Temporal Variability of Tropospheric NO₂ over Cities in the United States, Western Europe and China from 2005 to 2022

Anthropogenic Nitrogen oxides (NO_x = NO₂ + NO) emissions are highly concentrated in urban area, and the weekly cycles, seasonal patterns and long-term trends of tropospheric NO₂ columns over cities differ from region to region due to different emission sectoral compositions and human activities. In this study, we used satellite observed tropospheric NO₂ column data to compare the long- and short-term NO₂ column density time series over cities in the United Sates (the U.S.), western Europe and China. The results showed that in all the targeted cities, the outbreak of the Corona Virus Disease in 2019 (COVID-19) moved the December peak of the city-level NO₂ columns forward to November and October or even earlier in 2020 and 2021. On weekly level, cities in the U.S. show the lowest NO₂ columns ratio on weekend/work day, then come the western European cities, and a weak weekly pattern is seen in Chinese cities. For all the cites, we find a higher weekend/work day NO₂ ratio in cold seasons than in warm seasons, indicating a higher contribution from NO_x emission sectors of residential, industry and power plants in the warm seasons. In the long-term, NO₂ columns over the U.S. and western European cities declined by a fraction twice that of the regional mean level from 2004 to 2021. In China, NO₂ columns started to decrease since 2012, at a similar rate between the city and regional level. This work confirms the importance to quantify and control NO_x emissions from cities.