOx into the atmosphere, induce a weekly cycle of NO₂ abundances in cities. Comprehensive analysis of the 2005-2017 OMI NO₂ dataset reveals significant weekly cycles in 115 of the 274 cities considered. These results are corroborated by a full year of high-resolution TROPOMI NO₂ 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 NO₂, whereas NO₂ lifetime changes also play a minor role. In particular, the Sunday NO₂ 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.