Trend reversal from source region to remote tropospheric NO₂ columns

Global tropospheric nitrogen dioxide (NO₂) changes have different or even opposite impacts on the photochemical formation of ozone in different regions under different weather and emission condition. However, the changes over regions affected by different levels of human activities are not well known. By using the Ozone Monitoring Instrument (OMI) measurements, we analyzed spatial and temporal variability of tropospheric NO₂ vertical column densities (VCDs) from the megacity to the background regions during 2005-2019. Consistent with previous research, our results show a rapid decline of tropospheric NO₂ column density over regions strongly affected by human activities, especially for source regions. The decline rates of annual mean NO₂ VCDs are up to - 2.44% year^-1, - 2.37% year^-1, and - 1.43% year^-1 over megacities of the USA, Europe, and China, respectively. However, the decreasing rate has slowed, and even reversed to an increasing trend, of tropospheric NO₂ from megacities to developing and remote regions, especially over ocean and background areas less affected by anthropogenic activity. From 2005 to 2019, the NO₂ VCDs over the ocean and background areas increased for all seasons, with the statistically significant (p < 0.05) trends of 1.15%/0.74% year^-1 (MAM), 1.20%/1.06% year^-1 (JJA), 1.16%/0.82% year^-1 (SON), and 0.68%/0.65% year^-1 (DJF), respectively, for ocean/background region. Such decreasing/increasing trends of tropospheric NO₂ over sources/remote regions may prevent the ozone air pollution to be effectively resolved to achieve air quality goals worldwide.