cng-term variations and trends of tropospheric and ground-level NO2 over typical coastal areas.

• The global tropospheric NO 2 columns have generally decreased in the past 18 years. • Several hotspots tropospheric NO 2 are observed over industrialized and highly populated coastal zone areas. • Atmospheric nitrogen deposition plays an important role in nitrogen pollution in Bohai Sea. • The highest tropospheric NO 2 occurs in winter, and the highest near-surface NO 2 occurs at 08:00 and 20:00 LT. • NO 2 trends observed from ground-based measurements and satellite-derived retrievals is consistent. Tropospheric and ground-level levels of nitrogen dioxide (NO 2) may have changed due to environmental policies and technological changes. Here, multiyear ground-based measurement (2014–2022) and satellite-derived retrieval (October 2004 to May 2023) datasets of NO 2 concentrations are used to study the long-term ground-level and tropospheric NO 2 patterns and trends at four time scales (interannual, quarterly, monthly, and hourly) over the three regions with the highest global NO 2 concentrations. Several tropospheric NO 2 hotspots are observed over industrialized and highly populated coastal zone areas. Over the past 18 years, these hotpots have generally decreased due to the implementation of strong and effective environmental protection policies. A significant trend reversal was observed in 2011 in East-central China. The maximum values of tropospheric NO 2 occurred in winter over East-central China, Western Europe and East-central USA, with concentrations of 13.010, 4.243 and 4.149 × 1015 molecules cm−2, respectively. The Bohai Sea exhibited disproportionately high tropospheric NO 2 levels, suggesting it as a significant contributor to regional nitrogen pollution. Ground-level NO2 shows similar temporal patterns to tropospheric concentrations, with over 82.8 % of sites in East-central China exceeding 30 μg m−3, compared to 12.8 % in Western Europe and 3.8 % in East-central USA. The analysis of diurnal variations in ground-level NO 2 concentrations shows a significantly decrease during the daytime compared to nighttime, reflecting that the peak occurs at approximately 08:00 and 20:00 LT, while the lowest concentrations at around 12:00 LT. These insights clarify the impacts of environmental policies on NO 2 levels and identify critical areas for targeted pollution control, enhancing our understanding of NO 2 dynamics in high-pollution regions globally. [ABSTRACT FROM AUTHOR]